In a statement published in January 2023 the US CDC and FDA indicated that CDC’s Vaccine Safety Datalink (VSD) “met the statistical criteria” to investigate a potential “safety concern for ischaemic stroke” in people over 65 who received the Pfizer/BioNTech COVID-19 Vaccine, Bivalent. An ischaemic stroke occurs after the brain’s blood supply is disrupted or decreased. Brain cells can die within minutes if they are denied oxygen or nutrients.  

Vaccine Safety 

The CDC and FDA emphasise that “transparency and vaccine safety are top priorities”. This is demonstrated in the “multiple, complementary safety monitoring systems” that are used to “detect possible safety signals for vaccines”. One such safety system is the VSD, which offers “near real-time surveillance”.  

“All signals require further investigation and confirmation from the formal epidemiologic studies.” 

The CDC also describes the sharing of responsibility among monitoring systems. If one detects a signal, the rest are checked to “validate whether the signal represents an actual concern with the vaccine”.  

The Bivalent vaccine  

The signal was identified with the Pfizer-BioNTech COVID-19 Vaccine, Bivalent, but not its Moderna counterpart. This prompted “additional investigation” into the possibility of a safety concern for ischemic stroke. The investigation explored whether recipients of the vaccine were more likely to have an ischemic stroke in the 21 days following vaccination, compared with the following days up to 42 days after vaccination.

The CDC states that “no other safety systems have shown a similar signal” and “multiple subsequent analyses have not validated this signal”

“There may also be other confounding factors contributing to the signal identified in the VSD that merit further investigation”.  


The following points were highlighted by the CDC to demonstrate that it is confident in the current vaccination schedule: 

  • A large study of bivalent vaccines using the Centres for Medicare and Medicaid Services database revealed “no increased risk of ischaemic stroke”. 
  • A preliminary study using the Veterans Affairs database indicated no increased risk of ischaemic stroke following the bivalent vaccine. 
  • The CDC and FDA managed Vaccine Adverse Event Reporting System (VAERS) has “not seen an increase in reporting of ischaemic strokes” following the vaccine. 
  • Pfizer-BioNTech’s global safety database has “not indicated a signal for ischaemic stroke” with the vaccine. 
  • Other countries have “not observed an increased risk for ischaemic stroke” with these vaccines. 

Thus, the conclusion is that the signal probably does not represent a “true clinical risk”. However, the CDC decided to share their information with the public as has been done in the past. Furthermore, the recommendation is that there should be “no change in vaccination practice”.  

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