Global yellow fever vaccination coverage from 1970 to 2016: an adjusted retrospective analysis

Abstract

Background: Substantial outbreaks of yellow fever in Angola and Brazil in the past 2 years, combined with global shortages in vaccine stockpiles, highlight a pressing need to assess present control strategies. The aims of this study were to estimate global yellow fever vaccination coverage from 1970 through to 2016 at high spatial resolution and to calculate the number of individuals still requiring vaccination to reach population coverage thresholds for outbreak prevention.

Methods: For this adjusted retrospective analysis, we compiled data from a range of sources (eg, WHO reports and health-service-provider registeries) reporting on yellow fever vaccination activities between May 1, 1939, and Oct 29, 2016. To account for uncertainty in how vaccine campaigns were targeted, we calculated three population coverage values to encompass alternative scenarios. We combined these data with demographic information and tracked vaccination coverage through time to estimate the proportion of the population who had ever received a yellow fever vaccine for each second level administrative division across countries at risk of yellow fever virus transmission from 1970 to 2016.

Findings: Overall, substantial increases in vaccine coverage have occurred since 1970, but notable gaps still exist in contemporary coverage within yellow fever risk zones. We estimate that between 393·7 million and 472·9 million people still require vaccination in areas at risk of yellow fever virus transmission to achieve the 80% population coverage threshold recommended by WHO; this represents between 43% and 52% of the population within yellow fever risk zones, compared with between 66% and 76% of the population who would have required vaccination in 1970.

Interpretation: Our results highlight important gaps in yellow fever vaccination coverage, can contribute to improved quantification of outbreak risk, and help to guide planning of future vaccination efforts and emergency stockpiling.

Funding: The Rhodes Trust, Bill & Melinda Gates Foundation, the Wellcome Trust, the National Library of Medicine of the National Institutes of Health, the European Union's Horizon 2020 research and innovation programme.

Figure 1

Yellow fever vaccination coverage from 1970 to 2016 Estimated proportion of the population across all age cohorts who have ever received a yellow fever vaccine at the beginning of each decade (and in 2016) at the second administrative level for countries at risk of yellow fever virus transmission, based on the untargeted, unbiased vaccination-targeting scenario.

Figure 2

Contemporary yellow fever vaccination coverage scenarios Estimated proportion of the population in 2016 across all age cohorts who have ever received a yellow fever vaccine at the second administrative level in countries at risk of yellow fever virus transmission. Vaccination coverage was calculated using three alternative vaccination-targeting scenarios: (A) targeted; (B) untargeted, unbiased; and (C) untargeted, biased. (D) Percentage difference in coverage between targeting scenarios A and C.

Figure 3

National yellow fever vaccination coverage by age group Estimated proportion of individuals within each age group in 2016 who have ever received a yellow fever vaccine for each country at risk of yellow fever virus transmission, based on the untargeted, unbiased vaccination-targeting scenario. Plots for the beginning of each decade from 1970 to 2010 are provided in the appendix (pp 10–11).