ST. LOUIS — A study published in the Annals of Internal Medicine found that receiving a COVID-19 vaccine and an influenza vaccine on the same day was not associated with an increased risk of adverse events compared to receiving only a flu vaccine. The findings come from a target trial emulation conducted by Ziyad Al-Aly and colleagues.

The research indicated no increased risk for serious, life-threatening, or clinically important adverse events when both vaccines were coadministered. The study also reported no increased risk for less severe or self-limiting adverse events. Al-Aly, a physician at Washington University in St. Louis, said, "We looked at 46 different outcomes -- literally leaving no stone unturned -- and we came up empty-handed."

Data came from electronic health records obtained from the U.S. Department of Veterans Affairs. Researchers assessed 46 individual adverse events. While coadministration showed a slightly higher risk of syncope, with a risk ratio of 1.09 (95% CI 1.02-1.17), before correction for multiple comparisons, and a slightly lower risk for tinnitus, with a risk ratio of 0.95 (95% CI 0.92-0.99), no risks remained statistically important after this correction.

The study spanned three COVID-19 vaccine formulation eras between September 2022 and August 2025, which included bivalent, XBB-adapted, and KP-adapted vaccines. In each era, no notably increased risk of any individual adverse events was observed in the coadministration group compared to the flu vaccine-only group. Myocarditis risks were minimal in both groups; the risk difference for myocarditis was 1.11 (95% CI -0.87 to 5.26).

The coadministration group consisted of 705,124 individuals who received both vaccines on the same day, while the flu vaccine-only group included 1,813,205 people. The mean age in the coadministration group was 69.7 years, and 68.4 years in the flu vaccine-only group. The study population was predominantly male, at 92.7% in both groups, and between 72% and 78% white.

Within 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. Al-Aly said, "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." The researchers did not measure short-term adverse events such as transient fever or injection site pain.

Why It Matters

The study provides information regarding the safety of receiving both COVID-19 and influenza vaccines simultaneously. This information can assist in public health recommendations and individual vaccination decisions. The use of a large dataset from the U.S. Department of Veterans Affairs and a target trial emulation design contributes to the scope of the findings regarding coadministration and adverse events across different vaccine formulations and over time. The explicit examination of 46 different outcomes and the correction for multiple comparisons aimed to identify any potential risks.