Two vaccines against the Zaire strain of Ebola virus are immunogenic and safe in both adults and children, according to initial results from two phases II clinical trials published in the New England Journal of Medicine.

Ad26.ZEBOV (Ad26) followed by MVA-BN-Filo (MVA) 56 days later and rVSVΔG-ZEBOV-GP (rVSV) given with or without a booster dose 56 days later produced antibody responses as early as two weeks following administration of the first dose of vaccine which lasted for at least one year.

Launched in 2017, the PREVAC study aims to compare the safety and efficacy of three different experimental regimens of the Ebola Zaire vaccine versus placebo.

Overall, 2801 volunteers at six sites in Guinea, Liberia, Sierra Leone, and Mali, were recruited into two, randomized phase II clinical trials: one involving 1400 adults (median age of 27 years, 45% women) and the other involving 1401 children aged one year or older (median age of 8 years, 46% female).

Participants received either Ad26 (supplied by Janssen) followed by MVA (supplied by Bavarian Nordic) eight weeks later, one dose of rVSV (supplied by Merck Sharp & Dohme) followed by placebo eight weeks later, or two doses of rVSV separated by eight weeks.

Reported side effects, such as fever and headaches, were mild, temporary, and similar to previous vaccine studies.

Although the findings are promising, the authors emphasize that they were unable to assess how these vaccines might protect against EVD directly as no cases of the disease were recorded in the trial, and the exact level of antibody response needed to induce protective immunity is still unknown.