Recent headlines announced that European countries have halted use of the AstraZeneca Covid-19 vaccine because of fears that it causes clots. This was first done by Norway and Denmark, and more recently they were joined by Germany, France, Italy and Spain. The basis for this decision was the report of 37 clotting illnesses among some 17,000,000 people who had received the vaccine. The conditions reported were 15 people with clots in the legs (deep venous thrombosis, or DVT) and 22 with clots that had travelled to the lungs (pulmonary embolism, or PE).
Is this a rational decision? To help you think about it, you must realize there is a difference between correlation and causation. Let’s say you wash and polish your car and it rains, or a bird deposits its droppings on your windshield. Did the act of washing your car cause it to rain or motivate the bird to alter its flight path? Probably not. Or postulate that in years in which the American League team wins the World Series, the stock market goes up. Would you put all your savings in the market when the Red Sox win a World Series (if they ever do again)? I hope not.
Vaccines prevent or at least dramatically reduce the incidence of the infection against which they are targeted. They do not confer immortality or prevent unrelated illnesses (or make you taller, smarter or better looking).
How common are clotting diseases in general? Best estimates are that DVT and PE occur in about 1 person in a thousand per year. These conditions are very rare in children and get commoner as we age. This means that if you were to follow 17 million Europeans for a year, you would expect 17,000 cases of DVT and PE. Let’s be very cautious and estimate that of the vaccinated population, only an average of two weeks had passed since they were vaccinated. This would lead you to expect 654 cases to occur (17,000/26). I could argue that the numbers suggest that the vaccine protects against clotting rather than causing it. I will not, but I will argue that stopping vaccination based on a fear of clotting is irrational and dangerous.
Israel leads the world in vaccination, having administered 106 doses for every 100 people in the country. (They are using the 2-shot Pfizer vaccine.) They have also seen a halving of new cases in the past month. The larger European countries have been near the back of the pack in vaccine administration and are now seeing a spike in cases and imposing new lock downs. Which strategy would you prefer?
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