1. The vaccination resulted in adverse events that were grade 2 or lower with no serious adverse events.
2. Serum titers peaked a few weeks after vaccination and persisted for at least one year.
Evidence Rating Level: 1 (Excellent)
Study Rundown: A number of Zika virus vaccine candidates have advanced past the preclinical stage, but none have been approved for public use. Ad26.ZIKV.001 is a replication-incompetent human adenovirus serotype 26 (Ad26) vector that encodes the Zika envelope and membrane proteins. This trial was conducted to confirm the safety, reactogenicity, and immunogenicity of this vaccine candidate. While local and systemic adverse events were reported significantly higher in the vaccine group compared to the placebo group, no vaccine-related adverse events were serious. The vaccine elicited a strong immune response with more than 90% of participants seroconverting within one month. Furthermore, all regimens had similar durability of response at one year. The study was limited by being conducted in a nonendemic area; therefore, safety and immunogenicity could not be assessed in an exposed population. These findings both highlight Ad26.ZIKV.001 is a promising candidate in case of Zika virus resurgence.
Relevant Reading: Zika vaccines and therapeutics: landscape analysis and challenges ahead
In-Depth [randomized controlled trial]: This phase 1 study enrolled 100 adults at two sites in the United States. Participants between 18 and 50 years of age with no history of flavivirus infection or vaccination were included in the study. Participants that withdrew consent or moved from the area were excluded from the study. The participants were randomly assigned in a 1:1:1:1:1 ratio to receive: low dose/low dose, low dose/placebo, high dose/high dose, high dose/placebo, and placebo/placebo, respectively. The low dose (LD) sample contained 5×1010 viral particles while the high dose (HD) sample contained 1×1011 viral particles. After the first vaccination, 75% of participants in the combined LD group, 88% in the combined HD group, and 45% in the placebo-only group reported local adverse events (AEs). Similar percentages of each group (73%, 83%, and 45%, respectively) also experienced solicited systemic AEs. After the second vaccination, 68% of participants in the LD/LD group and 78% in the HD/HD group experienced local AEs as compared with only 17% in the placebo-only group. Solicited systemic AEs were also more frequent in the LD/LD (58%) and HD/HD (56%) groups compared to the placebo group (39%). All AEs related to the second vaccination were grade 2 or lower. Seroconversion occurred within 30 days in 88% (35 of 40) of the participants in the combined LD group and 94% (36 of 38) of participants in the combined HD group. Taken together, the vaccine was well tolerated by the participants with no occurrence of serious AEs in the treated group.
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