A study confirms a small risk of blood clots after vaccines, but not as high as the risk Covid brings.

The largest published study to date of certain post-vaccine side effects found that people were at slightly higher than normal risk of blood clots after receiving an AstraZeneca or Pfizer-BioNTech shot. But the same clotting conditions were substantially more likely to occur — and over longer periods — among people infected with the coronavirus, the study found.

Coupled with another study this week from Israel, the data, published on Thursday night in The British Medical Journal, added to the growing evidence that though the coronavirus vaccines are associated with certain rare side effects, those risks are dwarfed by the risks from Covid-19.

The study was based on the electronic health records of more than 29 million people in England. It went beyond previous analyses in finding a link not only between very rare clotting conditions and the AstraZeneca vaccine, but also between those conditions and the Pfizer vaccine. Earlier studies had detected increased clotting risks after the AstraZeneca vaccine, but not after the Pfizer shot.

In interviews, the new paper’s co-authors said that the numbers of those cases they detected — involving clots blocking a vein that drains blood from the brain — were small enough that further studies were needed. Even the increased risk of those clots was far outweighed by people’s chances of developing them after contracting the virus itself, the study said.

“Although there are some risks, clearly the risks of these events are very rare,” said Aziz Sheikh, a co-author of the study and a professor of primary care research at the University of Edinburgh in Scotland. “And the biggest point is that the risks associated with Covid-19 are orders of magnitude higher, really.”

The study examined the electronic health records of people given a first dose of a coronavirus vaccine during the first five months of England’s inoculation campaign. Of those 29 million people, nearly 1.8 million people also tested positive for the coronavirus before or after being vaccinated. The study compared the risk of blood clots shortly after vaccination to the risk during other periods, as well as to the weeks after someone developed Covid-19.

After a first shot of the AstraZeneca vaccine, people were at slightly increased risk of certain blood clots, as well as a condition characterized by a low level of platelets that can leave them prone to abnormal bleeding. A first shot of the Pfizer vaccine appeared to put people at slightly elevated risk of strokes caused by blockages within a blood vessel.

And both vaccines were linked, though at low numbers, to the very rare clots that prevent blood from draining out of the brain.

Even so, those risks were far smaller than those linked to developing Covid-19. For example, the authors said, for every 10 million people given a first dose of the AstraZeneca vaccine, about 66 more people than normal would develop clots starting in a vein. But among the same number infected with the virus itself, 12,614 more people than normal would develop those clots.

In the United States, 300,000 to 600,000 people a year develop blood clots in their lungs or in leg veins or other parts of the body, according to the C.D.C. With nearly a million people a day now getting vaccinated, some of those clots will occur in those receiving the shots just by coincidence, unrelated to the vaccine.

Understand Vaccine and Mask Mandates in the U.S.

    • Vaccine rules. On Aug. 23, the Food and Drug Administration granted full approval to Pfizer-BioNTech’s coronavirus vaccine for people 16 and up, paving the way for an increase in mandates in both the public and private sectors. Private companies have been increasingly mandating vaccines for employees. Such mandates are legally allowed and have been upheld in court challenges.
    • Mask rules. The Centers for Disease Control and Prevention in July recommended that all Americans, regardless of vaccination status, wear masks in indoor public places within areas experiencing outbreaks, a reversal of the guidance it offered in May. See where the C.D.C. guidance would apply, and where states have instituted their own mask policies. The battle over masks has become contentious in some states, with some local leaders defying state bans.
    • College and universities. More than 400 colleges and universities are requiring students to be vaccinated against Covid-19. Almost all are in states that voted for President Biden.
    • Schools. Both California and New York City have introduced vaccine mandates for education staff. A survey released in August found that many American parents of school-age children are opposed to mandated vaccines for students, but were more supportive of mask mandates for students, teachers and staff members who do not have their shots.  
    • Hospitals and medical centers. Many hospitals and major health systems are requiring employees to get a Covid-19 vaccine, citing rising caseloads fueled by the Delta variant and stubbornly low vaccination rates in their communities, even within their work force.
    • New York City. Proof of vaccination is required of workers and customers for indoor dining, gyms, performances and other indoor situations, although enforcement does not begin until Sept. 13. Teachers and other education workers in the city’s vast school system will need to have at least one vaccine dose by Sept. 27, without the option of weekly testing. City hospital workers must also get a vaccine or be subjected to weekly testing. Similar rules are in place for New York State employees.
    • At the federal level. The Pentagon announced that it would seek to make coronavirus vaccinations mandatory for the country’s 1.3 million active-duty troops “no later” than the middle of September. President Biden announced that all civilian federal employees would have to be vaccinated against the coronavirus or submit to regular testing, social distancing, mask requirements and restrictions on most travel.

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