Tipsheet

Here's Why the CDC Just Launched 'Additional Investigation' of COVID Vaccine Safety

Spencer Brown
Spencer Brown  |  January 13, 2023 3:30 PM

The Centers for Disease Control and Prevention (CDC) announced on Friday afternoon — in something of a pre-weekend news dump — that the agency's Vaccine Safety Datalink (VSD) has "met the statistical criteria to prompt additional investigation into whether there was a safety concern for ischemic stroke in people ages 65 and older who received the Pfizer-BioNTech COVID-19 Vaccine, Bivalent."

It's worth noting that the CDC's VSD is different from its Vaccine Adverse Event Reporting System (VAERS). 

The CDC's Friday updated emphasized that "[t]o date, no other safety systems have shown a similar signal and multiple subsequent analyses have not validated this signal" and that "[n]o change is recommended in COVID-19 vaccination practice" at this time while an additional investigation is conducted. 

The CDC explains more about the signal that met its threshold for additional investigation:

Rapid-response investigation of the signal in the VSD raised a question of whether people 65 and older who have received the Pfizer-BioNTech COVID-19 Vaccine, Bivalent were more likely to have an ischemic stroke in the 21 days following vaccination compared with days 22-44 following vaccination.

This preliminary signal has not been identified with the Moderna COVID-19 Vaccine, Bivalent. There also may be other confounding factors contributing to the signal identified in the VSD that merit further investigation.

Furthermore, it is important to note that, to date, no other safety systems have shown a similar signal and multiple subsequent analyses have not validated this signal:

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

Although the totality of the data currently suggests that it is very unlikely that the signal in VSD represents a true clinical risk, we believe it is important to share this information with the public, as we have in the past, when one of our safety monitoring systems detects a signal. CDC and FDA will continue to evaluate additional data from these and other vaccine safety systems. These data and additional analyses will be discussed at the upcoming January 26 meeting of the FDA’s Vaccines and Related Biological Products Advisory Committee.

So while the CDC is firm in its belief that a "true clinical risk" does not exist in terms of ischemic stroke in the three weeks after a person over 65 received the Pfizer COVID vaccine, there have been enough reports for a signal to trigger further investigation which has not yet allowed the potential risk to be ruled out. 

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