On Jan. 13 2023, the U.S. Food and Drug Administration (FDA) and the U.S. Centers for U.S. Disease Control and Prevention (CDC)) issued a public statement regarding the identification of a preliminary safety signal for ischemic stroke among adults over the age of 65 that occurs between one to 21 days after receiving Pfizer/BioNTech’s monovalent Comirnaty messenger RNA (mRNA) COVID-19 shot and the WT/OMI BA.4/BA.5 bivalent version of it. The safety signal was detected in the CDC’s Vaccine Safety Datalink (VSD), however, a primary analysis conducted by federal public health officials did not find elevated stroke risk following COVID bivalent shots.1
The VSD, which is a closed database operated by the CDC containing health maintenance organization medical records,2 also detected a risk of stroke in people who were given r both the monovalent Comirnaty shot and/or the bivalent WT/OMI BA.4/BA.5 shot plus a high-dose or adjuvanted influenza vaccine on the same day, a risk that was higher than the risk of stroke following receipt of either the monovalent Comirnaty shot or bivalent WT/OMI BA.4/BA.5 shot given alone.
The FDA recently conducted a secondary analysis, which showed an increased risk of non-hemorrhagic stroke (NHS) following administration of high-dose or adjuvanted influenza vaccine.3
Primary Analysis Found Elevated Risk of Stroke in Elderly After COVID Shots
The first CDC analysis evaluated the risk of the stroke following a COVID bivalent shot. The study’s sample population included 5,397,278 adults over 65 years old, who were given either the monovalent Comirnaty bivalent WT/OMI BA.4/BA.5 shot or Moderna/NIAID’s bivalent Spikevax shot.
There were 2,886 cases of NHS; 2,641 cases of transient ischemic attack (TIA); 4,788 cases of NHS/TIA, and 808 cases of hemorrhagic stroke (HS) across both manufacturers of the COVID shots. Approximately 10-15 percent of the sample cases claimed they had COVID disease between 31 to 365 days prior to suffering a stroke, and 34-45 percent had been given a high-dose or adjuvanted influenza vaccination at the same time.4
The findings from the primary analysis showed that there were no statistically significant associations of an increased risk of stroke following administration of the COVID bivalent shots. However, among those people who received the monovalent Comirnaty and bivalent WT/OMI BA.4/BA.5 shot, an increased risk for non-hemorrhagic stroke was observed in people over the age of 85 years old. In addition, among those people who were given the bivalent Spikevax, an increased risk was observed in the 65-74 year-old age group for both NHS and TIA.
Among those who simultaneously received the high-dose/adjuvanted influenza vaccine, as well as monovalent Comirnaty and bivalent WT/OMI BA.4/BA.5 shots, an increased risk of NHS was found. Among those who simultaneously received high-dose/adjuvanted influenza and bivalent Spikevax shots, an increased risk for TIA was found.5
Secondary Analysis Revealed Significantly Increased Risk of Stroke After High-dose/Adjuvanted Influenza Shots
Given that the VSD primary analysis showed an elevated risk of stroke when the bivalent COVID shot and influenza vaccine were given simultaneously, the researchers conducted a secondary analysis to assess the risk of stroke following the administration of high-dose/adjuvanted influenza vaccine during the same period of the study.
The study sample included 6,961,413 eligible Medicare patients who were given high-dose/adjuvanted influenza vaccine. There were 5,497 cases of NHS, 4,871 cases of TIA; 9,065 cases of NHS/TIA, and 1,498 cases of hemorrhagic stroke. The results of the analysis showed a statistically significant increased risk of NHS following high-dose/adjuvanted influenza vaccine, suggesting that the high dose adjuvanted influenza vaccine may be responsible for the increase in strokes observed in the study.6
It was found that for those people, who received both COVID shots and an influenza vaccine, there was a 20 percent increase in the risk of ischemic stroke with the bivalent WT/OMI BA.4/BA.5 shot, and a 35 percent increased risk of transient ischemic stroke after the bivalent Spikevax shot.7
Vaccine Policy Remains Unchanged Despite Study Finding Increased Stroke Risk After COVID and High Dose/Adjuvanted Influenza Shots
Although the results of the study apparently will not result in changes in the CDC’s vaccine recommendations, federal officials are being urged to further explore the reported association in larger studies since the current study is observational and cannot prove causality.8
Although the FDA would not allow for the researchers to be interviewed by reporters, Cherie Duvall-Jones, a spokeswoman for the FDA, made a statement saying the agency was “making this information known at this time through publication of this paper for transparency.”9
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1 Lu Y. et al. Evaluation of Stroke Risk Following COVID-19 mRNA Bivalent Vaccines Among U.S. Adults Aged ≥65 Years. medRxiv Oct. 15, 2023.
2 CDC. Vaccine Safety Datalink (VSD). Aug. 2, 2023.
3 Lu Y. et al. Evaluation of Stroke Risk Following COVID-19 mRNA Bivalent Vaccines Among U.S. Adults Aged ≥65 Years. medRxiv Oct. 15, 2023.
7 Mandavilli A. Covid Shots May Slightly Raise Stroke Risk in the Oldest Recipients. The New York Times Oct. 24, 2023.