The good news is that the systems for monitoring vaccine safety are working well. The bad news is that it looks increasingly likely that a vaccine technology that has come of age during the pandemic carries a very small risk of causing blood clots for some people.
Nearly seven million people in the United States have received the Johnson & Johnson vaccine. Of those, six have developed a rare blood clot disorder.
The US federal health agencies have responded by recommending that its use be paused while those cases are investigated.
What is striking is that the same condition has been reported in people in the European Union and the UK who were given the Oxford-AstraZeneca vaccine.
On both sides of the Atlantic, people have developed cerebral venous sinus thrombosis (CVST) — an extremely rare form of clot that prevents blood draining from the brain.
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This was combined with a condition called thrombocytopenia, which involves abnormally low levels of platelets. The combination is very rare and counterintuitive — platelets are a kind of blood cell required for clots to form.
With the Johnson & Johnson vaccine in the US, all the six recipients who developed CVST were women between the ages of 18 and 48. The condition developed within a couple of weeks of vaccination. Similar patterns have been connected to the Oxford jab.
Professor Adam Finn, of the University of Bristol, who sits on the Joint Committee on Vaccination and Immunisation, which formulates immunisation policy, said of the Johnson & Johnson reports: “The numbers of cases are, once again, very small but the combination of severe thrombosis with thrombocytopenia is characteristic of the cases that have been reported previously [with AstraZeneca] . . . Given the importance of these vaccines for the timely control of the pandemic, investigation of this phenomenon is now an extremely urgent international priority.”
The similarities are suspicious because the Oxford-AstraZeneca and Johnson & Johnson vaccines use the same basic technology.
They each inject a virus — an adenovirus — that has been modified so that it carries a piece of genetic code taken from the coronavirus. This causes cells to churn out copies of the spike protein that the coronavirus uses to infect its hosts.
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There’s no obvious reason why an adenovirus vaccine should cause CVST. That there is a causal link between the clots and the AstraZeneca vaccine still has not been proven beyond doubt, though on April 7 the European Medicines Agency, which had been careful not to draw quick conclusions, said that there was “a probable causal association” between the syndrome and the AstraZeneca jab.
We are now at the stage where scientists are sketching out plausible mechanisms.
A study published last week in the New England Journal of Medicine described how patients who had suffered the clots in Germany and Austria had unusual antibodies (not antibodies against the coronavirus) in their blood. In other circumstances these antibodies are known to trigger clotting in a process that can use up the body’s platelets and can block blood vessels. The idea here is that the vaccine could possibly be causing an aberrant immune response.
Another team of researchers suggested in the same journal that some of the adenovirus particles in a dose of vaccine might break apart and release some of the genetic material they carry. This could conceivably set off a chain reaction that results in clotting.
For now, though, this is speculation.
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Even if a causal link is confirmed, the vaccines will almost certainly remain a valuable tool, because the risks older people face from the coronavirus will be much greater than the danger posed by these very rare clots.
In the UK, it is recommended that people under 30 should be offered alternatives to the AstraZeneca vaccine. (The Johnson & Johnson vaccine is not yet approved in the UK.) But even for this age group, the risks are finely balanced and small.
The chances of serious harm because of the vaccine for people in their twenties is about 1.1 in 100,000, according to Professor David Spiegelhalter, an expert in risk at the University of Cambridge.
Their risk of being admitted to intensive care because of Covid-19 in the next 16 weeks is estimated to range from 0.8 in 100,000 to about 6.9 in 100,000, depending on the amount of virus to which a person is exposed.
For some context, 1 in 100,000 is about the risk a woman faces, every week, of developing a blood clot if she is taking the contraceptive pill. It’s also the level of risk you face of being killed or seriously injured during a 200-mile drive.
