It’s extremely important that epidemiologists (and for those trying to understand epidemiology) understand the difference between correlation and causation.
When I teach this topic to my graduate students, I start with a classic example…When ice cream sale go up, drownings increase. Does ice cream cause drowning? No. It’s tempting to assume that one pattern causes another. However, correlation might be coincidental or it might be a result of both patterns being caused by a third factor. The third variable here is a hot summer day, which boosts ice cream sales AND swimming, and thus drownings. In other words, correlation (ice cream sales and drownings) does not imply causation (ice cream sales cause drownings).
The same can be applied to a wide variety of public health issues, including the latest hot topic…deaths and vaccines.
As of this morning, Norway reported that 33 people (aged 75+ years) died a few days following their COVID19 immunization. Germany is also investigating 10 deaths. After reading headlines, it’s easy to make the assumption that the vaccine caused the fatal outcome. However, there is a very important third factor… frail, older adults die, and die more often. In Norway, an average of 45 people die each day in nursing homes because of underlying issues. The 33 deaths after vaccination do not represent an excess of deaths. When there’s a mass vaccination campaign, the overlap of vaccination and death is going to happen. It just is.
However, I don’t want to undermine the importance of investigating causation. Norway is investigating 13 of these deaths. And while the Norwegian Medicines Agency and the National Institute of Public Health are still not certain of causality, they did state the possibility that the vaccines’ side effects (like fever) may have exacerbated underlying medical issues that are not dangerous in fitter, younger populations. Dr. Madsen (Director of the Norwegian Medicines Agency) stated: “We are not alarmed or worried about this, because these are very rare occurrences and they occurred in very frail patients with very serious disease”. As of today, 48,000 Norwegian nursing home residents have been vaccinated and did not die.
These deaths will have no impact on Norwegian Medicines Agency’s vaccination strategy. They are still going to first offer vaccinations to those 75+ in nursing homes. But they do urge 80+ patients and their families to discuss the risks and benefits of vaccination with their doctor prior to vaccination.
Update: Many of you are confusing temporal correlation with direct causation. COVID19 and death is a direct causation evidenced by excess death analyses (search “excess deaths” in my blog). In other words, if COVID19 didn’t infect the patient, they would still be living.