BACKGROUND

At the beginning of the 2018-2020 outbreak of Ebola virus disease (EVD) in eastern Democratic Republic of Congo (DRC), no vaccine had been licensed. However, cluster-randomized evidence from Guinea in 2015 had indicated that ring vaccination around new cases (targeting contacts and contacts-of-contacts) with the use of single-dose live-replicating rVSV-ZEBOV-GP vaccine reduced EVD rates starting 10 days after vaccination. Thus, ring vaccination was added to the standard control measures for that outbreak.

METHODS

In this study, we evaluated the incidence of EVD within the first 9 days after vaccination (when little protection was expected from case isolation or ring vaccination), during days 10 to 29, and at later time periods. We established 1853 rings around new cases or clusters within 21 days after symptom onset in the index case and offered vaccination to the ring members. Vaccinees were monitored for EVD onset until the end of the outbreak in mid-2020.

RESULTS

From August 8, 2018, to January 14, 2020, we vaccinated 265,183 participants. Of these vaccinees, 102,515 were monitored on days 0, 3, and 21 for safety. Among the contacts and contacts-of-contacts, 434 cases of EVD (0.2 per ring) were diagnosed, almost all within 0 to 9 days (380 cases) or 10 to 29 days (32 cases) after vaccination. An additional 22 cases were diagnosed after day 29 during an average of 170 more days of follow-up. The sooner that control measures (including ring vaccination) began after EVD onset in the index case, the sooner EVD rates fell among contacts. In each subgroup, EVD rates fell suddenly around day 10. Among the contacts and contacts-of-contacts who were still disease-free at day 10, the EVD onset rate during days 10 to 29 was 0.16 per 1000 (in 32 of 194,019 participants). This rate was much lower than the rate of 4.64 per 1000 (in 21 of 4528 participants) that had been seen among similarly defined ring members in Guinea, in whom standard control measures had been promptly initiated but vaccination was delayed until 21 days after ring formation (rate ratio, 0.04; 95% confidence interval, 0.02 to 0.06). No safety concerns with the vaccine were identified.

CONCLUSIONS

Nonrandomized evidence regarding standard EVD control measures plus ring vaccination in eastern DRC reinforces the earlier randomized evidence from Guinea of vaccine efficacy against EVD onset 10 or more days after vaccination.