Two experimental Ebola vaccines are safe and can elicit an immune response that lasts for at least one year, a large clinical trial in Liberia has shown.
The findings, published in the New England Journal of Medicine, are based on a study of 1,500 adults that began during the West Africa Ebola outbreak. This clinical trial has yielded valuable information that is essential for the continued development of these two Ebola vaccine candidates and also demonstrates that well- designed, ethically sound clinical research can be conducted during an epidemic.
The trial enrolled men and women ages 18 and older with no reported history of Ebola virus disease at Redemption Hospital in Monrovia. Three groups of 500 volunteers received one of the vaccine candidates or a placebo (saline injection). Participants provided blood samples before vaccination and again at one week, one month, six months and one-year post-vaccination.
Investigators then tested each of these samples for antibodies to the Ebola virus. Responses at one week were modest with both vaccines.
However, by one month, 71 percent of cAd3-EBOZ recipients and 84 percent of rVSV-ZEBOV recipients developed an antibody response compared to 3 percent of placebo recipients. At one year, antibody responses were largely maintained in both groups: 64 percent of cAd3-EBOZ recipients and 80 percent of rVSV-ZEBOV recipients had an antibody response compared with seven percent of placebo recipients.
Some participants who received the investigational vaccines experienced mild to moderate side effects that resolved, such as a headache, muscle pain, feverishness and fatigue. Overall, researchers did not identify any major safety concerns related to the vaccines. Most of the serious medical issues reported during the trial were due to malaria.
At the beginning of the trial investigators found that four percent of participants already had a certain threshold of Ebola antibodies indicative of past Ebola infection – but no known history of Ebola virus disease.
Researchers also found unexpectedly that the proportion of participants developing malaria by one year was lower for participants who received the investigational vaccines as compared with those receiving placebo, particularly among the rVSV-ZEBOV recipients. Future studies are needed to determine if this is a chance finding or if it has some significance related to cross-reactive immunity.