Large study involving 2.5 million U.S. veterans shows no increased risk of serious side effects from same-day vaccination

A single visit to a pharmacy or healthcare centre could help people prepare for two major seasonal illnesses — COVID-19 and influenza — without adding extra safety concerns, according to a new study.

The research, published in the Annals of Internal Medicine, found that receiving a COVID-19 vaccine and a flu shot during the same appointment does not increase the risk of serious health problems compared with getting only a flu vaccine.

The study team, led by Yan Xie from the VA St. Louis Health Care System and Washington University School of Medicine, analysed health records of around 2.5 million U.S. veterans.

Researchers compared two groups — more than 700,000 adults who received both COVID-19 and flu vaccines on the same day, and nearly 1.8 million people who received only a flu vaccine.

The participants were followed for 90 days after vaccination, with researchers tracking 46 possible health events, including mild reactions as well as serious conditions involving the heart, blood clotting system, nervous system and immune system.

The findings showed that people who received both vaccines together did not have a higher risk of serious adverse events compared with those who received only the flu shot.

The study included different updated COVID-19 vaccine formulations administered between autumn 2022 and summer 2025, providing researchers with data across multiple vaccine versions.

According to the researchers, the findings add further evidence supporting the short-term safety of co-administering COVID-19 and influenza vaccines. The results may also help healthcare authorities develop future vaccination recommendations and encourage more people to stay protected during the respiratory virus season.

Experts say combining vaccination visits could also make it easier for people to complete recommended immunisations, especially during busy winter months when infections from respiratory viruses often increase.

The study reinforces the message that timely vaccination remains an important tool in reducing the burden of COVID-19 and seasonal flu, while making preventive healthcare more convenient for millions of people.