Simultaneous administration of a COVID-19 mRNA booster and an influenza vaccine may slightly increase the likelihood of systemic reactions, according to a study published in JAMA Network Open, but these reactions are mostly mild. Anne M. Hause, PhD, and colleagues evaluated adverse events and health impacts associated with simultaneous COVID-19 mRNA boosters and seasonal influenza vaccines in the US population. Self-reported vaccine data from 92,023 individuals aged 12 and older were collected on days 0-7 after vaccination. In that period, 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 mRNA booster vaccines were slightly more likely to report any systemic reaction in the week following simultaneous vaccination, although most reactions were mild.