CDC Questions Flu Vaccine’s Role in COVID-19 Booster Stroke Risk

The latest data and analysis were presented at the Vaccines and Related Biological Products Advisory Committee meeting on January 26, 2023.

As of January 11, 49.5 million COVID-19 mRNA bivalent booster doses have been administered to people 5 years or older in the US. This includes 21.3 million doses in people ages 65 years or older.

On January 13, the CDC and FDA announced they identified a safety signal from the Vaccine Safety Datalink (VSD) indicating a possible ischemic stroke risk associated with the Pfizer-BioNTech COVID-19 bivalent vaccine. The signal raised a question on whether adults 65 years and older were at higher risk of ischemic stroke in days one to 21 following vaccination compared to 22 to 42 days after receiving the booster shot.


At the Vaccines and Related Biological Products Advisory Committee meeting on January 26, Nicola Klein, M.D., Ph.D, from the Kaiser Permanente Northern California Vaccine Study Center, presented the latest analysis on the ongoing investigations into the stroke risk safety signal.

The CDC first observed a statistical safety signal on November 27, 2022. According to Klein, this signal has persisted for 7 weeks. However, for the first time, no safety signals were detected this week.

According to data through January 8, in adults 65 years and older, 130 stroke events occurred after the Pfizer-BioNTech booster. In addition, 33 stroke events occurred in people ages 18-64. However, CDC data also shows the number of older adults receiving the booster has steadily decreased since September 2022.

Their findings suggest that more stroke events occurred during days one to 21 post-vaccination than days 22 to 42 after receiving the shot. Moreover, a significant cluster of stroke events occurred 11 to 22 days after receiving the booster.

During the presentation, Klein noted that among the small subset of charts reviewed, most confirmed stroke cases occurred when the COVID-19 booster was co-administered with a high dose or adjuvant flu vaccine.

For example, in a preliminary review of 22 stroke cases in people 65 years or older on days 11 to 21 after receiving the booster, none of the individuals had a previous history of transient ischemic attack (TIA), and 64% had received the flu vaccine on the same day as the COVID booster (13 high-dose flu vaccines and one adjuvant flu vaccine).

Further data shows that 59% of the people who experienced a stroke were discharged home, 18% were discharged with home health, 9% were discharged to a skilled nursing facility, and 14% (three of the 22) died. The CDC notes that one death was likely related to the stroke.

In addition, analysis of a small sampling of older adults showed that three people experienced a stroke after receiving the Pfizer booster and a standard dose flu vaccine on the same day. In contrast, 40 people who received the Pfizer booster and a high-dose or adjuvant flu vaccine on the same day experienced a stroke. Moreover, 60 older adults experienced a stroke after only receiving the COVID booster.


The CDC notes that the small number of strokes and imprecise rate ratios limit some analyses.

The agency also says it will continue to monitor the data and explanations for the VSD safety signal and consider expanding reviews to all VSD sites. In addition, it will also examine the possible decreased rate of stroke in the three to six weeks following vaccination.

The CDC continues to recommend that everyone eligible for the COVID-19 mRNA bivalent booster or flu vaccine get vaccinated. However, the CDC and the FDA are engaged in analyses investigating the possible stroke risks of same-day COVID-19 booster and flu vaccination.


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