The unfortunate post-vaccinated death of a 48 yr old woman last week has raised concerns about the safety of the AstraZeneca vaccine (Vaxzevria). Here, I offer my interpretation of the risk in terms of micromort, and then my analysis of risk and benefit of the vaccination program.
Quantifying risk in micromort
The micromort concept was advanced by Ronald A. Howard in the 1980s for decision making. Basically, micromort (micro-mortality) is a way of representing probability of mortality for small and acute risks. Howard defined that a micromort is one-in-a-million chance of dying. So, if an activity that is associated with a probability of 10 deaths per 1 million, then the activity has 10 micromorts. Similarly, a 1% risk of death is equivalent to 10,000 micromorts. Micromort is a useful way to understand and convey microrisk associated with daily activities.
We now apply the idea of micromort to quantify the risk of death from blood clot associated with AstraZeneca vaccine (Vaxzevria). According to the European Medicine Agency (EMA), until 22/3/2021, 25 million people had received Vaxzevria; among whom, 18 had died from blood clot . Thus, micromort for blood clot associated with Vaxzevria is 18 / 25 = 0.72.
In Australia, there have been 885,000 Vaxzevria shots, and among whom 3 cases of blood clot have been recorded . Unfortunately, there was 1 death from blood clot. Thus, the micromort associated with blood clot in Australia is 0.89.
The data so far seem to suggest that among vaccinated people, micromort associated with blood clot is about 1.
What about micromort associated with Covid-19? We know that the risk of mortality associated with Covid-19 is highly variable between populations. According to the World Health Organization, the case fatality rate ranges between 0.001% and 1.63% . If we take the average mortality risk of 1%, that means it is equivalent to 10,000 micromorts.
An analysis in the United States found that the mortality risk associated with Covid-19 is increased with advancing age. For example, among those aged 70 years and older, the micromort associated with Covid-19 is 54,000, which is greater than the average micromort in the general population (without Covid-19) of 38704 .
In summary, these data show that the mortality risk associated with blood clot after vaccination is only 1 micromort (equal to the background risk in the general population), and it is only 1/10000 of the micromort associated with Covid-19. In other words, vaccination against Covid-19 is safe.
Benefit and harm
Another way to assess the benefit and harm of vaccination is by considering the risk of hospitalization and and mortality.
Imagine that we have a community of 100,000 people who are infected by Sars-Cov-2, and that they are not vaccinated. According to data from WHO , 20,000 individuals will be hospitalized. The risk of mortality among hospitalized patients with Covid-19 is highly variable between settings [6–7], but on average, it is around 10%. In other words, we expect that 2000 patients would die from Covid-19.
It should be remembered that Covid-19 is actually associated with an increased risk of blood clot. A review in Thrombosis Research suggests that the risk of blood clot among Covid-19 patients ranges between 5% to 42% . Let us take the lower value of 5% as an incidence, then we would expect to see 5000 patients with blood clot. It is curious that virtually all media outlets have until now ignored this fact.
Now, if the 100,000 people are vaccinated, then what would happen? We know that vaccines can reduce the risk of hospitalization by as high as 94% . So, we would expect to see 1200 vaccinated patients hospitalized, and assuming that the average mortality rate is still 10%, we would expect to observe 120 deaths. According to EMA, we would expect to see 1 patient with blood clot .
The above consideration suggests that in every 100,000 individuals, vaccines could save 1880 lives, and that the risk of blood clot among vaccinated people is not higher, if not lower, than non-vaccinated patients with Covid-19. This analysis clearly shows that the benefit of vaccination greatly outweighs its risk.