COVID-19 Mini-Update: Is there a Correlation between Getting the Flu Vaccine and COVID-19 cases?

Above is the flu vaccination rates for 2018 across states. The darker color is higher percentages of the population getting the vaccine. The highest states are RI and Mass with about 60% of the population vaccinated. The lowest are Nevada, Wyoming, and Florida, with about 40%. You’ll notice on the map of COVID-19 cases and deaths per 1000 persons that in general, though, states with lower flu vaccine are also states that haven’t been hit as hard by COVID-19 (and vice versa). Aha! Some may say, the anti-vaccination folks are right! Hold on, slow down…

Is this Causality or Just Correlation?

Many events share dependence with a third, less directly-related event. This can result in something called a spurious correlation. Many times, these spurious correlations can be uncovered quite easily once the “excitement” at the correlation has passed. A textbook example is the correlation between ice cream sales and drownings. This is true. Drownings are always highest in months that have the greatest number of ice cream sales. Of course, thinking about this, you will realize that these are really two independent events that both share a correlation with this thing we call “summer”! There are whole websites dedicated to the comedy value of uncovering spurious correlations. Here’s one of those sites.

Back to the Flu Vaccine – is this a spurious correlation?

Lets mull this over a bit. Our null hypothesis would be that there is no causal relationship between getting the flu vaccine and getting COVID-19. We would have to disprove this null hypothesis in a statistically-relevant way to prove that there IS a causal relationship (i.e., the flu vaccine causes someone to be more susceptible to COVID-19 or some such).

This would likely be very hard to do, because one would have to overcome the challenge of multiple third events correlated with both Influenza Vaccination rates as well as COVID-19 infections. One of these events may be that some regions have a perception of historically higher incidences of infectious diseases (like influenza and COVID-19) and have learned to get the flu shot. This seems very likely to me. Conversely, regions that traditionally don’t perceive great challenges with the infectious diseases won’t get flu vaccines as readily as those regions that do. These same regions that don’t traditionally get the flu as badly and therefore don’t get the flu vaccine, are also not getting hit hard by COVID-19 for the same reasons that they don’t get the flu as badly! Population density, cultural expressions, collectivist vs. individualist tendencies, and other factors may be at play, but from a scientific standpoint, if one wished to set up experiments/studies to determine if our null hypothesis was correct or not, they would have to control for these pretty large variables.

Funny Correlations

Here’s a funny example from https://www.tylervigen.com/spurious-correlations