Pfizer's bivalent COVID-19 shot 'very unlikely' to carry increased stroke risk for seniors: US officials

US federal health officials said it is "very unlikely" that Pfizer's bivalent COVID-19 shot carries an increased risk of stroke for people aged 65 and older, after it launched an investigation into a preliminary safety concern detected by one of its monitoring systems.

The US Centers for Disease Control and Prevention (CDC) said in a statement on Friday (Jan 13) that no other surveillance system has detected a similar safety concern so far.

"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 ischaemic stroke," said the CDC.

It noted that other countries have not observed an increased risk for ischaemic stroke with the updated bivalent vaccines, adding that the matter requires more investigation.

"Often these safety systems detect signals that could be due to factors other than the vaccine itself," the CDC added.

According to The Washington Post, the issue was first detected in late November.

The CDC’s vaccine safety datalink (VSD), a near real-time surveillance system, uncovered a possible safety issue in which people aged 65 and older were more likely to have an ischaemic stroke 21 days after receiving the Pfizer-BioNTech bivalent shot, compared with days 22 to 44.

An ischaemic stroke, also known as brain ischaemia, is caused by blockages in arteries that carry blood to the brain.

"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," said the CDC.

"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."

Pfizer and BioNTech said in a statement that they have been made aware of limited reports of ischaemic strokes in people 65 and older following vaccination with their updated shot.

"Neither Pfizer and BioNTech nor the CDC or FDA (Food and Drug Administration) have observed similar findings across numerous other monitoring systems in the US and globally and there is no evidence to conclude that ischaemic stroke is associated with the use of the companies' COVID-19 vaccines," the companies added.

This safety concern has not been identified with Moderna's bivalent shot.

Both the CDC and FDA continue to recommend that everyone aged six months and older stay up-to-date with their COVID-19 vaccination.