ST. LOUIS — A new study published in the Annals of Internal Medicine found that receiving COVID-19 and flu vaccines on the same day was not linked to an increased risk of adverse events when compared to receiving only a flu shot. The research, conducted by Ziyad Al-Aly, MD, of Washington University in St. Louis, and his colleagues, utilized electronic health data from the Department of Veterans Affairs (VA).

The study examined 46 individual adverse events and found that coadministration of COVID-19 and influenza vaccines was not associated with increased risks for serious, life-threatening, clinically relevant, or less severe adverse events. "We looked at 46 different outcomes -- literally leaving no stone unturned -- and we came up empty-handed," Al-Aly said.

The study identified a slightly higher risk for syncope (fainting) with coadministration, showing a risk ratio of 1.09 (95% CI 1.02-1.17). Conversely, tinnitus showed a slightly lower risk with coadministration, with a risk ratio of 0.95 (95% CI 0.92-0.99). After statistical correction for multiple comparisons, none of these risks remained statistically notable. "Even in today's world, where people have been exposed to multiple prior COVID-19 vaccines and their immune systems have been shaped by at least one COVID-19 infection, vaccines have a reassuring record of safety," Al-Aly said.

The researchers conducted a target trial emulation using VA electronic health data to compare same-day coadministration of both vaccines against flu vaccines alone. The coadministration group included 705,124 individuals, while 1,813,205 people received only a flu shot. The average age in the coadministration group was 69.7 years, and it was 68.4 years in the flu vaccine-only group. The coadministration group was 92.7% male, and between 72-78% of participants in both groups were white.

The study encompassed data from three COVID-19 vaccine formulation eras, spanning from September 2022 to August 2025, including bivalent, XBB-adapted, and KP-adapted vaccines. In the coadministration group, 60.4% received the Moderna vaccine and 39.2% received the Pfizer vaccine, with 0.46% receiving other formulations. High-dose flu vaccines were administered to 58.8% of the coadministration group and 51.1% of the flu vaccine-only group. Risks of myocarditis were small in both groups, and the difference in myocarditis risk had wide confidence bounds, including the null (risk difference 1.11, 95% CI -0.87 to 5.26).

The study's limitations include the demographic characteristics of the VA population, which is predominantly older, male, and white, potentially affecting the generalizability of the findings. The study also did not assess short-term adverse events like transient fever or injection site pain, which may not have resulted in clinical encounters.