Scientists in America are investigating a potential link between Pfizer’s new Covid booster and the heightened risk of stroke in people aged 65 and over.
This comes after a possible link was found between the updated dose and a higher risk of "ischemic" in the three to four weeks following vaccination.
Officials from the Centres for Disease Control and Prevention and the US Food and Drug Administration say that despite the link being found, other systems that are used to track vaccine outcomes have not established a similar connection.
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This has led them to conclude that this may be an anomaly rather than a cause for alarm among those who have received - or are due to receive - the dose.
However, in a joint statement from the CDC and FDA experts said: "It is very unlikely that the signal would represent a true clinical risk, we believe it is important to share this information with the public."
The agencies have issued similar statements in the past when a safety monitoring system detects a signal.
An ischemic stroke or brain ischemia is caused by a clog in arteries that bring blood to the brain.
The possible link was detected by the CDC’s Vaccine Safety Datalink (VSD), a real-time surveillance system.
Officials said although it met the criteria required for investigation seniors should still have their boosters, which are known to give good protection against Covid.
Data from elsewhere, including the Centres for Medicare and Medicaid Services, the Department of Veterans Affairs, have not shown up any such risk, including research from other countries. The FDA and CDC said that there may be other factors contributing to the signal that might "merit further investigation".
The Centres for Disease Control and Prevention maid it clear that the link applied only to the Pfizer bivalent booster and not Moderna's. It stressed that no other age group appeared to be affected.
Pfizer/BioNtech and Moderna's bivalent shots, which target the original coronavirus and the Omicron sub-variants, are authorised for use in children of six months old and above.
The updated shots were brought in specifically to directly target the Omicron variant. Both the CDC and FDA continue to recommend everyone aged six months and older stay up-to-date with their vaccinations.
Despite US government officials pushing these vaccines, they say they have been disappointed in the low rate of take-up among the population. One doctor, Ashish Jha, the governmental Covid response coordinator, even warned that citizens in the States who had not received the bivalent jab would be vulnerable to the budding XBB.1.5 variant.
The CDC was keen to stress that no change in the practice of vaccinations was necessary. It said data had:
Shown updated Covid-19 vaccines reduce the risk of hospitalisation from the disease nearly 3-fold compared with those previously vaccinated who had not received the updated vaccine.
Shown the updated vaccine also reduces risk of death from Covid-19 by nearly 19-fold compared to the unvaccinated.
Demonstrated more than 80 per cent protection against severe disease and death from the bivalent vaccine compared with those not in receipt of the bivalent vaccine.
The latest available data showed 50 million Americans had the updated booster - about 15 percent of the population. Around 700,000 Americans have an ischemic stroke annually, accounting for nearly 90 per cent of all strokes. A stroke is fatal in a fifth of cases.
Last year, the CDC said young males who had the mRNA vaccines - either the Pfizer or Moderna shot could be at an increased risk of heart inflammation but continued to recommend the vaccine despite the possible risks.
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