Merck's Ebola Vaccine Showed 84% Effectiveness During Large Outbreak In Africa

Merck & Co Inc’s MRK rVSV-ZEBOV vaccine is the only WHO-prequalified vaccine endorsed by the Strategic Advisory Group of Experts on Immunization for use in Ebola outbreaks caused by the Zaire ebolavirus.

The FDA approved the vaccine for adults in December 2019 and for use in children 12 months and older in August 2023.

Researchers conducted a retrospective test-negative analysis to estimate the effectiveness of rVSV-ZEBOV vaccination against Ebola virus disease during the 2018–20 epidemic in the Democratic Republic of the Congo.

The data showed that 1,273 (4·8%) of 26,438 eligible individuals were positive for Ebola virus disease (cases), and 25,165 (95·2%) were negative (controls).

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40 (3·1%) cases and 1271 (5·1%) controls were reported as being vaccinated at least ten days before symptom onset.

After selecting individuals who reported exposure to an individual with Ebola virus disease within 21 days before symptom onset and matching, the analysis datasets comprised a median of 309 cases and 309 controls.

Ten days or more after vaccination, the effectiveness of rVSV-ZEBOV against Ebola virus disease was estimated to be 84%.

The researchers add that the analysis provides the first real-world effectiveness estimates for the rVSV-ZEBOV Ebola vaccine during a large outbreak.

The findings confirm the vaccine is highly effective against Ebola virus disease and support its use in outbreaks, even in challenging environments like the Eastern Democratic Republic of the Congo.

The real-world effectiveness of rVSV-ZEBOV vaccination is expected to be less than 100%.

During the 2018–20 outbreak in the Democratic Republic of the Congo, national and international agencies recorded confirmed cases of Ebola virus disease in vaccinated individuals, including individuals who were reported as having been vaccinated ten days or more before symptom onset.

Preliminary, unadjusted analyses estimated vaccine effectiveness to be 98%. Other studies have shown that rVSV-ZEBOV vaccination reduces the risk of death in patients with Ebola virus disease who were vaccinated before and even after exposure.

Price Action: MRK stock is up 0.16% to $116.22 at the last check on Thursday.

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