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Combined flu and COVID vaccines associated with mild reactions

A new study examined the incidence of mild reactions after individuals have received the flu and COVID-19 vaccines at the same time. The researchers did not find anything to be of concern, but I always like to get ahead of a story and discuss it so I (and you) are prepared to deal with the inevitable anti-vaccine meme or trope.

Since COVID-19 cases will probably be increasing this fall (in fact, it’s already increasing), we will probably need another booster this fall. And that’s about the time most of us get our flu shot, so researchers wanted to know if there were any issues when getting both the flu and COVID-19 vaccines. Spoiler alert — not really.

woman lying on bed while blowing her nose
Photo by Andrea Piacquadio on

COVID-19 and flu vaccines paper

In a paper published on 15 July 2022 in JAMA Network Open, corresponding author Anne M. Hause, Ph.D., MSPH, and colleagues utilized data from the CDC’s v-safe program to develop a retrospective cohort study that examined reports of adverse events in the 7 days after concurrent receipt of a flu vaccine with either the Pfizer or the Moderna booster. The study used self-reported data from 981,099 people after vaccination. Data were collected from 22 September 2021 to 1 May 2022 via the voluntary application v-safe, a smartphone-based monitoring system.

Here are some of the results:

  • Approximately 8% more individuals reported adverse events after the flu and Pfizer COVID-19 booster vaccines compared to the booster alone (adjusted odds ratio = 1.08).
  • Approximately 11% more reports with the flu and Moderna booster compared to the booster alone (adjusted OR=1.11).
  • The excess systemic reactions resulted in mostly mild symptoms including fatigue, headache, and myalgia.

In other words, there was just a minor increase in reported adverse effects when both vaccines were given at the same time. And the vast majority of the effects were relatively minor. Furthermore, we might be observing the nocebo effect, which is the opposite of a placebo effect, where a negative outcome occurs due to a belief that the intervention will cause harm — in other words, the adverse effect may be in their heads.

Furthermore, there were three major limitations to the study:

  1. Since the study was based on self-reports to v-safe, the generalizability of the results to the overall population may not be accurate.
  2. v-safe was not designed to collect data on rare, unsolicited adverse events.
  3. There were no records on the type of influenza vaccine used, which might have been useful in determining if one or more of the flu vaccines caused a higher risk of adverse events in combination with the COVID-19 vaccines.

The authors concluded:

…compared with administration of COVID-19 mRNA booster vaccines alone, simultaneous administration of COVID-19 mRNA booster and seasonal influenza vaccines was associated with significant increases in reports of systemic reactions during days 0 to 7 following vaccination.

In an accompanying editorial in JAMA Network Open, Kathryn Hall, Ph.D., MPH, molecular biologist at Brigham and Women’s Hospital in Boston, and colleagues wrote:

During the COVID-19 pandemic low vaccination confidence emerged as a significant barrier to the uptake of vaccines and a threat to individual health, public health, and health equity. The reasons for low vaccine confidence are multiple and complex. In addition to mistrust of health systems, the pharmaceutical industry and the government, fear regarding adverse events and safety was a factor in low vaccine confidence and unwillingness to accept COVID-19 vaccination. Although adverse events, including local and systemic reactions, are often mild and transient, they may still significantly affect willingness to accept future vaccination. Informing patients about potential adverse events can substantially affect anxiety related to adverse events and in turn their experience of them.

The issue becomes that not only is there growing resistance to the COVID-19 vaccines but also to the flu vaccines, which could lead to a “flurona” outbreak or epidemic of both a new variant of COVID-19 along with a new flu variant. And reports of very mild adverse events are often exaggerated which increases reluctance to get either or both vaccines. It is frustrating.

Nevertheless, this study shows that the increase in risks of adverse events is tiny and the reported events are minor and transient. There are no worries here.


Michael Simpson

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