(HealthDay News) — Simultaneous administration of COVID-19 mRNA booster and influenza vaccines may increase the likelihood of systemic reactions, according to a study published online July 15 in JAMA Network Open.

Anne M. Hause, PhD, from the US Centers for Disease Control and Prevention in Atlanta, and colleagues evaluated adverse events and health impacts associated with simultaneously administering COVID-19 mRNA booster and seasonal influenza vaccines in the US population. Self-reported vaccine data from 92,023 individuals aged 12 years and older were collected on days 0 to 7 after vaccination between Sept. 22, 2021, and May 1, 2022, through v-safe, a voluntary smartphone-based monitoring system.

The researchers found that in the week following vaccination, any systemic reactions were reported by 58.9% of 61,390 respondents who simultaneously received the Pfizer-BioNTech booster and influenza vaccines and 68.6% of 30,633 respondents who simultaneously received the Moderna booster and influenza vaccines.

Compared with respondents who received only a COVID-19 mRNA vaccine booster, those who simultaneously received influenza and Pfizer-BioNTech booster vaccines or influenza and Moderna booster vaccines were slightly more likely to report any systemic reaction in the week following simultaneous vaccination, although most reactions were mild.

“These results may help better characterize the outcomes associated with simultaneously administered COVID-19 booster and influenza vaccines in the US population,” the authors write.

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