Results from a randomized control trial published Nov. 6 in JAMA Network Open show that participants who received mRNA COVID-19 and inactivated influenza vaccines simultaneously had no more adverse effects than those who received the two shots sequentially, 1 or 2 weeks apart.

The study included 335 people who were vaccinated from October 2021 to June 2023 at one of three U.S. study sites. All participants were ages 5 years and older and intended to receive both flu and mRNA COVID-19 vaccines. The average age of participants was 33.4 years, and 63% were female.

In total, 169 received the vaccines simultaneously, while 166 received them sequentially.

The authors said the main outcomes of the study were reactogenicity, including fever, chills, muscle aches, and/or joint pain of moderate or greater severity within seven days after vaccination. Serious adverse events and adverse events of special interest were assessed for 121 days.

More than half of participants (57.0%) reported a history of SARS-CoV-2 infection or had detectable antibodies to the virus at the time of study enrollment.

Good safety data

The authors found that the proportion of study participants with the primary composite reactogenicity outcome in the simultaneous group (25.6%) was noninferior to the proportion in the sequential group (31.3%).

"Fewer than 13% of participants in either group (simultaneous, 14 [8.3%]; sequential, 21 [12.6%]) had a severe reaction for any of the solicited reactions, and no participants sought medical attention for a solicited reaction," the authors wrote.

Similarly, there was no difference between the two groups in health-related quality of life indexes.

"This trial lends support to the option of simultaneous administration of these vaccines, which is a strategy to achieve high levels of vaccination coverage during anticipated periods of increased influenza and SARS-CoV-2 virus transmission," the authors concluded.