An experimental Ebola virus vaccine was 100% effective in preventing the disease after 10 days in the 5837 people who received it in Guinea and Sierra Leone. Meanwhile, 23 of the 4507 people who did not receive the vaccine contracted the virus in the same timeframe.
These findings, published online December 22 in the Lancet, are the final results from a phase 3 trial of a recombinant, replication-competent vesicular stomatitis virus–based candidate vaccine expressing a surface glycoprotein, dubbed rVSV-ZEBOV. The same issue also includes successful results from a phase 2 trial, led by Feng-Cai Zhu, MSc, from the Jiangsu Provincial Center for Disease Control and Prevention in Nanjing, China, and Alie H Wurie, MSc, from the Ministry of Health and Sanitation in Freetown, Sierra Leone, of another vaccine candidate: recombinant adenovirus type-5 vector-based vaccine expressing the glycoprotein of Ebola Zaire Makona Variant.
The rVSV-ZEBOV trial was conducted as the Ebola epidemic in that area was on the decline, so the investigators, led by Ana Maria Henao-Restrepo, MD, from the World Health Organization, Geneva, Switzerland, enrolled subjects who were close contacts, as well as contacts of contacts, of people who had contracted the disease.
In this ring vaccination strategy, all the person's contacts would be randomly assigned, as a group, to be offered the vaccine either immediately or after a 21-day delay. People were eligible to receive the vaccine if they were aged at least 18 years and were not pregnant, breast-feeding, or severely ill. After the vaccine began to show effectiveness, an independent safety board recommended offering the vaccine immediately to all eligible contacts and to extend the eligibility criteria to include children aged 6 years and older.
In total, the researchers randomly assigned 51 clusters containing 4539 people to receive the vaccine immediately, 47 clusters of 4557 people to receive the vaccine after a delay, and 19 clusters of 2745 people to the nonrandomized part of the study. For outcomes, the investigators only considered cases of Ebola virus that occurred 10 days or more after randomization.
By 21 days after vaccination, no cases of Ebola virus occurred in clusters in which some of the people had been vaccinated. The investigators say this suggests that the vaccine "seemed to have contributed to interrupt Ebola transmission" in those groups. The effectiveness of ring vaccination was 70.1%, with 52.1% of people vaccinated.
Adverse events occurred in 53.9% of people within 14 days of vaccination. Most (87.5%) adverse events were mild, 11.0% were moderate, and 1.2% were severe. Children most commonly reported headache (52.6%), fatigue (11.3%), and injection site pain (9.3%) within 3 days of vaccination. Adults reported headache (25.0%), fatigue (19.0%), and muscle pain (13.2%). Of the 80 serious adverse events, half were Ebola virus disease, which was not considered to be caused by the vaccine because it occurred in the first 10 days. Four people were involved in road traffic accidents. Two had serious adverse reactions attributed to the vaccine: one febrile reaction and one case of anaphylaxis. A third had influenza-like symptoms, which the researchers judged to be possibly related.
In an attempt to keep the clusters as similar as possible, the researchers stratified them according to size (20 people or more counted as a large cluster) and urban vs rural locations. The immediate vaccination group had more high-risk contacts in their clusters. The nonrandomized clusters had higher uptake of vaccine, possibly because of public knowledge of the interim results and possibly because children could also be included.
Because of difficulties in implementing the trial ("[a] devastating outbreak of Ebola virus disease is clearly not the ideal situation for doing a vaccine trial," the authors note), the investigators did not attempt to collect samples from vaccinated people for immunological analysis. Still, they observe, "The findings...showed that it is feasible to undertake efficacy trials in the challenging circumstances of epidemics."
The results of this trial agree with those of the phase 2 PREVAIL trial, in which 94% of 500 individuals showed seroconversion after a month. Previous animal studies also showed what Dr Henao-Restrepo and colleagues deem "consistently high and rapid protection."
"Although rVSV-ZEBOV seems to be highly efficacious and safe in the context of an outbreak, some questions remain," Thomas W Geisbert, PhD, from the University of Texas Medical Branch at Galveston, writes in a related commentary. Dr Geisbert was not involved in the trial. He notes that the vaccine's long-term efficacy is unknown, as is whether future formulations could reduce the number of adverse events without reducing efficacy. In a phase 1 trials of a lower dose of this same vaccine, 13 of 51 subjects reported oligoarthritis.
Phase 2 Trial Also Promising, but Immunity Short-Lived
Another trial, conducted in healthy volunteers in Sierra Leone, found that a recombinant adenovirus type-5–based vaccine was safe and highly immunogenic. A phase 1 trial had previously been performed in Chinese volunteers. This phase 2 trial involved 500 participants at a single center and was a randomized, double-blind, placebo-controlled trial.
Of those participants, 250 received high-dose vaccine with 1.6 × 1011 viral particles, 125 received a low dose vaccine with 8 × 1010 particles, and 125 received a placebo with just vaccine excipients and no viral particles.
Immune responses were similar for both versions of the vaccine. The glycoprotein-specific antibodies peaked at day 28 with a geometric mean titer of 1471.8 (95% confidence interval, 1151.0 - 1881.8) in the low-dose group and 2043.1 (95% confidence interval, 1762.4 - 2368.4) in the high-dose group vs 6.0 to 6.8 in the placebo group. This response decreased by 85% 6 months after the injection. Because of the short duration of the response, the authors suggest a booster may be needed.
The immune response was similar among both versions of the vaccine, but the lower dose was associated with fewer injection site reactions.
"Taking vaccine profiles, manufacturing costs, and production capacity into consideration, 8.0 × 1010 viral particles seem to be an optimal dose, since it could induce a high level of glycoprotein-specific antibody responses and confer substantial protection to vaccinated individuals, at least in the short term," the authors write. They note that vaccine efficacy trials will be difficult to conduct without an ongoing Ebola outbreak.
"Concerns remain about the immunogenicity and the nature and durability of protection when implementing mass vaccination campaigns in future outbreak settings in Africa," Martin P. Grobusch, MD, PhD, and Abraham Goorhuis, MD, PhD from the Center of Tropical Medicine and Travel Medicine of the University of Amsterdam write in an accompanying comment. The immune response was not as strong in this study as in the previous trial with Chinese volunteers, and even though Dr Zhu and colleagues recommend that a booster vaccine may be necessary, Dr Grobusch and Dr Goorhuis note that we do not know how effective that booster might be.