key: cord-0890936-hf54ic40
authors: Kendall, M.; Milsom, L.; Abeler-Dorner, L.; Wymant, C.; Ferretti, L.; Briers, M.; Holmes, C.; Bonsall, D.; Abeler, J.; Fraser, C.
title: COVID-19 incidence and R decreased on the Isle of Wight after the launch of the Test, Trace, Isolate programme
date: 2020-07-14
journal: nan
DOI: 10.1101/2020.07.12.20151753
sha: 6f88725264189fabb8909bcd48b14fe0a28b647b
doc_id: 890936
cord_uid: hf54ic40

In May 2020 the UK introduced a Test, Trace, Isolate programme in response to the COVID-19 pandemic. The programme was first rolled out on the Isle of Wight and included Version 1 of the NHS contact tracing app. We used COVID-19 daily case data to infer incidence of new infections and estimate the reproduction number R for each of 150 Upper Tier Local Authorities in England, and at the National level, before and after the launch of the programme on the Isle of Wight. We used Bayesian and Maximum-Likelihood methods to estimate R, and compared the Isle of Wight to other areas using a synthetic control method. We observed significant decreases in incidence and R on the Isle of Wight immediately after the launch. These results are robust across each of our approaches. Our results show that the sub-epidemic on the Isle of Wight was controlled significantly more effectively than the sub-epidemics of most other Upper Tier Local Authorities, changing from having the third highest reproduction number R (of 150) before the intervention to the tenth lowest afterwards. The data is not yet available to establish a causal link. However, the findings highlight the need for further research to determine the causes of this reduction, as these might translate into local and national non-pharmaceutical intervention strategies in the period before a treatment or vaccination becomes available.

As part of efforts to control the spread of COVID-19 using non-pharmaceutical interventions, many countries are prioritising community testing, case isolation, contact tracing, and quarantining of contacts of cases [3] . Contact tracing usually requires individually questioning index cases (individuals identified as having , relying on their memory of past close proximity contact events [4] ; contact tracing may also be conducted via smartphone contact tracing apps which can be used to pass anonymised notifications between new index cases and 1 their past contacts [5, 6] . These two approaches are complementary: the former approach relies on noticing and remembering contacts and an efficient infrastructure of trained public health officials; the latter relies on establishing a well-calibrated new technology and appreciable population uptake and adherence. The former is a more familiar intervention and a phone call may be more effective once received, but the speed of notification of the latter may be critical given the speed of COVID-19 transmission [5] . In May 2020, a pilot of this combined approach was conducted on the Isle of Wight, an island to the south of England with 141,536 inhabitants, of whom 24% are over the age of 65 [7] .

The UK national policy of "Test, Trace, Isolate" (TTI) included community testing and contact tracing conducted by Public Health England using the Contact Tracing and Advisory Service (CTAS). On the Isle of Wight it also included version 1 of the NHS contact tracing app. The app was configured for people to report two characteristic symptoms of COVID-19: cough and fever. Shortly after reporting symptoms, the app anonymously notified other app users who had been in recent contact with the individual, and offered generic guidance on how to reduce risks of transmission to others (though no instructions to self-isolate or quarantine).

The Isle of Wight TTI programme was launched on 5 May 2020, with the app made available for download for the general public from 7 May. The app was downloaded by more than 54,000 people on the island during the trial period, corresponding to 38% of the population [8] . On 18 May community testing was introduced nationwide, initially for all people above the age of 5, then on 28 May for everyone. Also on 28 May, manual contact tracing through the national contact tracing service was re-introduced nationwide. At the time of writing, there is no contact tracing app nationwide. The full timeline is shown in Figure 1 .

Between the 6th May and 26 May, 160 cases on the Isle of Wight were reported to manual contact tracing, resulting in 163 individuals receiving a notification and request to self-isolate, and during the same period 1,524 people reported symptoms to the app resulting in 1,188 receiving an exposure notification.

In this study we report trends in incidence on the Isle of Wight before and during the period of this pilot, and compare these trends with other areas of England. We focus on total incidence, per capita incidence, and the reproduction number R. We do not analyse data on the function of the contact tracing intervention; we rather explore whether the intervention might have been associated with declines in rates of new infection that stand out compared to trends in other areas of England.

New laboratory-confirmed COVID-19 cases are reported daily for each of 150 areas of England, specifically Upper-tier Local Authorities (UTLAs), of which the Isle of Wight is one. There are two categories of case data: "Pillar 1" data which consists of tests conducted in hospitals, for those with a clinical need and health and care workers, and "Pillar 2" data from tests in the wider population [9] . Pillar 1 and Pillar 2 data represent tests taken at different stages of individuals' disease progression, which needs to be considered for analysis. Estimates of the total population size for each UTLA were obtained from the Office for National Statistics [ 10 ] .

We used a back-calculation approach to estimate the distribution of the time of infection in Pillar 1 data, assuming that most Pillar 1 tests are derived from hospital patients. We used a lognormal distribution with mean 5.42 days and standard deviation 2.7 days for the incubation period following a meta-analysis [11] , and a gamma distribution with mean 5.14 days and standard deviation 4.2 days for the time from symptom onset to hospitalisation [12] . Analyses were truncated at 14 June as calculation beyond this date would have been affected by missing data 25-29 June [13] .

The generation time distribution is the probability distribution of times elapsed between an individual becoming infected and their transmitting to someone else. We used a gamma distribution with mean 5.5 days and standard deviation 2.14 days as obtained by Ferretti et al. (in prep.) combining data from [5, 14, 15, 16] . This distribution was converted into a discrete distribution on integer time intervals, i.e. 0,1,2,3,... days post infection.

We used Bayesian estimation of the instantaneous reproduction number [17] as implemented in the software package EpiEstim [18, 19] . To reduce oversensitivity to the priors when the number of infections is very low, we used the posterior mode rather than the posterior mean as the central estimate.

We combined R with incidence per capita to provide a simple "nowcast" for each UTLA. For each day we multiplied the estimated R value by the mean incidence per capita for the week beginning three days before and ending three days afterwards.

We used a 'synthetic control' approach [20, 21] to construct a comparison area for the Isle of Wight. The approach creates a weighted average of other areas in England that matches the Isle of Wight in the average R before the introduction of the pilot, as well as in distributions of age and ethnicity. To choose the weights, we used a cross validation with a 19-day training period followed by a 19-day validation period. Significance was determined using a permutation test.

Maximum-Likelihood estimation of R Pillar 2 daily case data based on community testing was not publicly available at the time of writing for individual UTLAs, and the combined dataset available presents challenges for accurate inference of incidence of new infections, so we did not perform the Bayesian EpiEstim analysis on these datasets. Instead we used a Maximum-Likelihood estimation of the growth rate r over defined time periods which clearly post-date the TTI interventions, and from these estimated an average value of R for the Isle of Wight and at a national level [22] . We used publicly available data for the Pillar 1 and combined pillars comparisons, and for Pillar 2 we made use of Isle of Wight data obtained via private communication with Public Health England. We also used Pillar 1 data for each UTLA to compare values of R in time periods before and after the TTI interventions. This simple change-point analysis acts as a robustness check for our earlier analyses as it does not depend on the same modelling assumptions.

In Figure 2 we show the number of new confirmed daily cases on the Isle of Wight and "nationally", by individual pillars (Figures 2A and 2B ) and using the combined pillars data released on 2 July 2020 ( Figures 2C and 2D) . By "nationally" we refer to the data available: England for Pillar 1, the "UK except Wales" for Pillar 2, and England for the combined pillars data. Pillar 1 data is available for the other nations of the UK over the same time period but suffered from sporadic reporting which made incidence and R difficult to estimate with confidence. We note that although it has become common to see Pillar 1 and 2 data displayed in stacked histograms such as these, interpreting true incidence from these plots is non-trivial because of the different lag times on testing infected individuals in the two pillars. Nevertheless, there is a clear trend of declining incidence through May and June, even in the context of increasing availability of tests.

To compare between UTLAs with accurate timing information we used Pillar 1 daily case data on the Isle of Wight ( Figure 3A , green) and inferred the incidence of new infections ( Figure 3A , red). The number of new infections each day on the island was generally decreasing from mid-April, although we do not have the data to be able to deduce whether this was entirely the impact of the lockdown or whether the result is exaggerated by lack of test availability. We hypothesise that the apparent uptick in new infections in late April is largely an artefact of the increased availability of tests after the TTI intervention. We see a particular decline in incidence straight after the launch of the TTI intervention on 5 May (dashed line). Since the back-calculation relies on using around 10 days of future data, the inferred incidence over the final 10 days is increasingly prone to underestimation and is shown shaded in grey in Figure 3A . In Figure 3B and Figures 4-6 we truncate our results on 14 June for this reason. The trend in 4 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted July 14, 2020. . incidence looks encouraging for the Isle of Wight but it is important to compare the incidence with other areas in England that did not see the launch of the TTI programme on 5 May. Figure  3B shows the inferred incidence curves scaled per capita for each of the UTLAs, with the Isle of Wight shown in red. We see that in mid-April per capita incidence on the Isle of Wight was around average for English UTLAs, but by late May the incidence on the Isle of Wight was very low. COVID-19 incidence therefore decreased more sharply on the Isle of Wight than in other areas of England.

R decreased after the TTI intervention on the Isle of Wight more sharply than the national trend The value of the reproductive number R over time is shown in Figure 4 . In these figures we report for each date the estimate of R for the week which starts 3 days before and ends 3 days afterwards. On the Isle of Wight, R declined rapidly after the TTI programme's introduction (dashed line) from a value of 1.0 on 5 May to 0.25 on 23 May. It then followed a fluctuating and gradually rising trend to 0.54 on 14 June. However, incidence was very low during the period 23 May -14 June ( Figure 3A ) so the confidence intervals on R are very wide ( Figure 4A , grey). In Figure 4B we plot the Isle of Wight's time-varying reproduction number R alongside those of the other UTLAs. The decline from 5 May to 23 May was much sharper than that of a typical UTLA. In Figure 4C we compare R on the Isle of Wight (red) with the average R of other UTLAs and in Figure 4D as a centile of all UTLAs. On 5 May the reproduction number R on the Isle of Wight was high compared to the national average, shown in Figure 4C by the red line nearing the top of the blue 95% range and in Figure 4D ranked in the 89th centile. By 23 May it was well below the 5% range ( Figure 4C , blue) and ranked lowest of all UTLAs 19-21 May ( Figure 4D ). R therefore decreased more sharply on the Isle of Wight than in other areas of England.

In the final few weeks of the study an increasing majority of cases were recorded as Pillar 2 ( Figure 2B ), and Pillar 1 case numbers in all UTLAs decreased to nearly zero. We do not know how consistent the magnitude of this shift from Pillar 1 to Pillar 2 case numbers was across UTLAs. The effect of the shift is that incidence and R calculated only from Pillar 1 decreased rapidly to zero for all UTLAs ( Figures 3B and 4C ). The centile ranking of the Isle of Wight ( Figure 4D ) therefore became higher but less meaningful (the statistical test for deviation loses power).

The "nowcast" for each UTLA ( Figure 5A ) shows that throughout April the epidemic on the Isle of Wight had a size and growth comparable to that of an average UTLA in England, between the 25th and 75th centiles ( Figure 5B ). Following the TTI intervention the prospects for the Isle of Wight looked significantly better than those for the majority of UTLAs, reaching the second centile in late May. In the final few weeks of the study, incidence and R were both very low leading to stochasticity and thus ceasing to be useful measures to compare epidemic trends across areas.

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. .

Our results comparing incidence, R and nowcasts across UTLAs using Pillar 1 data (Figures 3B, 4B and 5A) are available in the EpiNow-C19 web application [1] where they can be explored in more detail. These are provided alongside a daily extension of the analyses using the available combined pillars dataset, noting that this data provides a better estimate of the number of infections than Pillar 1 data alone but presents challenges for estimating timings and R.

The Isle of Wight has a lower R compared to the rest of England after the intervention using a synthetic control approach Since the demographics and trajectories of the sub-epidemics vary across UTLAs we used a 'synthetic control' approach [20] to create a more comparable control group for the Isle of Wight. We combined data from all other English UTLAs and weighted their contributions to match the evolution of R on the Isle of Wight before the introduction of the TTI pilot ( Figure 6 , left hand panels). The choice of matching variables is to some extent subjective and can influence the result of the analysis. We therefore consider two different scenarios, first using weekly averages of the estimates of R alone, and second using additional demographic information. In both cases we use a cross validation approach to guard against overfitting. Table S1 shows the weights on the matching variables for the two scenarios. When we add population density and measures of deprivation to Scenario 2, almost no weight is allocated to these variables and the overall results remain unchanged (data not shown).

Both synthetic control scenarios show that R is lower for the Isle of Wight than for its synthetic control during almost the entire period after the introduction of the TTI pilot. To establish the statistical significance of this result we perform a permutation test by constructing synthetic controls for all other UTLAs in England. We compute the difference between each UTLA and its synthetic control ( Figure 6 , right hand panels). Using this method we find that the difference for the IoW is statistically significant for part of May in both scenarios at the 5% level (see Supplementary Material for more details).

We used three different datasets (Pillar 1, Pillar 2 and the combined data) to obtain Maximum-Likelihood estimates of R after the TTI interventions. Figure 7A shows the results across the different pillar datasets. The reproduction number R on the Isle of Wight was consistently lower than the "national" rate, by which we mean England for Pillar 1 and the combined dataset, and the UK except Wales for Pillar 2 data. The national estimates have tight confidence intervals ( Figure 7A , error bars) thanks to being informed and constrained by more data. R was lower for the Isle of Wight than nationally for data from Pillar 1 ( , 0 p = 6 × 1 −6 likelihood ratio test), from Pillar 2 ( ) and from both combined ( ). We 0 p = 5 × 1 −4 2 0 p = × 1 −11 also used Pillar 1 data to compare R on the Isle of Wight to R for each UTLA individually before the intervention from 1 March ( Figure 7B ) and afterwards ( Figure 7C ). By this estimation the Isle of Wight experienced a striking reduction in its reproduction number, going from 1.3 in the period before the TTI intervention to 0.5 in the study period afterwards. This is a particularly 6 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. . https://doi.org/10.1101/2020.07.12.20151753 doi: medRxiv preprint notable change when compared with other UTLAs as it went from having the third highest reproduction number before the intervention to the tenth lowest afterwards.

We analysed the course of the COVID-19 epidemic on the Isle of Wight before and after the launch of the Test, Trace, Isolate (TTI) programme including a test version of the mobile phone contact tracing app and compared it to the epidemic in other areas of England and the UK. We focussed on total incidence, per capita incidence, and the effective reproduction number R and compared each using different approaches. We used an improved method to calculate R by taking into account variable lag times between infection and case counts; the method can be generalised to account for different distributions of lag times across data sources.

All analyses paint a consistent and encouraging picture. In mid-April the Isle of Wight was experiencing moderate-to-high incidence per capita and a higher reproduction number R than a typical UTLA. The launch of TTI on the Isle of Wight was followed by a short phase in which case numbers increased, as expected with increased testing. However, the inferred incidence of new infections and the reproduction number R declined sharply to below the English average, and below predicted levels for a synthetic control, immediately upon introduction of TTI. This finding is consistent across our analyses of Pillar 1, Pillar 2 and the combined pillars datasets, and in both our Bayesian and Maximum-Likelihood analyses.

These results are correlative: rapid decreases in the rate of new infections on the Isle of Wight could be explained by multiple causes. Comparisons are statistically significant for every way in which we analysed the data, suggesting that the effect of chance fluctuations alone is unlikely. The introduction of TTI was accompanied by community discussions and national publicity. The success could be due to the attention that the start of the programme on the Isle of Wight received leading to increased care and physical distancing, although changes in movement are not apparent from Google mobility data [23] (Supplementary Figure S1) . All inhabitants received a letter and an invitation to download the app and contact tracers might have been highly motivated to make the first phase of the launch a success. The start of the programme and the testing of the app might have led to a positive community response and an increased awareness. A hospital-based app usability study concluded that the app had been embraced and adopted well during the initial trial period, whilst making recommendations for improvements for future versions [24] . Contact tracing could have contributed to the drop in infections, both through the manual/CTAS system, and through the use of the app by 38% of inhabitants. Alternatively, the success in reducing infections on the Isle of Wight might be attributable to reasons other than the launch of the TTI programme.

The national TTI programme was launched on 28 May during a period of gradual relaxation of lockdown measures which began on 10 May. It is possible that this is why we did not see a 7 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. . strong decrease in incidence and R at this point; it was also towards the end of our period of study so there was little opportunity to observe a more gradual decline.

This study has limitations. Data from the Contact Tracing and Advisory Service (CTAS) and time series of cases traced by the app are not yet publicly available. This study is therefore unable to evaluate the effect of the app independently of the effect of the wider TTI programme. We did not adjust for changes in testing practice, and so likely overestimated the reproduction number over time when widespread community testing became available in the week beginning 5th May in the Isle of Wight, and in other areas in the week beginning 18 May. We hypothesise that these biases become less pronounced after one week. The Isle of Wight's synthetic controls are not perfect matches, as is evident from several time periods that are significantly different between the Isle of Wight and its synthetic control before the start of the TTI pilot. This is, however, not surprising given that the Isle of Wight's unusual epidemic trajectory and geography make it harder to find a perfect synthetic control.

We are currently using the Public Health England data which is published daily [2] to provide an approximate surveillance and nowcasting application EpiNow-C19 [1] . To improve the accuracy of this and to build on the results of this study, we recommend that local area data is made publicly available for all regions of the UK, either separated by Pillar or with similarly informative timing information such as the date of onset of symptoms. We encourage further analyses comparing sub-epidemics across geographical areas as the TTI programme evolves, with focused data collection that allows evaluations of the separate TTI components. In particular, knowing the proportion of cases from individuals who were already quarantined at the point of diagnosis is a strong indicator of the effectiveness of TTI. Tracking the progression of local sub-epidemics and identifying the key determinants of successful suppression will be crucial for informing local and national non-pharmaceutical intervention strategies.

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. . https://doi.org/10.1101/2020.07.12.20151753 doi: medRxiv preprint . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. .

12 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. . https://doi.org/10.1101/2020.07.12.20151753 doi: medRxiv preprint A Isle of Wight (separate pillars) B "Nationally" (separate pillars) C Isle of Wight (combined pillars) D "Nationally" (combined pillars)

13 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. 

14 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. 

15 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. 16 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. 18 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. . https://doi.org/10.1101/2020.07.12.20151753 doi: medRxiv preprint

We used publicly available reports of Pillar 1 and Pillar 2 data until 30 June 2020 [9, 10] . Pillar 1 data was available by Upper Tier Local Authority (UTLA) whereas Pillar 2 was reported as a national daily total. On 1 July 2020, Public Health England stopped reporting Pillar 1 cases alone, and on 2 July after a deduplication procedure they began publishing daily Pillar 1 and 2 case numbers combined at the levels of countries, regions, UTLAs and Lower Tier Local Authorities (LTLAs) [25, 26] . Although Pillar 1 data does not give a complete picture of confirmed cases and is restricted to the UTLA level, we focus on Pillar 1 cases for our estimation of incidence and the time changing reproduction number R(t) because these are the most consistently reported over time and across areas. The combined pillars dataset presents challenges for interpreting incidence of new infections because of differences in timing: cases are reported by date of swab test, which for a Pillar 2 test is likely to be shortly after symptom onset so around 4-7 days post-infection, but for Pillar 1 is likely to be shortly after admission to hospital so around 7-12 days post-infection [12] . Pillar 2 daily case data for the Isle of Wight was obtained via private communication with Public Health England. This data can be approximated by subtracting the Pillar 1 data from the combined data; the differences caused by the deduplication procedure are slight as can be seen by comparing Figures 2A and 2C .

For each of the 150 areas of England under study, we have a time-series of reported cases , C t where is the number of cases newly diagnosed via hospital testing, where the swab was C t taken on day t . We used a back-calculation approach to infer the time-series of infection times of these cases . Let be the probability distribution of times elapsed s between being I t ζ s infected and being swab-tested in hospital. For a case infected at time t-s the probability that they are swab-tested at time t is . Conversely, for a case swab-tested at time t , the probability ζ s that they were infected at time t-s is , i.e. the backwards-in-time delay distribution

is not in general equal to the forwards-in-time distribution. However, we make the simplifying assumption that incidence changes sufficiently slowly across the range of (about a week) ζ s that , and so we estimate that, of the people who became Pillar 1 confirmed

For we took the convolution of the incubation period distribution and the distribution for time ζ s from symptoms to hospital visit, making the assumption that the majority of the Pillar 1 tests were for patients. For the incubation period we used meta-analysis results indicating a lognormal distribution with mean 5.42 days and standard deviation 2.7 days [14] (specifically, the results excluding those of Ma et al for which bias was detected [14] ). For time from symptoms to hospital visit time we used a gamma distribution with mean 5.14 days and standard deviation 4.2 days [12] .

Back-calculation of incidence leads to right-censoring towards the present by an amount corresponding to roughly the mean of , and so we truncate analyses 10 days before the last ζ s date of "reliable" data. That is, we truncate analyses at 14 June because accurate estimates after this time would require complete data beyond 24 June, but case numbers within the last 5 days of the dataset (25-29 June) should be considered incomplete [9] .

The time-varying effective reproduction number is a standardized measure for the rate of (t) R spread of an infection at time t. Several definitions exist; here we used the instantaneous reproduction number [17] . Use of this measure and back-calculation from reporting dates were recently recommended by a survey of methods [27] and has been implemented across many different countries for COVID-19 [28] . A Bayesian method of estimation of this measure is implemented in the software package EpiEstim [18, 19] . Specifically, our definition of

where is the number of inferred infections on day t and is the 

To reduce oversensitivity to the priors when the number of infections is very low, instead (a, ) b of reporting as the central estimate of the posterior mean (which is and is the default in (t) R b a′ ′ EpiEstim) we report instead, as the central estimate, the posterior mode, namely . The a )b ( ′ − 1 ′ choice of posterior mode as a central tendency is common in Bayesian statistics, and reduces the tendency to obtain high estimates of when there are very few reported cases in an (t) R area. We report the usual 95% credibility intervals.

The future course of an epidemic depends on both the current number of infectious individuals and the effective reproduction number R. In the last month of this study the Isle of Wight had a consistent pattern of recording just one Pillar 1 case every 5-7 days. Although we know that there were also Pillar 2 cases, the Pillar 1 case count is consistent with a single chain of transmission where one individual infects exactly one other: a reproduction number of 1. However, the short-term forecast is clearly better for the island than it would be for another UTLA with a reproduction number of 1 but many Pillar 1 cases per 100,000 population. We combine R with incidence per capita to provide a simple "nowcast" for each UTLA in the following way: for each UTLA on each date we consider the week which starts 3 days before 20 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. . and ends 3 days afterwards. We multiply the estimated value of R in that week by the mean incidence per capita for that week to provide a simple projection of the expected number of new cases per capita per day in that area in the near future.

The 'synthetic control' method was developed to evaluate the effect of a treatment on the aggregate level, e.g., regions, when only one region is treated [20, 21] . The method provides a data-driven manner of constructing a more similar comparison unit. The synthetic control is constructed as a weighted average of all 'donor' (control, untreated) units. The vector W of donor unit weights is chosen based on matching variables X such that the outcome variable of the synthetic control most closely resembles the outcome variable of the treated unit in the pre-treatment period. In our setting, the Isle of Wight is the treated region and the treatment is the introduction of the TTI pilot. The donor units are all other UTLAs in England. The outcome variable Y is the daily value of R, estimated as described above. As matching variables X we use age bands, ethnicity shares in the population, and weekly averages of the estimated values of R.

We used the synth package in Stata to construct the synthetic control. The weights are chosen over T days and N = 149 donor units. Define Y treat as a ( T ⨉ 1) vector of the treatment unit's outcome variable, i.e., R on the Isle of Wight. Y donor is a ( T ⨉ N ) matrix of the donor units' outcome variable. There are J matching variables. Define X treat = ( X treat 1 , …, X treat J )' as a ( J ⨉ 1) vector of matching variables for the treatment unit and similarly X donor as the corresponding ( J ⨉ N ) matrix of matching variables for the donor units. Define W as an ( N ⨉ 1) vector of weights that sum to one: W is a possible synthetic control . Finally, define V as a ( J ⨉ J ) diagonal matrix of matching variable weights. Thus W contains the weights across donor units and V contains the weights across matching variables. Then the optimal synthetic control W* and the associated symmetric matrix V* solve the following minimisation problems:

To provide robustness against overfitting, we determined the matching variables weights ( V ) by cross validation. Specifically, we split the pre-treatment period into two equal-length periods, the training period and the validation period, so T = 19 days for each of the two periods . The minimisation was conducted twice, first to find a using training period data and a W train V val using validation period data, and again using data from the validation period to find by W * setting found in the first minimisation.

To test whether the difference between the Isle of Wight data and its synthetic control is statistically different from zero, we used a permutation test. First, we created synthetic controls for all 150 areas in England and calculated the difference between the outcome variable in each area and its synthetic control. The Isle of Wight was deemed significantly 21 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 14, 2020. . different from its synthetic control at the 5% level if it was in the 2.5 centile of the areas with the largest negative or the 2.5 centile of the areas with the largest positive difference to their respective synthetic control. 

We estimated R(t) for Pillar 1 data because it is comparable across time and space; however, valuable information is present in Pillar 2 data based on community testing. Because this was not collected consistently over time, and community testing was introduced on the Isle of Wight before the rest of the country (Figure 1 ), we performed three separate analyses using Pillar 1 data, Pillar 2 data and the combined dataset, estimating for each the exponential rate of change r over defined time periods of analysis that clearly pre-date and post-date the introduction of TTI. For a given series of cases in a time period , we estimated the rate of change by C t t ,

22 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted July 14, 2020. 

respect to the intercept and the slope r . We used the likelihood ratio test to compare the K 0 slope between two time series (that for the Isle of Wight versus that for elsewhere): we compared the likelihoods obtained from a model constrained to use the same slope r for the two series versus a model with freedom to select a different slope for each series. Ranges t ,

were chosen so as to represent the time period of interest, taking into account delays in reporting of cases relative to underlying infections, namely 5 days for Pillar 2 cases and the combined data, and 10 days for Pillar 1 cases alone. We also remove the final 5 days of data available for each data set since these may be incomplete [9] . The start and end dates are given explicitly in Table S2 

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted July 14, 2020. . https://doi.org/10.1101/2020.07.12.20151753 doi: medRxiv preprint Figure S1 : Google mobility data shows similar trends for the Isle of Wight (solid) and the UK (dashed) over the periods of lockdown and easing of restrictions.

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted July 14, 2020. . https://doi.org/10.1101/2020.07.12.20151753 doi: medRxiv preprint

EpiNow-C19: local epidemic surveillance and nowcasting. Shiny web application

COVID-19) in the UK. Public Health England and NHSX

Surveillance strategies for COVID-19 human infection World Health Organisation

Quantifying SARS-CoV-2 Transmission Suggests Epidemic Control with Digital Contact Tracing

Can phone apps slow the spread of the coronavirus?

Results of first week of test and trace are in -app is still out, Digital Health

COVID-19 testing data: methodology note Department of Health and Social Care

The incubation period of COVID-19: A rapid systematic review and meta-analysis of observational research

Challenges in control of Covid-19: short doubling time and long delay to effect of interventions

COVID-19) cases in the UK: About the data

Yuling Xing, Shunqing Xu. Transmission of corona virus disease 2019 during the incubation period may lead to a quarantine loophole

Leung Temporal dynamics in viral shedding and transmissibility of COVID-19

Hsien-Ho Lin, for the Taiwan COVID-19 Outbreak Investigation Team. Contact Tracing Assessment of COVID-19 Transmission Dynamics in Taiwan and Risk at Different Exposure Periods Before and After Symptom Onset

Estimating Individual and Household Reproduction Numbers in an Emerging Epidemic

A New Framework and Software to Estimate Time-Varying Reproduction Numbers During Epidemics

EpiEstim: Estimate Time Varying Reproduction Numbers from Epidemic Curves

The Economic Costs of Conflict: A Case Study of the Basque Country

Synthetic Control Methods for Comparative Case Studies: Estimating the Effect of California's Tobacco Control Program

How generation intervals shape the relationship between growth rates and reproductive numbers

Google COVID-19 Community Mobility Reports

Test, track, and trace: How is the NHSX Covid app performing in a hospital setting?

COVID-19) cases in the UK . Department of Health and Social Care and Public Health England

Coronavirus (COVID-19) UK Historical Data. GitHub repository with data collated from Department of Health and Social Care and Public Health England

Practical considerations for measuring the effective reproductive number, Rt

Estimating the time-varying reproduction number of SARS-CoV-2 using national and subnational case counts