The world of public health was ready for something like this. They just didn’t know what that something would be. “When these vaccines were released to the world’s population, to billions of people, we knew there would be unanticipated side effects,” says Eric Topol, director of the Scripps Research Translational Institute. “These are so rare – the one from Johnson & Johnson is one in a million, the AstraZeneca is one in a hundred thousand, roughly. It’s rather good. It doesn’t bring any comfort to one, but it should bring a lot of comfort to most. “
Yet it won’t be easy to get this message across, especially when the numbers don’t tell the whole story yet. Regulators and vaccine researchers are still waiting for the most basic information about the mechanism behind clots, which subpopulations might be most vulnerable, and the true frequency of these dangerous clots.
The J&J and AstraZeneca vaccines both encode the “spike protein” on the surface of the virus that causes Covid-19. Vaccines serve this piece of biochemical code to a person’s immune system via a different virus, especially adenoviruses that have been changed into “vectors” that will fly the virological cargo. This association—Between blood clots and Covid vaccines that use an adenovirus vector – seems pretty clear.
While so far the J&J vaccine has only accounted for 5% of total vaccinations given in the United States, those numbers are on the rise. These clots are so strange that more may have gone unnoticed, or more may be yet to come – meaning epidemiologists might not know the actual number of people with the problem (the numerator, so to speak) or the actual number of people who have received the J&J vaccine in the past two weeks (the denominator). And these types of clots – usually without a low platelet count – are already a rare but known complication of Covid-19, too, so even getting a background rate isn’t easy.
With only six events to work on, no one can say whether the fact that all affected were women under the age of 50 is significant or not. Maybe it’s just a coincidence, statistical noise. Or maybe young women are more vulnerable. If that’s true, their risk isn’t one in a million – because they were a smaller subgroup of people vaccinated overall, it’s an order of magnitude larger. Or maybe there is another risk factor, as yet unidentified.
Now, maybe that risk is still worth it compared to getting Covid-19. This is what European regulators concluded about the risks and benefits of AstraZeneca vaccine, at least for certain groups of people. “I think the uniqueness of the symptom constellation and the fact that it appeared preferentially in a low risk group for severe disease, that motivates some of the considerations,” says Natalie Dean, a biostatistician at the University of Florida. . “It all comes down to a calculation of risks and benefits, and these occur at the subpopulation level, not just at the entire population level.”
So why haven’t these regulators asked their epidemiologists to do this calculation in the background, quietly, rather than rushing towards this national break? The real question the FDA and its advisers will need to answer is the same as with any medical intervention: How do the risks compare to the benefits? In the case of the J&J vaccine, the benefits are obvious, but the risks are not, at least not entirely. And the risks and benefits are different for people of different age groups, in places with different levels of Covid-19 prevalence. Earlier this month, researchers at the Winton Center for Risk and Evidence Communication attempted to run these numbers for the AstraZeneca vaccine. For young people at low risk of exposure to the virus, the vaccine could have caused 1.1 blood clots per 100,000 people, approximately, and prevented only 0.8 emergency room visits linked to Covid. But in people aged 60 to 69 at high risk, the vaccine could have caused just 0.2 clots per 100,000 and prevented nearly 128 emergency room admissions. Seems like a good deal.