Exploring the Link Between Covid and Influenza Co-vaccination and Stroke Risk: New Insights and Implications
In January of this year, an initial analysis by the Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) raised concerns about a potential increased risk of stroke after Covid (bivalent mRNA) and influenza (adjuvanted or high-dose) co-vaccination. The study indicated a 1.47 times increased risk of ischemic stroke following Pfizer's bivalent mRNA vaccination in individuals over 65 years, based on data from the U.S. Vaccine Safety Datalink (VSD) database.
The analysis further revealed that the risk was specifically observed in individuals who received the bivalent mRNA vaccine along with influenza vaccination on the same day, resulting in a 2-times increased risk. However, no changes in risk were observed in those who received bivalent mRNA without influenza vaccination, bivalent mRNA + standard-dose influenza vaccination, or influenza vaccination alone.
The findings prompted concerns and warranted further investigation into this potential link between co-vaccination and stroke risk. However, as this analysis was the first of its kind, experts advised caution and the need for additional studies to validate the results.
Fast forward six months, and a new study has emerged to provide additional insights into this matter. Titled "BA.1 Bivalent COVID-19 Vaccine Use and Stroke in England," the study was conducted by Andrews et al. from the U.K. Health Security Agency and London School of Hygiene and Tropical Medicine and was published in the Journal of American Medical Association (JAMA) Network in June 2023.
The study utilized a self-controlled case series design similar to the CDC's analysis, comparing stroke risk in the 1-21 day period post-vaccination (risk period) with the 22-42 day period (control period).
The study focused on a population of 14.6 million people aged over 50 years who received the bivalent mRNA vaccine in the U.K. National Immunisation Management System. Among these individuals, 983 (11.7%) were co-vaccinated with adjuvanted influenza vaccine on the same day, with the majority (over 94%) being over 65 years old. Throughout the study period, 6,882 cases of ischemic stroke and 1,510 cases of hemorrhagic stroke were recorded.
The results of the study indicated that for individuals over 50 years who received bivalent mRNA vaccine alone or in combination with adjuvanted influenza vaccine, there were no significant changes in the relative incidence or risk of ischemic or hemorrhagic stroke during the risk vs. control period. These results were consistent for individuals over 65 years who received either Pfizer's or Moderna's bivalent mRNA vaccine along with adjuvanted influenza vaccine.
While this study provides reassurance, it is essential to consider its limitations when interpreting the results. Firstly, the study focused on the BA.1 Omicron variant, while the U.S. CDC study targeted the BA.5 variant. Nevertheless, there is no evidence to suggest that the safety of these two bivalent mRNA vaccines would differ significantly.
Secondly, only a small percentage of participants in the U.K. study underwent Covid and influenza co-vaccination on the same day, similar to the U.S. CDC study's limitations.
Thirdly, both studies assumed that the risk of stroke would be acute, occurring within 21 days of vaccination, considering the typical duration of vaccine immune responses.
In conclusion, while the U.K. and U.S. CDC studies share similarities in design, they yielded conflicting findings. Therefore, it is possible that the increased stroke risk reported in the U.S. CDC study may have been a false alarm. Additionally, Health Canada and the Public Health Agency of Canada (PHAC) reported no elevated risk or signal for vascular events like stroke in over 7 million bivalent mRNA vaccine recipients in Canada as of January 2023, although influenza co-vaccination was not considered in this data.
Considering the complexities of these studies and the potential influence of confounding variables, it is more plausible that factors other than co-vaccination are at play. Behavioral differences and statistical fluctuations may have influenced the study results. As the scientific community continues to investigate, it is crucial to remain vigilant in understanding vaccine safety and efficacy thoroughly.