On January 13, the Centers for Disease Control and Prevention (CDC) announced that a vaccine safety monitoring system called the Vaccine Safety Datalink picked up a signal possibly linking Pfizer’s COVID-19 bivalent vaccine with an increased risk of stroke in people 65 and older. Here’s what you need to know:

  • What’s a safety signal and what does it mean? A “safety signal” occurs when vaccine monitoring systems pick up on an adverse medical event after vaccination at a rate higher than statistically expected. When a system “signals” a possible adverse event, researchers review other monitoring systems to check if other data are signaling the same risk. The Vaccine Safety Datalink, a collaborative database involving the CDC and healthcare organizations, includes electronic health records on 12 million people, and it is one of several independent vaccine safety monitoring systems. To date, no such possible risk of stroke among people age 65 and older has been detected by these other monitoring systems.
  • After extensive review of the latest vaccine safety data, federal health officials have said it’s very unlikely that there’s a true clinical risk of stroke associated with Pfizer’s COVID-19 booster. After the possible risk was detected in the Vaccine Safety Datalink, no other systems independently monitoring COVID-19 vaccine safety have observed any correlation between Pfizer’s updated bivalent vaccine and an increased risk of stroke. This safety signal has not been seen with Moderna’s bivalent COVID-19 vaccine.
  • The safety system that monitors COVID-19 vaccine safety is the most extensive in U.S. history. According to safety experts, safety signals occur frequently, which is a sign that the safety system monitoring COVID-19 vaccines is effective and sensitive enough to detect potential concerns and safety risks. The CDC will continue to monitor vaccine safety systems for any updates.
  • The COVID-19 vaccines and updated boosters are safe. Getting vaccinated and boosted remains our best defense against serious illness and hospitalization due to COVID-19. The CDC continues to recommend everyone age 6 months and older stay up to date with COVID-19 vaccination, including those who are eligible for an updated bivalent booster. As the virus continues to evolve and new variants emerge, the data consistently show that COVID-19 vaccines have saved tens of millions of lives, are safe, and continue to be effective at preventing severe illness. 

For more messaging guidance on COVID-19 vaccine safety, see our Talking Points on Vaccine Safety and Effectiveness

Updated on January 19, 2023