Assessing the impact of preventive mass vaccination campaigns on yellow fever outbreaks in Africa: A population-level self-controlled case series study
File(s)
Published version
Author(s)
Type
Journal Article
Abstract
Background
The Eliminate Yellow fever Epidemics (EYE) strategy was launched in 2017 in response to the resurgence of yellow fever in Africa and the Americas. The strategy relies on several vaccination activities, including preventive mass vaccination campaigns (PMVCs). How
ever, to what extent PMVCs are associated with a decreased risk of outbreak has not yet been quantified.
Methods and findings
We used the self-controlled case series (SCCS) method to assess the association between the occurrence of yellow fever outbreaks and the implementation of PMVCs at the province level in the African endemic region. As all time-invariant confounders are implicitly controlled for in the SCCS method, this method is an alternative to classical cohort or case–control study designs when the risk of residual confounding is high, in particular confounding by indication. The locations and dates of outbreaks were identified from international epidemiological records, and information on PMVCs was provided by coordinators of vaccination activities and international funders. The study sample consisted of provinces that were both affected by an outbreak and targeted for a PMVC between 2005and 2018. We compared the incidence of outbreaks before and after the implementation of a PMVC. The sensitivity
of our estimates to a range of assumptions was explored, and the results of the SCCS method were compared to those obtained through a retrospective cohort study design. We further derived the number of yellow fever outbreaks that have been prevented by PMVCs.
The study sample consisted of 33 provinces from 11 African countries. Among these, the first outbreak occurred during the pre-PMVC period in 26 (79%) provinces, and during the post-PMVC period in 7(21%) provinces. At the province level, the post-PMVC period was associated with an 86% reduction (95% CI 66% to94%, p<0.001)in the risk of outbreak as compared to the pre-PMVC period. This negative association between exposure to PMVCs and outbreak was robustly observed across a range of sensitivity analyses, especially when using quantitative estimates of vaccination coverage as an alternative exposure measure, or when varying the observation period. In contrast, the results of the cohort-style analyses
were highly sensitive to the choice of covariates included in the model. Based on the SCCS
results, we estimated that PMVCs were associated with a 34% (95% CI 22%to 45%) reduction in the number of outbreaks in Africa from 2005 to 2018. A limitation of our study is the fact that it does not account for potential time-varying confounders, such as changing environmental drivers of yellow fever and possibly improved disease surveillance.
Conclusions
In this study, we provide new empirical evidence of the high preventive impact of PMVCs on yellow fever outbreaks. This study illustrates that the SCCS method can be advantageously applied at the population level in order to evaluate a public health intervention.
The Eliminate Yellow fever Epidemics (EYE) strategy was launched in 2017 in response to the resurgence of yellow fever in Africa and the Americas. The strategy relies on several vaccination activities, including preventive mass vaccination campaigns (PMVCs). How
ever, to what extent PMVCs are associated with a decreased risk of outbreak has not yet been quantified.
Methods and findings
We used the self-controlled case series (SCCS) method to assess the association between the occurrence of yellow fever outbreaks and the implementation of PMVCs at the province level in the African endemic region. As all time-invariant confounders are implicitly controlled for in the SCCS method, this method is an alternative to classical cohort or case–control study designs when the risk of residual confounding is high, in particular confounding by indication. The locations and dates of outbreaks were identified from international epidemiological records, and information on PMVCs was provided by coordinators of vaccination activities and international funders. The study sample consisted of provinces that were both affected by an outbreak and targeted for a PMVC between 2005and 2018. We compared the incidence of outbreaks before and after the implementation of a PMVC. The sensitivity
of our estimates to a range of assumptions was explored, and the results of the SCCS method were compared to those obtained through a retrospective cohort study design. We further derived the number of yellow fever outbreaks that have been prevented by PMVCs.
The study sample consisted of 33 provinces from 11 African countries. Among these, the first outbreak occurred during the pre-PMVC period in 26 (79%) provinces, and during the post-PMVC period in 7(21%) provinces. At the province level, the post-PMVC period was associated with an 86% reduction (95% CI 66% to94%, p<0.001)in the risk of outbreak as compared to the pre-PMVC period. This negative association between exposure to PMVCs and outbreak was robustly observed across a range of sensitivity analyses, especially when using quantitative estimates of vaccination coverage as an alternative exposure measure, or when varying the observation period. In contrast, the results of the cohort-style analyses
were highly sensitive to the choice of covariates included in the model. Based on the SCCS
results, we estimated that PMVCs were associated with a 34% (95% CI 22%to 45%) reduction in the number of outbreaks in Africa from 2005 to 2018. A limitation of our study is the fact that it does not account for potential time-varying confounders, such as changing environmental drivers of yellow fever and possibly improved disease surveillance.
Conclusions
In this study, we provide new empirical evidence of the high preventive impact of PMVCs on yellow fever outbreaks. This study illustrates that the SCCS method can be advantageously applied at the population level in order to evaluate a public health intervention.
Editor(s)
Grais, Rebecca Freeman
Date Issued
2021-02-18
Date Acceptance
2020-12-15
Citation
PLoS Medicine, 2021, 18 (2)
ISSN
1549-1277
Publisher
Public Library of Science (PLoS)
Journal / Book Title
PLoS Medicine
Volume
18
Issue
2
Copyright Statement
© 2021 Jean et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
License URL
Identifier
https://www.ncbi.nlm.nih.gov/pubmed/33600451
PII: PMEDICINE-D-20-03496
Subjects
COHORT
General & Internal Medicine
Life Sciences & Biomedicine
Medicine, General & Internal
RISK
Science & Technology
Publication Status
Published
Coverage Spatial
United States
Article Number
e1003523
Date Publish Online
2020-02-18