Atlanta—Pharmacists might have heard complaints from recipients about systemic reactions when they got COVID-19 mRNA booster and seasonal influenza vaccines at the same time. Reports like that led a CDC team to try to determine if those adverse effects were more common after the dual vaccines than COVID-19 mRNA booster alone.

Their results, published in the Journal of the American Medical Association Network Open, suggest the complaints might be warranted. The cohort study of self-reported data from 981,099 vaccine recipients aged 12 years or older indicated that simultaneous administration of a COVID-19 mRNA booster dose and an influenza vaccine was associated with 8% to 11% increases, respectively, in systemic reaction compared with COVID-19 mRNA booster alone. The authors said the differences were statistically significant.

Embarking on the study, the researchers pointed out, "COVID-19 and seasonal influenza vaccines are essential in preventing respiratory infections and their potentially severe complications. Simultaneous administration of vaccines is efficient and may improve coverage with each vaccine. However, the safety of simultaneous administration of COVID-19 and influenza vaccines has not been well described."

For their retrospective cohort study, self-reported vaccine data were collected on Days 0 to 7 after vaccination from September 22, 2021, through May 1, 2022, through CDC's v-safe, a voluntary smartphone-based monitoring system. Included in the study were people who voluntarily registered in v-safe following COVID-19 vaccination.

The study gathered information on local injection site, systemic reactions (e.g., fatigue, headache, and myalgia), and other effects reported by v-safe respondents in the week following a COVID-19 mRNA booster vaccination.

Of the more than 981,000 v-safe registrants, 9.4% reported receiving simultaneous administration of COVID-19 mRNA booster and seasonal influenza vaccines. Of those, 59.7% were female, 39.4% were male, and sex was unknown for 0.9%.

"In the week following vaccination, any systemic reactions were reported by 36,144 (58.9%) of 61,390 respondents who simultaneously received Pfizer-BioNTech booster and influenza vaccines and 21,027 (68.6%) of 30,633 respondents who simultaneously received Moderna booster and influenza vaccines," the researchers advised. "Respondents who simultaneously received influenza and Pfizer-BioNTech booster vaccines (aOR, 1.08; 95% CI, 1.06-1.10) or influenza and Moderna booster vaccines (aOR, 1.11; 95% CI, 1.08-1.14) were slightly more likely to report any systemic reaction in the week following simultaneous vaccination than respondents who received only a COVID-19 mRNA vaccine booster."

Noting that simultaneous administration of COVID-19 mRNA booster and seasonal influenza vaccines was associated with significant increases in reports of systemic reactions during the first week after vaccination, the authors suggested their results might help better characterize the outcomes associated with simultaneously administered COVID-19 booster and influenza vaccines in the U.S. population.

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