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 ischemic stroke," said the CDC.

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

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

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

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

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

An ischemic stroke, also known as brain ischemia, 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.