Effect of same-arm versus cross-arm administration of sequential doses of BNT162b2 on short-term vaccine effectiveness—a retrospective cohort study

Objective: We sought to test whether administering the second BNT162B2 vaccine dose on the cross-arm or the same arm as the first dose creates a more robust local and systemic immune response leading to favourable clinical results. Methods: A retrospective cohort study was conducted on all Clalit Health Services members who received the BNT162b2 vaccine between December 2020 and December 2021. The primary endpoint was a positive RT-PCR test result for SARS-CoV-2 38 days after administration of the second dose. Results: During the study, 2 678 226 Clalit Health Services members received both doses of the BNT162b vaccine and were eligible for analysis. Of these, 2 367 694 (88.41%) received the first two doses of the vaccine on the same arm. The primary endpoint was observed in 2061 (0.077%) participants. The primary endpoint was observed less frequently in the same-arm versus the cross-arm group (1760/2 365 934 and 301/310 231 respectively), with an adjusted OR of 0.83 (95% CI, 0.73–0.94; p 0.004). Discussion: Administration of the first and second BNT162b2 vaccine doses in the same arm might increase vaccine effectiveness in the short term possibly because of more robust local lymph node activation. This easy intervention could have a public health impact on the implementation of future mRNA vaccines. Further studies are needed to assess the long-term effectiveness of our findings.