On Friday, the US Centers for Disease Control and Prevention (CDC) and Food and Drug Administration (FDA) warned that the updated Covid-19 shot made by US drugmaker Pfizer Inc and its German partner BioNTech could be linked to a type of brain stroke in older adults. CNN said that the CDC still advises people to stay up-to-date with their Covid-19 vaccines.

US health officials looked at preliminary data and came to these conclusions, Reuters said.

The safety monitoring system said that a CDC vaccine database had found a possible safety issue: people 65 and older were more likely to have an ischemic stroke 21 days after getting the Pfizer/BioNTech bivalent shot than between days 22 and 44.

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Ischemic strokes are caused by obstructions in the blood vessels that supply the brain. Also known as cerebral ischemia.

The FDA and CDC stated that other big trials, the CDC’s Vaccine Adverse Event Reporting System, other nations’ databases, and Pfizer-databases BioNTech’s did not signal this safety concern because further investigation is necessary.

“At this time, the totality of the data suggests that it is very unlikely that the signal in VSD (Vaccine Safety Datalink) represents a real clinical risk,” said a health official. “However, we think it is important for the public to know this information.”

In a statement, Pfizer and BioNTech said that they have heard of a small number of cases of ischemic strokes in people 65 and older who got their updated shot.

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The companies also said, “Neither Pfizer and BioNTech nor the CDC or FDA have seen similar findings across many other monitoring systems in the US and around the world, and there is no evidence to conclude that ischemic stroke is linked to the use of the companies’ COVID-19 vaccines.”

This safety concern has not been found with Moderna’s bivalent shot, and the CDC and FDA continue to recommend that everyone 6 months and older get a COVID-19 shot.