Young women who received at least one dose of the Covid vaccine made by AstraZeneca may have been more likely to die of a heart problem in the 12 weeks after their vaccination, according to an analysis of immunization and death records in Britain released on Monday.
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. By that time, the young women who were immunized would have been mainly health care workers or those who were medically vulnerable, because people at high risk of Covid from their age, health or employment were vaccinated first. So the results of the study may not apply to the general population.
“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 results were published on Monday in the journal Nature Communications.
The analysis found six cardiac-related deaths per 100,000 young women who received at least one dose of the vaccine in Britain. In these women, the cardiac-related death was 3.5 times more likely in the 12 weeks following vaccination than after the 12-week period. Clots that block blood flow can cause a heart attack or stroke.
The researchers did not find a significantly elevated risk of death in any other subgroup or with the Pfizer-BioNTech mRNA vaccine, which was also widely used in Britain. And the study did not prove that the vaccines caused the deaths.
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The benefits of Covid vaccines still vastly outweigh the risks, and the incidence of harmful events after vaccination remains very low, the researchers and other experts emphasized.
In the analysis, Covid was associated with one additional death for every 12,000 unvaccinated young people and one additional death for every 56,000 vaccinated young people.
“When you look at vaccination side effects, I think it’s very important to also look at the benefits,” Dr. Nafilyan said.
The researchers linked immunization records to deaths from any cause recorded in people ages 12 to 29. They looked at data starting on Dec. 8, 2020, when the vaccines were rolled out in Britain.
The mortality data came from two independent sources: deaths registered by June 8, 2022, and hospital deaths by March 31, 2022.
The team found a very small increase in deaths in the 12 weeks after vaccination in young men who received a mRNA vaccine, but said that 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.
The analysis does not conclusively link the vaccines to the deaths, experts cautioned.
“It’s enough to catch my interest and say we should study this more,” said Daniel Salmon, director of the Institute for Vaccine Safety at Johns Hopkins Bloomberg School of Public Health. 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 said.
Other studies have linked vaccine side effects to specific 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 introduction, the AstraZeneca vaccine was linked to a rare blood-clotting disorder, particularly in young women in Britain and other countries in Europe. The vaccine was never approved for use in the United States, but in December 2020, the Food and Drug Administration authorized a similar vaccine made by Johnson & Johnson.
In April 2021, the F.D.A. 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.
A year later, the 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. By then, the agency had received reports of 60 cases of the clotting disorder and nine deaths, out of 18 million doses administered.
“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 the Smidt Heart Institute at Cedars Sinai in Los Angeles, said of mRNA and non-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.”