key: cord-0742340-5b6365x3
authors: Liu, Ying; Rocklöv, Joacim
title: What is the reproductive number of yellow fever?
date: 2020-09-05
journal: J Travel Med
DOI: 10.1093/jtm/taaa156
sha: 5a98c395628787b815707fe91fc0d50b7fe70457
doc_id: 742340
cord_uid: 5b6365x3

Teaser Our review found the average reproductive number R(0) for yellow fever to be 4.81 with a median of 4.21.

Yellow fever is a viral vector-borne disease caused by the yellow fever virus with its geographic distribution currently limited to sub-Saharan Africa and South America. The case fatality rate of hospitalized severe yellow fever is above 40%. 1 The basic reproductive number, R 0 , can be used to characterize the epidemic potential of a pathogen by assessing the number of secondary cases that would be generated by one infectious case if it was to be introduced into an immunologically naïve population. R 0 values that are larger than 1 indicate epidemic growth; values around 1 represent endemicity and for values below 1, the outbreak is declining and the number of new infections will be decreasing in subsequent generations. We conducted a review of published peer-reviewed literature on the estimates of the basic reproductive number of yellow fever and discuss the implications for herd immunity in relation to the critical vaccination levels.

We conducted searches on PubMed and Web of Science with the following search terms 'yellow fever AND (R 0 OR basic reproductive number)'. The restriction on published article language was English. We included all publications from 1950 until August 2020. Our review excluded the estimates of the effective reproductive number that depends on the background level of immunity.

A total of 31 studies were identified through the literature search based on these search terms. We excluded 23 studies because of ineligible or incomplete outcome data. Eight studies were included in the final analysis. Overall, 11 data points were collated from the included studies. R 0 estimates were derived for a variety of countries, study years and methods as provided in Table 1 . The estimates range from 1.35 to 11. The average R 0 was 4.81 with a median of 4.21 and an interquartile range of 2.19.

The R 0 estimates appear to vary between studies. Partly, this can be related to methodological differences, but also different local susceptibility and exposure to vectors, i.e. which could be emphasized during due El Nino period and in warmer climate. A relationship between R 0 and climate has been observed for other viruses transmitted by the same vector (Liu et al., 2020 in Supplementary data).

The R 0 is an important number for elimination and it should be considered at a high average/aggregation level over time and space as it is the long-term elimination that is being considered.

With increasing global travel patterns (at least before the COVID-19 pandemic), the risk of importation of yellow fever to vulnerable countries where the vector is present but no adequate vaccination coverage exists is high. 10 The critical vaccination level corresponds to the proportion of population that need to be vaccinated to achieve herd immunity assuming the population is vaccinated at random and that the population is mixing homogenously. Therefore, in the hypothetical situation when a vaccine is 100% effective (i.e. E = 1), the critical vaccination level equals the herd immunity level,

Assuming a vaccine efficacy of 99% [30 days after vaccination (WHO, 2019 in Supplementary data)], we calculated that the critical vaccine coverage levels need to be between 26.2, 77.0 and 91.8% according to the minimum, median and maximum R 0 values, respectively. Reaching very high V c levels, such as 91.8%, for herd immunity is logistically not feasible in many current settings.

We conclude that vaccine coverage thresholds may vary between areas and countries as the basic reproductive number can vary substantially in different localities.

Supplementary data are available at JTM online. 

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