Adverse Events After Same-Day COVID-19 and Influenza Vaccination Versus Influenza Vaccination Alone

BACKGROUND: Safety studies of the COVID-19 vaccine have identified some adverse events. Yet newer variant-updated formulations, along with increased hybrid immunity, may change these risks. Early-era safety data may not reflect experience with updated formulations in more immune-experienced populations. OBJECTIVE: To evaluate 90-day risks for adverse events after coadministration of COVID-19 and influenza vaccines compared with influenza vaccination alone, across bivalent, XBB-adapted, and KP-adapted COVID-19 vaccine periods. DESIGN: Target trial emulation using electronic health care data. SETTING: U.S. Department of Veterans Affairs. PARTICIPANTS: = 1 813 205) between 1 September 2022 and 26 August 2025. INTERVENTION: Receipt of both COVID-19 and seasonal influenza vaccines versus receipt of only an influenza vaccine. MEASUREMENTS: 90-day risks for 46 prespecified individual adverse events grouped into 3 composite outcomes (tier 1, serious or life-threatening; tier 2, clinically significant; tier 3, less severe or self-limiting), using weighted discrete-time survival models. RESULTS: For all 3 composite outcomes, risks were similar between groups: tier 1 (risk ratio [RR], 1.03 [95% CI, 0.99 to 1.09]), tier 2 (RR, 0.99 [CI, 0.96 to 1.03]), and tier 3 (RR, 0.99 [CI, 0.96 to 1.02]). Of the 46 individual adverse events, 2 tier-3 risks had nominal statistical significance: syncope (RR, 1.09 [CI, 1.02 to 1.17]) and tinnitus (RR, 0.95 [CI, 0.92 to 0.99]); no risks were statistically significant after correcting for multiple comparisons. For all risks in tier 1 or tier 2, confidence bounds included 1.0 (no effect). In period-stratified analyses, neither composite (tier) nor individual event estimates supported differences in risks between groups. LIMITATION: Generalizability and potential unmeasured confounding. CONCLUSION: Same-day coadministration of COVID-19 and influenza vaccines was not associated with an increased risk for adverse events in 3 updated-formulation periods. These findings support the short-term safety of coadministration. PRIMARY FUNDING SOURCE: U.S. Department of Veterans Affairs.