On April 7 both Britain’s health officials and the European Medicines Agency (EMA), announced there is strong evidence that AstraZeneca’s Covid-19 vaccine may be linked with very rare blood clots, often in the brain or the abdomen, reports The Economist.
The EMA experts reached their conclusion based on a review of 86 reported cases, 18 of which were fatal. Britain’s experts reached the same conclusion from data on 79 cases, 19 of which were fatal.
Both concluded that the vaccine’s benefits outweigh the potential risk of the clots.
However, Britain’s officials said that for people under 30 the risks and benefits from the vaccine were “finely balanced”, so a different jab may be preferable.
The investigation of the suspected clots from the AstraZeneca jab is a prime example of the challenge of determining a vaccine’s side effects from the unrelated medical emergencies that happen to millions of people every day.
The first signs emerged in late February, when doctors in several European countries noticed clusters of blood clots in people recently given the AstraZeneca jab, some of whom died.
Most were women under 60 because many EU countries were at first not convinced that the jab worked in the elderly and used it largely for essential workers, such as nurses, teachers, and social-care workers – professions in which most employees are women under 60.
The EMA’s data as of March 22 suggested that the rate of brain clots in people under the age of 60 who had had AstraZeneca’s vaccine was one in 100,000 – higher than would be expected normally.
As doctors began to look more closely, they discovered that many patients with suspected clots from the vaccine had unusually low levels of platelets.
Platelets are fragments of cells that float in the blood. Their job is to form clots where the skin is cut. Low platelet levels therefore usually result in uncontrolled bleeding, not clots.
Britain’s medical regulators searched their data on vaccinated people for the unusual combination of clots and low platelet counts. They found four cases per million people vaccinated, a rate several times lower than in the EU.
One explanation is that Britain, unlike the rest of Europe, had used the jab primarily in older people.
The rate at which the clots occurred in Britain declined steadily with age. Importantly, Britain’s experts found that the clots occurred as much in men as they did in women.
This combination of blood clots and low platelet counts resembles a condition seen in some people who are given heparin, a drug used widely to treat blood clots, and doctors know how to diagnose and treat it.
For unknown reasons, some people develop an immune reaction to heparin, which results in blood clotting so serious that it depletes their platelets. The same reaction appears to be provoked by the vaccine.
Medical societies in several countries have already issued guidelines to doctors on how to spot and treat this rare reaction to the AstraZeneca vaccine.
With vigilance and appropriate care, the extremely rare deaths that may result from it will become even rarer.