European health regulators have advised that there may be a link between the Covid-19 vaccine developed by AstraZeneca PLC and the University of Oxford and very rare blood clots, and some governments have recommended restricting its distribution among younger people as a result. The potential side effect is rare -- so rare it is only likely to be picked up in a real-world rollout of the vaccine rather than in earlier clinical trials involving many fewer people. The concern is limited to the AstraZeneca vaccine, which isn't yet authorized for use in the U.S. Here is what you need to know.
What decisions have European regulators made on the AstraZeneca vaccine?
The European Medicines Agency and the U.K.'s Medicines and Healthcare products Regulatory Agency on Wednesday issued new guidance on the use of the AstraZeneca vaccine. Both regulators said there might be a link between the vaccine and very rare blood clots, some of which have been fatal. The EMA advised European Union governments to keep using the vaccine in all age groups -- though some governments have said they would limit the use of the shot to older people. In the U.K., a panel of scientists that advises on vaccines said adults under 30 years old should be offered an alternative vaccine if available. Both urged doctors to be alert for symptoms of these clots in anyone newly vaccinated, especially if associated with another unusual condition involving low levels of platelets, a type of blood cell.
Why did they reach different conclusions?
Behind both decisions is an assessment of the trade-off between the benefits of vaccination against Covid-19 and the risk of suffering an extremely rare side effect soon after getting a shot. The risk of severe illness and death from Covid-19 rises steeply with age. That means for older people, the risks are more heavily weighted in favor of vaccination. For younger people, who are much less likely to fall gravely ill with Covid-19, the risks are more finely balanced, especially because the latest evidence suggests these rare clots are slightly more likely to occur in younger people than older people.
A key thing to understand, though, is that the balance of these risks for young people in some parts of the EU and young people in the U.K. aren't necessarily the same, and can change over time. That helps explain why regulators made different recommendations.
So what kind of risks are we talking about?
Regulators said that as of April 4 there have been 169 cases of a type of blood clot in the brain called a cerebral venous sinus thrombosis, or CVST, in Europe since vaccinations began, and 53 cases of another rare clot. Through April 4, 34 million people had been vaccinated in the region, giving a crude frequency of six or seven cases per million vaccinations. Reviews of the medical literature suggest CVST would be expected to occur in around two to four people per million a year.
Reported cases vary considerably from country to country. The U.K. has recorded 44 cases of CVST out of more than 20 million vaccine doses. Germany has logged 31 cases out of 2.7 million vaccinations and Norway five out of just 120,000 shots. Regulators report higher incidences of clots among women than among men.
The infection fatality rate for Covid-19, meanwhile, is estimated to be around 1%, meaning that for every million people infected, 10,000 deaths would be expected. But for younger people, the risk is considerably lower. The IFR for someone aged 20 to 29 is estimated to be 0.03% to 0.04%, meaning that for every million people in that age group infected, 300 to 400 deaths would be expected.
That isn't the end of the story. In considering whether the risk of getting seriously ill and even dying from Covid-19 outweighs the risk of getting a blood clot as a side effect of vaccination, it is also worth considering the likelihood of catching the disease in the first place.
That is where the balance of risks behind these decisions is most pertinent. A University of Cambridge analysis shows that if Covid-19 infection is accelerating, with new cases being detected at a rate of 200 per 100,000 people a day, for instance, then vaccinating 100,000 20-to-29-year-olds would prevent seven intensive-care admissions over a four-month period. If daily incidence is lower, at 20 cases for every 100,000 people, roughly where the U.K. was in March, then vaccination would prevent 0.8 intensive-care admissions. The same analysis put the risk of a severe blood clot as a consequence of vaccination in people of the same age group at 1.1 per 100,000 doses administered.
How do those risks compare to other common risks?
The chance of dying in a road accident in 50 years of driving is one in 85, according to the British Medical Journal. Blood clots are a side effect of contraceptive pills, and over the course of a year can occur in between two and 12 of every 10,000 women taking them, depending on the medication in question. Studies cited by the U.S. Centers for Disease Control and Prevention suggest that there is a 0.3% chance of developing a symptomatic blood clot after a flight of longer than eight hours -- 30 per 10,000.
What else should be considered when weighing the risks of Covid-19 and vaccination?
While for young people most Covid-19 cases are mild, scientists are concerned about so-called long Covid, a puzzling and sometimes debilitating set of symptoms that can persist long after infection. Some studies have found around 10% of people infected are still suffering symptoms 12 weeks after infection. Another consideration is herd immunity. Preventing a virus spreading by ensuring sufficient numbers are immune not only protects against future outbreaks but shields those especially vulnerable to the virus.
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