The Ebola virus vaccine being used in the ongoing outbreak in the Democratic Republic of the Congo, or DRC, has been more than 97% effective, according to a preliminary estimate published by WHO.
The estimate comes from a ring trial of Merck’s experimental Ebola vaccine, V920, that is being conducted during the outbreak, in which contacts — and contacts of contacts — of confirmed cases are being offered vaccination. It was included in a report published on the heels of WHO’s announcement that it would not declare the outbreak, which now involves more than 1,200 confirmed or probable cases, an international public health emergency.
According to the report, between Aug. 1, 2018, and March 25, 2019, 71 cases of Ebola occurred among 93,965 at-risk contacts who were vaccinated in 679 rings. During the same time period, 880 cases occurred among at-risk individuals who were not vaccinated. Of the 71 cases in the first group, 15 experienced onset of symptoms 10 or more days after vaccination — the length of time after which vaccinated people are assumed to be protected, according to WHO.
Additionally, WHO reported that only 8.8% of the vaccinated rings reported Ebola cases and only 2.2% reported them 10 days or more after vaccination. According to the report, 76% of cases among vaccinated people occurred among high-risk contacts, and only two out of 68,279 vaccinated contacts of contacts developed Ebola, indicating to researchers that ring vaccination is effective for preventing cases.
The estimated attack rate for vaccinated individuals was about 0.017% compared with 0.656% in unvaccinated individuals, for an estimated vaccine efficacy of 97.5% (95% CI, 95.8-98.5), the report explained.
“This evidence is critical to the control of the current outbreak because it provides evidence on the role that ring vaccination with [V920] can play in an outbreak when implemented with the other key Ebola control measures,” it said.
“There is increasing evidence on the efficacy of this vaccine and therefore it is critical to offer this vaccine to all people at risk of Ebola (contacts and contacts of contacts). Even those people at risk already infected at the time of vaccination may have greater chances of survival. This is a particularly important finding if we consider that there are no licensed treatments for Ebola, although investigational therapeutics are being used” in a pioneering multidrug trial, the report concluded. – by Caitlyn Stulpin
WHO. Preliminary results on the efficacy of rVSV-ZEBOV-GP Ebola vaccine using the ring vaccination strategy in the control of an Ebola outbreak in the Democratic Republic of the Congo. https://www.who.int/csr/resources/publications/ebola/ebola-ring-vaccination-results-12-april-2019.pdf. Accessed April 15, 2019.
Disclosures: The authors report no relevant financial disclosures.