Interim Results of a Phase 1–2a Trial of Ad26.COV2.S Covid-19 Vaccine

The interim analysis of our phase 1–2a trial showed that the Ad26.COV2.S vaccine had an acceptable safety and reactogenicity profile and was immunogenic after a single vaccination with either the low or high dose. After the administration of the first dose, a trend toward a higher incidence of solicited systemic adverse events was noted with the higher vaccine dose, and a clear trend for decreasing grade 3 adverse events with increasing age was observed. The local and systemic reactions occurred on the day of immunization or the next day and generally resolved within 24 hours. The systemic reactions were very responsive to antipyretic drugs, and no need for the prophylactic use of such drugs was identified. After the second dose among participants between the ages of 18 and 55 years, the incidence of grade 3 solicited systemic adverse events was much lower than that after the first immunization in both the low-dose and high-dose groups, a finding that contrasts with observations with respect to messenger RNA–based vaccines, for which the second dose has been associated with increased reactogenicity.

https://www.nejm.org/doi/full/10.1056/NEJMoa2034201