Assessing the impact of preventive mass vaccination campaigns on yellow fever outbreaks in Africa : a population-level self-controlled case-series study

Abstract

Introduction: 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). However, by how much PMVCs decrease the risk of outbreak to occur has not yet been quantified. Methods: 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, the SCCS method is an alternative to classical cohort or case-control study designs when the risk of residual confounding is high. The location 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 2005 and 2018. We compared the relative 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. Results: The study sample consisted of 33 African provinces. Among these, outbreaks occurred during the pre-PMVC period in 26 (78.8%) provinces versus 7 (21.2%) occurring in the post-PMVC period. This corresponded to a significantly reduced incidence rate ratio of 0.14 (95% Confidence interval 0.06 to 0.34) for the post-PMVC versus pre-PMVC period. This estimate was robust across a range of sensitivity analyses, while 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 have reduced the number of outbreaks by 34% (22% to 45%) in Africa over the study period. Conclusion: Our estimates 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.

Using self-controlled case series to understand the relationship between conflict and cholera in Nigeria and the Democratic Republic of Congo

“…The recently developed percentage attributable fraction (PAF) equations ( 30 ) were adapted to the model results found here. The PAF values estimate the percentage of outbreaks that could be attributed to conflict at a national level, using the datasets and the IRR values from the model results (full equations in S3 Information).…”

“…To bridge this research gap, we aimed to investigate if the incidence of cholera outbreak onset was affected by conflicts in Nigeria and the DRC in the past 23 years. To test this, we used the Self-Controlled Case Series (SCCS) method [27], which has previously been used to test the effectiveness of drug and vaccine intervention on an individual level [28][29] with its used at a population level more recently explored [30]. This study is a novel application of these methods and has the potential to prove its effectiveness in other contexts.…”

“…To test this, we used the Self-Controlled Case Series (SCCS) method in a novel application and to understand its use in other contexts [27]. The SCCS methods have previously been used to test the effectiveness of drug and vaccine intervention on an individual [28][29] and population level [30]. Furthermore, we adapted the recently developed percentage attributable fraction (PAF) equations to the work presented here [30], to understand the proportion of cholera outbreaks attributable to conflict.…”

“…Previous uses of the method include testing the effectiveness of drug and vaccine intervention on an individual ( 28,29 ) and population level ( 30 ). Furthermore, we adapted the recently developed percentage attributable fraction (PAF) equations to the work presented here ( 30 ), to understand the proportion of cholera outbreaks attributable to conflict. Based on these results, we suggest mechanisms for which conflict is driving cholera and potential risk factors, building on previous research in this area.…”