Background: Although the utility of vaccines in combating coronavirus disease 2019 (COVID-19) cannot be overstated, the intensity and duration of immune response play a key role in maintaining their efficacy. This retrospective observational study therefore investigated whether ipsilateral or contralateral injection of two doses of vaccine can elicit a different humoral response. Methods: The study population consisted of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) seronegative workers who received the primary vaccination cycle with the mRNA-based Comirnaty COVID-19 vaccine (two doses 3 weeks apart). Patients were free to choose in which arm to receive the first and second vaccine doses. Blood was collected before the first and second vaccine doses, and one month after the second dose. Humoral response was assayed by measuring serum levels of total anti-SARS-CoV-2 antibodies (Abs) with Roche Elecsys Anti-SARS-CoV-2 S chemiluminescence immunoassay. Results: The study population consisted of 269 ostensible health care workers (median age, 44 years; 109 men). Of these, 253 (94.1%) received both doses of vaccine on the same arm (i.e., ipsilateral), while 16 (5.9%) received the second dose on a different arm (i.e., contralateral). After the second dose, significantly higher Abs levels were detected after ipsilateral vaccination compared to contralaterally administered vaccines (1,437 vs. 1,052 KBAU/L; P=0.047). The ratio of total anti-SARS-CoV-2 Abs levels after the second and first vaccine doses was significantly higher in ipsilaterally injected patients than in those contralaterally vaccinated (33.7 vs. 25.2). Conclusions: Injection of COVID-19 vaccine into the ipsilateral arm may be better at eliciting a stronger humoral immune response.
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