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Background Safety after the second dose of the SARS-CoV-2 vaccine remains to be elucidated, especially among individuals reporting adverse events after their first dose. This study aims to evaluate the impact of a delayed second dose on all-cause mortality and emergency services. Methods A territory-wide, retrospective cohort of people who had completed two doses of mRNA (BNT162b2) or inactivated SARS-CoV-2 (CoronaVac) vaccine between February 23 and July 3, 2021, in Hong Kong was analyzed, with linkage to electronic health records retrieved from the Hong Kong Hospital Authority. Vaccine recipients were classified as receiving a second dose within recommended intervals (21–28 days for BNT162b2; 14–28 days for CoronaVac) or delayed. Study outcomes were all-cause mortality, emergency department (ED) visits, and unscheduled hospitalizations within 28 days after the second dose of vaccination. Results Among 417,497 BNT162b2 and 354,283 CoronaVac second dose recipients, 3.8% and 28.5% received the second dose beyond the recommended intervals (mean 34.4 and 31.8 days), respectively. During the study period, there were &lt; 5 daily new cases of COVID-19 infections in the community. Delaying the second dose was not associated with all-cause mortality (hazard ratio [HR] = 1.185, 95% CI 0.478–2.937,  P  = 0.714), risk of ED visit (HR = 0.966, 95% CI 0.926–1.008,  P  = 0.113), and risk of unscheduled hospitalization (HR = 0.956, 95% CI 0.878–1.040,  P  = 0.294) compared to that within the recommended interval for CoronaVac recipients. No statistically significant differences in all-cause mortality (HR = 4.438, 95% CI 0.951–20.701,  P  = 0.058), ED visit (HR = 1.037, 95% CI 0.951–1.130,  P  = 0.411), and unscheduled hospitalization (HR = 1.054, 95% CI 0.867–1.281,  P  = 0.597) were identified between people who received a second dose of BNT162b2 within and beyond the recommended intervals. Conclusions No significant association between delayed second dose of BNT162b2 or CoronaVac and all-cause mortality, ED visit, and unscheduled hospitalization was observed in the present cohort. Regardless of the recommended or delayed schedule for SARS-CoV-2 vaccination, a second dose of both vaccines should be administered to obtain better protection against infection and serious disease. The second dose should be administered within the recommended interval following the manufacturer’s product information, until further studies support the benefits of delaying vaccination outweighing the risks.

bmc medicine

Impact of a delayed second dose of mRNA vaccine (BNT162b2) and inactivated SARS-CoV-2 vaccine (CoronaVac) on risks of all-cause mortality, emergency department visit, and unscheduled hospitalization