key: cord-0720200-meiho5v9
authors: KWOK, Kin On; MCNEIL, Edward B; TSOI, Margaret Ting Fong; WEI, Vivian Wan In; WONG, Samuel Yeung Shan; TANG, Julian Wei Tsz
title: Will achieving herd immunity be a road to success to end the COVID-19 pandemic?
date: 2021-06-10
journal: J Infect
DOI: 10.1016/j.jinf.2021.06.007
sha: b2515c7944c24d9f465177786959495e07bbf52d
doc_id: 720200
cord_uid: meiho5v9

As the COVID-19 pandemic continues, the availability of several different new vaccines, their varying supply levels, effectiveness, and immunity duration across different ethnic populations, together with natural infection rates, will have an impact on when each country can reach herd immunity (ranging from 15.3% to 77.1%). Here we estimate the population proportions still required to gain immunity (ranging from 0.01 to 48.8%) to reach an overall herd immunity level to stop the exponential virus spread in 32 selected countries.

The crucial question of what is the minimal vaccine coverage needed for different countries to achieve SARS-COV-2 herd immunity (i.e. that required to block exponential virus spread in a population) is an important one, when COVID-19 vaccine supplies are limited and unreliable, and different vaccines have different efficacies. With evidence demonstrating natural immunity effectiveness (i.e. immunity acquired after natural SARS-COV-2 infection), we can factor this into the minimum vaccine coverage required for any given population.

Much of the COVID-19 vaccine and incidence data can only be estimated from publicly available and various websites, but these can be combined to provide useful estimates of the required herd immunity level -and therefore the COVID-19 vaccine coverage still requiredfor different countries.

We revisit the calculation of and estimate the current immune proportion, , with the following formulae:

where is the basic reproductive number, and are the proportions of the population vaccinated with one and two doses, respectively, and are the overall real-world population effectiveness of the vaccine for one and two doses, respectively, is the proportion of confirmed cases, and is the proportion of the population who have naturallyinduced immunity against symptomatic SARS-COV-2 infection. From equations {1} and {2}, we define P as the proportion still required to gain immunity for the country to achieve herd immunity:

. A country with P > 0 indicates that its population had achieved herd immunity. All analyses were performed in R (version 4.1.0; R Foundation for Statistical Computing) ( Table 1) . 5% and 50.1%, respectively) . Surprisingly, countries in Asia such as Malaysia, Japan, and South Korea, regardless of level of vaccine availability, reported low single-digit values (5.6%, 5.1%, and 6.4%, respectively). Of the 32 study countries, 11 had achieved herd immunity, 6 others required P to be between 0.01% and 8.6% to reach the herd protection level, and the rest required proportions ranging from 11.1 to 48.8%. (Table 1, Figure 1 , and

Our study suggested that the majority of study populations had lower proportions that were immune compared to Israel, the exemplar in reducing the infection rate after successful vaccine deployment 5 . This might be partly attributable to the inequitable distribution of vaccines globally, which may be shaping different government policies on vaccination, but also cultural and socio-economic barriers leading to vaccine refusal and hesitancy, particularly amongst Asian and African populations. Thus, to improve COVID-19 vaccination coverage and raise the levels of population immunity, sufficient vaccine supplies need to be more reliable 6 , with improved, culturally sensitive, and appropriate communication to encourage their uptake. This will partly depend on whether we can successfully identify determinants of vaccine hesitancy and refusal amongst various

The exact proportion in any population that is required to achieve herd immunity to stop the spread of the virus will vary, depending on the virus variant circulating, as well as the natural degree of mixing in that population -which also depends on population density and mobility and so on. In addition, the duration of protection conferred by natural and vaccine-induced immunity is not well-established, and different vaccines may confer differing durations and degrees of humoral(B-cell) and cell-mediated(T-cell) immunity 8, 9 . It is also not known how long and effective the immunity conferred by mixed vaccine regimens and third dose boosters will be in different populations -including those of different ethnicities. Finally, children are still not routinely vaccinated as most COVID-19 vaccines are not yet licensed for this subgroup, particularly primary school children, which means they will mostly remain a susceptible population where any degree of herd immunity will be uncertain. Therefore, the precise level of population immunity required, as estimated by the equation of herd immunity, to 'end' the pandemic in each country and globally is difficult to determine.

From a practical viewpoint, estimates of will be considered to be transient and herd

immunity is likely to be a spectrum instead of a specific threshold that determines if and when the entire pandemic is over 10 Proportion of population already immune ( ) (red) and the additional proportion still required to achieve herd immunity ( ) (blue) in the 32 study populations stratified by vaccine availability for various key priority groups. With the most recent data for the numbers of vaccine doses given and naturally occurring COVID-19 cases, as reported from each country's population on 26th May, 2021 1 , assumed estimates for , , and to be 70% 2 , 88% 3 and 80% 4,5 , respectively, can be estimated. Percentages to the right of each bar represent the minimum proportion of the total population required to recover from COVID-19 to confer immunity with vaccine availability . 

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Herd Immunity Is Proving Elusive

 1 We first estimate with the exponential growth method 1 using COVID-19 case series from 21st January 2021 to 31st July 2020 (Figure 1 ) coupled with estimates of the serial interval 2 (mean = 4.7 days, standard deviation = 2.9 days). Each country's exponential phase was defined as the period from onset (the first day of a consecutive 3-day period with at least 3 cases) to the peak (maximum cases) of the first wave. The first wave was defined as the period from onset to the day when the number of cases decreased by more than 50% of the maximum up to that day for at least 3 consecutive days or did not exceed the maximum for 7 consecutive days. 2 Information updated on 26/5/2021 3 Three priority groups were key workers, clinically vulnerable people and the elderly.