Serological response and safety of heterologous ChAdOx1-nCoV-19/mRNA-1273 prime-boost vaccination with a twelve-week interval.

The appropriate interval between heterologous prime adenoviral vectored vaccination and boost mRNA vaccination remains unclear. We recruited 100 adult participants to receive a prime adenoviral vectored vaccine (ChAdOx1, AstraZeneca) and a boost mRNA vaccine (mRNA-1273, Moderna) 12 weeks apart and checked their serum SARS-CoV-2 anti-spike IgG titers and neutralizing antibody titers against B.1.1.7 (alpha) and B.1.617.2 (delta) variants on the 28th day after the boost dose. Results were compared with our previous study cohorts who received the same prime-boost vaccinations at 4- and 8-week intervals. Compared to other heterologous vaccination groups, the 12-week interval group had higher neutralizing antibody titers against SARS-CoV-2 variants than the 4-week interval group and was similar to the 8-week interval group at day 28. Adverse reactions after the boost dose were mild and transient. Our results support deploying viral vectored and mRNA vaccines in a flexible schedule with intervals from 8 to 12 weeks.