SOURCE: Modjarrad K, et al. Lancet 2018;391:563-571.

Zika virus infection during pregnancy can cause microcephaly and other serious neurologic defects. Protection from infection with Zika virus has been demonstrated in animal studies using a formalin-inactivated Zika virus vaccine derived from a 2015 Puerto Rican virus strain, similar to the 2015 Brazilian Zika virus strain. In preclinical trials in mice and nonhuman primates, two doses of vaccine (day 1 and day 29) produced high antibody levels within two weeks after the second dose.

Modjarrad et al reported on the first study in humans, using an aluminum hydroxide adjuvant Zika virus vaccine. Zika seronegative adults (n = 67) were randomized to Zika vaccine or placebo. Intramuscular vaccine was administered on day 1 and day 29. Efficacy was determined by the percent seroconversion (i.e., attainment of a microneutralization titre of ≥ 1:10).

By day 57, seroconversion had occurred in 92% of vaccine recipients. Tolerability of the vaccine was good, with only mild-moderate adverse events reported. These results compare well with trials of Japanese encephalitis virus and yellow fever virus that have used the same seroconversion status as a surrogate for protection from infection. Clinicians look forward to confirmation of vaccine efficacy in a large population.