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Respiratory epidemics and the seasons

By Skip Rigney
To say that people have been talking about coronavirus is the understatement of the year. Meanwhile, influenza season continues but may have peaked in February according to the Centers for Disease Control weekly influenza report (www.cdc.gov/flu/weekly/). A decrease in the number of cases of the flu in the spring is very typical and highlights an interesting link between influenza and the seasons, a link which may, or may not, also exist for the distinctly different coronavirus.
Here, “flu” and “influenza” refer to a specific set of viruses that have afflicted humans for hundreds if not thousands of years with a disease involving aches, fever, chills, coughing, and sneezing, and which can sometimes lead to severe respiratory complications, and even death. We’re not talking about what some people refer to as the “stomach flu,” and not the common cold.
Most years, influenza is very much a seasonal disease. Since 1982 flu activity in the U.S. most often peaked in February (15 seasons), followed by December (7 seasons), January (6 seasons) and March (6 seasons) according to the CDC.
This strong seasonal cycle is driven by a number of factors, some of which relate to the transmission of the virus from person-to-person. The CDC’s website states that an infected person may pass influenza to others who are in close contact (within about 6 feet), usually through respiratory droplets produced when the infected person coughs or sneezes. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. Less often, a person might become infected by touching a surface or object that has the virus on it and then touching their own mouth, nose, or possibly their eyes.
Undoubtedly one reason the flu is mainly a winter disease is simply because people tend to congregate together inside more during the colder months.
However, studies over the past fifteen years point to the humidity of the air as a crucial factor in the flu’s seasonal cycle.
As noted many times in this column, warm air can hold more water in gaseous form than cold air, so, as temperatures rise in the spring, the absolute humidity also usually increases. Not only is there evidence that the influenza virus itself is less stable in moist, warm air, the droplets containing the virus coughed or sneezed into the air don’t float around as long when the humidity is high. Some studies indicated that if the flu virus does make it into your upper respiratory tract, it’s less likely to grow when the air your breathing is relatively moist.
Will “coronavirus disease, 2019” (COVID-19) follow a similar seasonal cycle? While the CDC says that coronavirus is spread in virtually the same way as influenza, the two are very different viruses, and it is just not known yet whether the rate of spread of COVID-19 will be affected by springtime’s increased humidity, warmth, and sunlight in ways similar to influenza.
Unfortunately, even if spring weather works to slow the spread of COVID-19, the number of cases will almost certainly continue to grow in the coming weeks, perhaps dramatically in some areas, because this is a new virus for which no one has built up an immunity.