Scientists in the United States and Britain believe they may have identified a key step in how the Covid vaccine developed by AstraZeneca and Oxford University may cause an extremely rare but serious blood-clotting disorder.
The AstraZeneca vaccine was expected to be the workhorse of the world’s vaccination efforts, thanks in part to pledges to manufacture it on a nonprofit basis during the pandemic and to make it available cheaply in poorer countries. The company and its partners have distributed more than two billion doses worldwide.
But reports of the rare side effect — an autoimmune response that has led to dangerous and sometimes fatal clotting in a small number of cases — were among a series of setbacks that tarnished the shot’s reputation and caused many European countries to limit its use. It has yet to receive authorization in the United States, although the company has said that it hopes to achieve approval soon.
“We hope our findings can be used to better understand the rare side effects of these new vaccines — and potentially to design new and improved vaccines to turn the tide on this global pandemic,” Prof. Alan Parker of Cardiff University’s School of Medicine, one of the researchers involved in the study, said in a news release.
The findings, which were published on Thursday, in the journal Science Advances. suggest that the problem is linked to the vaccine’s use of another, harmless virus — an adenovirus — to deliver a coronavirus gene into human cells, in order to train the immune system to recognize and battle the virus.
The shot is injected into muscle tissue, but the report suggests that if the adenovirus leaks into the bloodstream it can bind to a protein in the blood called platelet factor 4, or PF4, which is involved in the natural clotting process.That process, in turn, could lead to the release of antibodies against the protein, causing platelets to cluster and blood clots to form in very rare cases, the authors of the article said.
“With a better understanding of the mechanism by which PF4 and adenoviruses interact there is an opportunity to engineer the capsid, or outer shell of the vaccine, to prevent this interaction occurring,” Dr. Parker said.
The Johnson & Johnson vaccine also uses an adenovirus, and has also been linked to the rare blood clotting disorder. The Pfizer or Moderna shots are based on a different technology, one that does not involve a helper virus.
Concerns about the rare side effect first emerged in March, causing many European nations began to rethink the vaccine’s use in some age groups.
The reaction was first discovered by scientists in Germany and Norway, but how or why it took place had remained a mystery.
Public health experts have expressed concern that the rare vaccine-related reactions have fueled hesitancy, particularly in Europe, and continue to emphasize that the AstraZeneca vaccine’s benefits far outweigh the risks.
In Britain, where the vaccine first went into use in January, the National Health Service reports that “the risk of this extremely rare side effect is around 1 in every 100,000 first doses” whereas the benefit of one dose leads to an 80 percent reduction in deaths. As of Aug. 11, the side effect had been linked to 73 deaths in Britain, according to the country’s Medicines and Healthcare Products Regulatory Agency. Britain has given more than 25 million first doses of the vaccine.