Vaccine-associated blood clot death: a micromort thinking

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 [1]. 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 [2]. 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% [3]. 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…