ABSTRACT
Importance: Antibody responses elicited by current messenger RNA (mRNA) COVID-19 vaccines decline rapidly and require repeated boosting. Objective: To evaluate the immunogenicity and durability of heterologous and homologous prime-boost regimens involving the adenovirus vector vaccine Ad26.COV2.S and the mRNA vaccine BNT162b2. Design, Setting, and Participants: In this cohort study at a single clinical site in Boston, Massachusetts, 68 individuals who were vaccinated at least 6 months previously with 2 immunizations of BNT162b2 were boosted with either Ad26.COV2.S or BNT162b2. Enrollment of participants occurred from August 12, 2021, to October 25, 2021, and this study involved 4 months of follow-up. Data analysis was performed from November 2021 to February 2022. Exposures: Participants who were previously vaccinated with BNT162b2 received a boost with either Ad26.COV2.S or BNT162b2. Main Outcomes and Measures: Humoral immune responses were assessed by neutralizing, binding, and functional antibody responses for 16 weeks following the boost. CD8+ and CD4+ T-cell responses were evaluated by intracellular cytokine staining assays. Results: Among 68 participants who were originally vaccinated with BNT162b2 and boosted with Ad26.COV2.S (41 participants; median [range] age, 36 [23-84] years) or BNT162b2 (27 participants; median [range] age, 35 [23-76] years), 56 participants (82%) were female, 7 (10%) were Asian, 4 (6%) were Black, 4 (6%) were Hispanic or Latino, 3 (4%) were more than 1 race, and 53 (78%) were White. Both vaccines were found to be associated with increased humoral and cellular immune responses, including against SARS-CoV-2 variants of concern. BNT162b2 boosting was associated with a rapid increase of Omicron neutralizing antibodies that peaked at a median (IQR) titer of 1018 (699-1646) at week 2 and declined by 6.9-fold to a median (IQR) titer of 148 (95-266) by week 16. Ad26.COV2.S boosting was associated with increased Omicron neutralizing antibodies titers that peaked at a median (IQR) of 859 (467-1838) week 4 and declined by 2.1-fold to a median (IQR) of 403 (208-1130) by week 16. Conclusions and Relevance: Heterologous Ad26.COV2.S boosting was associated with durable humoral and cellular immune responses in individuals who originally received the BNT162b2 vaccine. These data suggest potential benefits of heterologous prime-boost vaccine regimens for SARS-CoV-2.