Background: Because SARS-CoV-2 mutations and immunity wane over time, a third dose of heterologous COVID-19 vaccine is proposed for individuals primed with inactivated COVID-19 vaccine.

Research design and methods: We conducted a single-center, open-label trial to assess the safety, immunogenicity, and immune-persistence of a heterologous BBIBP-CorV/ZF2001 prime-boost vaccination in Chinese adults. 480 participants who had been primed with two doses of BBIBP-CorV, received a third dose of ZF2001 after an interval of 3-4, 5-6, or 7-9 months.

Results: The overall incidence of adverse reactions within 30 days after vaccination was 5.83%. No serious adverse reactions were reported. The respective geometric mean titers (GMTs) of neutralizing antibodies for 3-4, 5-6, and 7-9 months groups at baseline were 2.06, 2.02, and 2.10; which increased to 55.42, 63.45, and 62.06 on day 14; then decreased to 17.53, 23.79, and 26.73 on day 30; before finally waning to 8.29, 9.24, and 9.51 on day 180. After the booster, the three groups showed no significant differences in GMTs. GMTs were lower in older participants than younger participants.

Conclusions: A heterologous BBIBP-CorV/ZF2001 prime-boost vaccination was safe and immunogenic. Prime-boost intervals did not affect the immune response. The immune response was weaker in older adults than younger adults. CLINICAL TRIAL IDENTIFIER: NCT05205083.