Sunshine, Vitamin D, and Covid-19 mortality rates

We’ve been wondering about the effects of Vitamin D, which one mostly gets more of by exposure to sunshine, on Covid-19 mortality. Yesterday saw new news from Trinity College in Dublin on the point:

Whereas there are currently no results from randomized controlled trials to conclusively prove that vitamin D beneficially affects COVID-19 outcomes, there is strong circumstantial evidence of associations between vitamin D and the severity of COVID-19 responses, including death.

Here’s a recent study from Northwestern that points in the same direction:

Backman and his team were inspired to examine vitamin D levels after noticing unexplained differences in COVID-19 mortality rates from country to country. Some people hypothesized that differences in healthcare quality, age distributions in population, testing rates or different strains of the coronavirus might be responsible. But Backman remained skeptical.

“None of these factors appears to play a significant role,” Backman said. “The healthcare system in northern Italy is one of the best in the world. Differences in mortality exist even if one looks across the same age group. And, while the restrictions on testing do indeed vary, the disparities in mortality still exist even when we looked at countries or populations for which similar testing rates apply.

“Instead, we saw a significant correlation with vitamin D deficiency,” he said.

Google yields more such early data.

Now, in the United States there is a huge range in mortality rates from one state to the next. Using the Worldometers data from yesterday, in the 20 states with the most confirmed cases the mortality rate ranges from 1.67% in Tennessee to an astonishing 8.97% in Michigan. Even a few weeks ago, these disparities were easily dismissed as an artifact of testing rates — some states had high mortality rates because they had insufficiently tested and therefore the denominator was artificially small, the argument went. As testing has risen in to the hundreds of thousands even in the smaller of these Top 20 states, that explanation is weakening.

So why the huge differences in mortality rates?

We compared the mortality rates in the Top 20 states and their average hours of sunshine per year. The results were interesting, albeit subject to the qualifications below the scatterplot:

We did adjust the data in one respect: New York’s “hours of sunshine” calculation was based on readings in Syracuse, which is a very gloomy place. Since most of its deaths are in the tri-state region, we used New Jersey’s number for New York, which is probably a decent quick and dirty correction.

Obviously, there are many limits to this approach, the first being that hours of sunshine in a state is only a very loose proxy for Vitamin D levels. The actual Vitamin D readings for hospitalized patients who do and do not die would tell us a lot, and even better would be the levels from a sampling of mildly symptomatic or even the famous asymptomatic cohort.

There is also the problem that we used the annual hours of daylight, rather than during the two months during which the Covid-19 deaths actually occurred. We reason that the correlation could actually be stronger if we had the urge to unpack the sunshine data for those two months, insofar as many of the high mortality states in the north probably accrue most of their sunny hours in the summer and fall rather than in March and April, but who knows?

No doubt actual scientists could think of many other wrinkles. Regardless, understanding the massive differences in mid-pandemic mortality rates between American states will be fodder for countless graduate students in the years to come.