The four factors of coronavirus seasonality

Marc Lipsitch, professor of epidemiology and director of the Center for Communicable Disease Dynamics at Harvard University’s T.H. Chan School of Public Health, wrote recently, busting some of the myths around cold and flu seasonality.

He was writing not specifically about COVID-19, but about other, similar viruses, including coronaviruses and influenza. There are, he said, four factors to consider.

First, the environment. “In temperate countries, dry cold air = favorable conditions for flu transmission,” Lipsitch wrote, though “for coronaviruses, the relevance of this factor is unknown.” He points out COVID-19 did very well for itself in the warm, humid climate of Singapore, though a summer in Connecticut and a Singapore spring are very different. “We just don’t know,” he wrote.

Next, human behavior. We tend to be outside more in the summertime, and schools aren’t in session which means there may be less transmission in air-conditioned spaces. It certainly has made a difference in other pandemics. “The 2009 pandemic flu in the United States was very much decreased during the summer, and then came back rapidly in September,” Lipsitch wrote.

Then there’s the immune system. Melatonin and vitamin D both have been shown — at least anecdotally — to have some measure of value against severe respiratory illnesses, and both are naturally increased with greater exposure to the sun. That being said, Lipsitch wrote that neither melatonin nor vitamin d have been found to help with flu transmission. Still, he said, “this is a promising area for more study but at present its relevance seems uncertain.”

Finally there’s what Lipsitch called “depletion of susceptible hosts,” meaning that as more people are infected (and either succumb or survive) there are fewer available hosts for the disease. This is also a factor when considering seasonal infections, he said.

But, perhaps most importantly, Lipsitch wrote