More (mostly mild) side effects when flu vaccine given with COVID booster

Fatigued man on park bench
Fatigued man on park bench

Lassi Kurkijärvi / Flickr cc

Self-reported data from nearly 1 million Americans show an 8% to 11% higher rate of mostly mild systemic adverse events after simultaneous seasonal flu vaccine and mRNA COVID-19 booster (third) doses than with the COVID-19 booster alone.

In the study, published today in JAMA Network Open, the Centers for Disease Control and Prevention (CDC) COVID-19 Response Team and Emory University analyzed rates of systemic adverse events among 981,099 Americans aged 12 years and older in the week after either simultaneous flu and COVID-19 booster doses or the booster alone from Sep 22, 2021, to May 1, 2022. Volunteers responded to at least one health survey through the CDC's smartphone-based v-safe monitoring system during the study period.

The authors noted that current clinical CDC guidance states that COVID-19 vaccines may be given without regard to timing of other vaccines, including during the same vaccination visit. Authorization of COVID-19 vaccine boosters in the United States coincided with the recommended period for flu vaccination, increasing the odds of simultaneous receipt.

Fatigue, headache, muscle aches most reported

Of the 981,099 volunteers, 92,023 (9.4%) reported having received simultaneous flu and COVID-19 booster vaccination. Of these participants, 59.7% were female, 40.6% were aged 12 to 49 years, 25.8% were 50 to 64, 27.0% were 65 to 74, and 6.6% were 75 or older.

A total of 58.9% of 61,390 participants who simultaneously received the flu vaccine and the Pfizer/BioNTech COVID-19 booster and 68.6% of 30,633 of those who simultaneously received the flu vaccine and the Moderna booster reported systemic adverse reactions.

Participants who simultaneously received the flu and Pfizer vaccine (aOR, 1.08; 95% CI, 1.06 to 1.10) or the flu and Moderna vaccine (aOR, 1.11; 95% CI, 1.08 to 1.14) were slightly more likely to report a systemic reaction than those who received a COVID-19 booster alone.

Most reactions were mild or moderate and were reported the day after vaccination. The most common systemic reactions were fatigue, headache, and muscle aches.

Less than 1% sought hospital care

A total of 19.0% of respondents receiving simultaneous flu and COVID-19 booster vaccination with Pfizer and 26.8% of flu and Moderna recipients reported health effects. Among Pfizer recipients, 15.5% said they couldn't perform routine daily activities, as did 22.2% of Moderna recipients. But less than 1% needed hospital care.

Hospital admission data were available for 19 of 22 participants; of them, 10 said their hospitalization was unrelated to vaccination or was reported in error. The remaining patients indicated they were admitted for treatment of excessive vomiting, supraventricular tachycardia, abnormal heart rhythms, chest pain, an autoinflammatory disease exacerbation, kidney infection, low oxygen levels, or stroke.

Recipients of simultaneous flu and Pfizer boosters were not more likely than recipients of a booster alone to report a "health impact" within a week of vaccination (aOR, 0.99; 95% CI, 0.97 to 1.02), while flu and Moderna vaccine recipients were slightly more likely to report a health impact during the first week (aOR, 1.05; 95% CI, 1.02 to 1.08) than recipients of a Moderna booster alone.

The researchers said that while simultaneous administration of both vaccines is efficient and may increase uptake of the vaccines, it is associated with a higher risk of systemic adverse events.

But in a related commentary, Kathryn Hall, PhD, MPH, of Brigham and Women's Hospital; Valerie Stone, MD, MPH, of the Boston Public Health Commission; and Bisola Ojikuto, MD, MPH, of Massachusetts General Hospital, said that concomitant administration of the flu vaccine and COVID-19 booster could increase the flu vaccination rate, which was only 50.2% during the 2020 to 2021 season.

"Although logistical challenges may serve as barriers to the implementation of dual vaccination, organizing for provision of both influenza vaccination and COVID vaccination within clinical settings and at community-based nonclinical venues would be advantageous and may increase the likelihood of uptake," they wrote.

"Given the small increase in rate of adverse events reported by Hause et al, health care systems should be encouraged to develop routine and streamlined processes for coadministration of these vaccinations."

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