A paper in The Lancet Infectious Diseases in August 2024 presents the results of an observational study during the Ebola epidemic in the Democratic Republic of the Congo (DRC). The researchers evaluated the effectiveness of the only WHO prequalified vaccine recommended for use in outbreaks of Ebola virus, the recombinant vesicular stomatitis virus-Zaire Ebola virus (rVSV-ZEBOV) vaccine. This is the first work to provide estimates of the real-world effectiveness of the vaccine and confirms that it is “highly protective” against Ebola virus disease.
A tool against Ebola
Ebolaviruses are endemic in the Democratic Republic of the Congo (DRC), which had reported15 outbreaks by March 2024. The 10th of these was confirmed in August 2018 and was in northeastern provinces of North Kivu and Ituri, characterised by “chronic insecurity and conflict, political instability, mistrust in government, and high population mobility”. At the end of the outbreak in June 2020, 3,470 cases and 2,287 deaths were recorded; it was the largest reported outbreak in the country and the second-largest outbreak worldwide.
Merck’s recombinant vesicular stomatitis virus-Zaire Ebola virus single-dose vaccine (rVSV-ZEBOV, known as Ervebo) received WHO prequalification in November 2019 and is recommended by WHO’s Strategic Advisory Group of Experts on Immunisation (SAGE) for individuals at risk of exposure during outbreaks. It was deployed in the 10th ebolavirus outbreak in the DRC under the Expanded Access framework following the recommended strategy based on reactive ring vaccination and targeting of at-risk individuals. This was expanded under SAGE guidelines to include pregnant and breastfeeding women and infants between 6 and 12 months.
The study
The authors sought to retrospectively estimate the effectiveness of rVSV-ZEBOV vaccination against Ebola virus disease during the 2018-2020 outbreak in the DRC. They used a test-negative design; the study population comprised eligible individuals who were reported as having suspected Ebola virus disease at Ebola virus disease facilities. Standardised patient data were recorded by data managers at each Ebola virus disease facility and compiled into a centralised case management database weekly.
60,246 suspected cases were assessed for eligibility, among which 26,438 were eligible for inclusion. Among eligible individuals, 1,273 (4.8%) were Ebola virus disease-positive (cases) and 25,165 (95.2%) were Ebola virus disease-negative (controls). 333 (26.2%) of the cases were reported as being vaccinated; most were vaccinated fewer than 10 days before symptom onset. 4,855 (19.3%) of the controls were reported as being vaccinated.
The effectiveness of rVSV-ZEBOV vaccination against Ebola virus disease was estimated to be 84% at 10 or more days after vaccination. Stratified by sex, effectiveness was 80% for females and 86% for males. Stratified by age, effectiveness was 80% for children younger than 15 years, and 83% for adults. The effectiveness estimate was compatible with results from the Ebola Ça Suffit! ring vaccination trial but are lower than preliminary estimates from the 2018-2020 outbreak.
“Our results indicate that rVSV-ZEBOV is highly protective against Ebola virus disease and support its reactive, targeted use in people at risk of exposure during Ebola virus disease outbreaks.”
Dr Sophie Meakin, epidemiologist with Epicentre MSF, states that the study “dispels uncertainties about the vaccine’s actual effectiveness”.
“It is the first published study to evaluate the effectiveness of the rVSVΔG-ZEBOV-GP vaccine outside of a clinical trial. It was carried out during the second largest Ebola epidemic on record.”
The authors highlight the need for further work on the duration of protection and efficacy in populations that are susceptible to severe disease and outcomes. Professor Steve Ahuka, head of virology at Institut National de Recherche Biomédicale (INRB) and medical professor at the University of Kinshasa, commented on the importance of data collection during epidemics, amid ongoing challenges.
“These are unique opportunities to deepen our knowledge of often rare diseases, and thus improve the management of future epidemics, develop new control tools, and determine the best strategies for using them effectively.”
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