On the Seasonality of the Virus


What are the odds that the coronavirus will recede on its own during the spring because of warmer temperatures or a higher ultraviolet (UV) index? This has been a question from the beginning.

There has been some research in support of the idea that the warmer season would force the virus to retreat. And there has been other research that concluded that the virus would retreat but not disappear, that it would survive in the southern hemisphere and that it could then stage a comeback in the northern hemisphere in the fall when cooler temperatures return.

Single Variables

Looking at the United States state by state, we find little correlation between the number of deaths per capita and the UV index. For example, Wisconsin with a UV index of 4 in March has so far suffered 25 deaths per million inhabitants, but Rhode Island also with a UV index of 4 saw as many as 60 deaths per million. At one extreme, New York, New Jersey, Michigan, Connecticut and Massachusetts, all with a March UV index of 4, had over 100 deaths per million. At the other extreme, South Dakota also with a March UV index of 4 had only 7 deaths per million. (All deaths figures are as of 12th April 2020 per Worldometer).

Clearly the UV index on its own is not a determinant of the spread, whether we measure that spread by the number of deaths or by the number of confirmed cases.

Another important single variable is the international exposure of each state. This approach is based on the premise that a higher international exposure would lead to a wider spread of the virus. In order to run this exercise, we quantified international exposure as the number of international air passengers that arrived in each state in 2018, via its gateway airports. Here again we found no clear correlation by using this variable on its own.

For example, there were 3.7 million arrivals in Michigan through Detroit Airport and Michigan has so far had as many as 149 deaths per million. But California, Florida and Texas had 42.8 million, 36 million and 20.4 million international arrivals, each through its own multiple gateway airports. And none of these states has had more than 22 deaths per million so far.

So here again, international exposure alone does not provide a good indication. It is true that the virus is occurring in winter and early spring and the figures for annual international travel differ from month to month. It is likely that these sun belt states see many more international arrivals in the summer than in winter. Nonetheless the difference between 3.7 million in Michigan and 20+ million in CA, FL and TX means that it is not necessary to make the adjustment to monthly figures. In any given month, there is no doubt that Los Angeles plus San Francisco plus San Diego receive far more international passengers than Detroit does.

Multiple variables

The next task then was to combine the two variables and to see if states with 1) a low UV index and 2) a high international exposure have seen a larger number of cases and of deaths than states with only one or neither of these conditions.

The result here was more encouraging.

Read the rest of this piece at Populyst.

Sami J. Karam is the founder and editor of populyst.net and the creator of the populyst index™. populyst is about innovation, demography and society. Before populyst, he was the founder and manager of the Seven Global funds and a fund manager at leading asset managers in Boston and New York.

Photo credit: Lorie Shaull via Flickr under CC 2.0 License.