US Health Agencies Investigate Preliminary COVID-19 Vaccine Safety Signal In Older People

The U.S. Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) are investigating a "preliminary signal" of the likelihood of a stroke among elderly people who received a Covid-19 vaccine booster manufactured by Pfizer Inc PFE and BioNTech SE BNTX.

The CDC identified the safety signal through its vaccine-safety datalink system, or VSD, which it calls a "near real-time" monitoring system. The system triggered an investigation if people 65 years and above who received Pfizer's booster and afterward were more likely to have a stroke within 21 days after getting the shot than in days 22-42 after vaccination.

The CDC called the findings highly preliminary and said that "no other safety systems have shown a similar signal, and multiple subsequent analyses have not validated this signal." That included analysis of other large vaccine datasets from Medicare, the U.S. Department of Veterans Affairs, the companies' monitoring programs, and a vaccine side-effect reporting collection tool known as VAERS.

Related: US Health Official Says Updated COVID-19 Boosters Should Offer Protection Amid Tracking Emerging Variants.

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

"Often these safety systems detect signals that could be due to factors other than the vaccine itself," the CDC said. No such signal was found with Moderna Inc's MRNA updated booster.

The CDC said it wasn't making any changes to its vaccine recommendations.

"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," the CDC said.

Price Action: PFE shares closed at $47.70, and BNTX shares closed at $145.05 on Friday.