According to an analysis, young women who had received at least one dose from AstraZeneca’s Covid vaccine may have been more likely than others to develop a heart attack within the first 12 weeks following their vaccination. Analyse Tuesday’s release of death and immunization records in Britain.

Those findings carry a big caveat: Britain withdrew AstraZeneca’s vaccine use for young people under 30 in April 2021, citing the risk of rare but dangerous blood clots. Because people at highest risk of Covid due to their age, health, or employment, were vaccinated first, young women who were immunized in that period would have been mostly health care workers or medically vulnerable. Therefore, the findings of the study may not be applicable to all people.

“It could be that the people who are clinically extremely vulnerable are more susceptible to side effects from vaccination,” said Vahé Nafilyan, a senior statistician for Britain’s Office for National Statistics and one of the lead researchers on the study. The These results were published On Monday, in Nature Communications.

The United States never granted authorization for the AstraZeneca vaccination.

Six deaths from cardiac disease were found in 100,000 young British women who had received at least one dose. These women had a cardiac-related death rate of 3.5 times higher after 12 weeks than they did after the 12-week period. A stroke or heart attack can be caused by clots blocking blood flow.

Researchers found no evidence of a significantly increased risk of death for any other subgroups or the Pfizer BioNTech MRNA vaccine. The latter was widely used in Britain. It was not proven that the vaccines caused deaths.

Researchers and other experts highlighted that Covid vaccines have many benefits, outweighing the risks. Furthermore, there are very few adverse reactions after vaccination.

The analysis showed that Covid was associated to one additional death per 12,000 young people who were not vaccinated and one extra death for every 56,000 young people who were vaccinated.

“When you look at vaccination side effects, I think it’s very important to also look at the benefits,” Dr. Nafilyan said.

Researchers linked immunization records with deaths from any cause among people aged 12 to 29. They examined data that began on Dec. 8, 2019, when the vaccines in Britain were first introduced.

Two independent sources provided the mortality data: hospital deaths up to March 31, 2022 and deaths recorded by June 8, 2022.

However, the team discovered a small increase in deaths 12 weeks after vaccination among young men who received an mRNA vaccine. The finding was not statistically significant. Only young women — just over 177,000 — who received a dose of AstraZeneca had a higher risk of death.

Experts warn that the study does not establish a causal link between vaccines and deaths.

“It’s enough to catch my interest and say we should study this more,” Daniel Salmon, director of Johns Hopkins Bloomberg School of Public Health’s Institute for Vaccine Safety, stated this. But “I wouldn’t come close to drawing any causal conclusions.”

“Overall it’s fairly reassuring, but it does bring up some vaccines and some populations that deserve further study,” Dr. Salmon spoke.

Studies have also shown that vaccine side effects can be linked to subgroups. Data from several countries link the mRNA Covid vaccines to an elevated risk of myocarditis and pericarditis — inflammation of the heart or its outer lining — particularly in males between the ages of 12 and 29.

Within weeks of its release, AstraZeneca was linked with a rare blood-clotting disorder in young women in Britain, and other European countries. Although AstraZeneca’s vaccine did not clear U.S. regulatory approval, the Food and Drug Administration in December 2020 authorized a similar vaccine made by Johnson & Johnson.

F.D.A. called for a halt in the use of Johnson & Johnson vaccines on April 20, 2021. called for a pause in use of the Johnson & Johnson vaccine, following reports of a blood-clotting disorder in six American women. The agency withdrew the recommended pause 10 days later, and amended the vaccine’s label to warn about the risk.

One year later, F.D.A. again restricted the vaccine’s use, saying it should be offered only to people who could not or would not opt for one of the mRNA vaccines. Out of 18 million doses, 60 cases had been reported to the agency. Nine deaths were also reported.

“Fortunately, as more and more of these types of data are collected and become public, we can continue to rest assured that the rate of serious side effects — referred to as adverse events — is remarkably low for both types of vaccines,” Dr. Susan Cheng, a cardiologist, and epidemiologist at Cedars Sinai, Los Angeles, spoke of non-mRNA and mRNA vaccines.

“That said, while the rates of these adverse events remains extremely low,” She said: “they are important and they need to be counted and analyzed so that we can understand them better.”