Seasonality

In my earliest posts on the epidemic it was noted that coronaviruses are typically highly seasonal.  The default assumption regard the current strain should be that it is also seasonal.  So if we see a sharp decline in cases in the coming weeks, that is the most likely reason.  It also would mean that in the late fall, we would expect to see a re-emergence of cases, although I think those will be increasingly mild and asymptomatic and fewer in number, as the virus has already attacked most of the very susceptible population.  Our biggest concern should be to exhibit resoluteness against any attempt to re-impose business shutdowns and stay-at-home orders, even if there is a re-emergence.  This lockdown approach has proved itself to be extremely damaging and completely unnecessary.

A new paper gives some further evidence of the seasonality of the typical coronavirus strain.   (JID Paper)   The authors note the importance of understanding the epidemiology of those pre-existing strains in order to ascertain potential cross-reactive immune defenses, which might limit the susceptible population.  These researchers, based in Scotland, have been collecting information on common respiratory viruses from January 2005 through September 2017, including coronaviruses.  Samples from around 65,000 patients had been collected.  Some had more than one sample over the study period.  There were a total of about 74,000 samples tested.  Eliminating repeat episodes led to about 56,000 initial episodes of respiratory disease.  There was about a 4% overall rate of patient infection by seasonal coronaviruses, which corresponded to about 11% of all virus types detected in these patients.  Rhinoviruses and influenza viruses were more common than the seasonal coronavirus.  There tended to be more cases among children under the age of 5 and the elderly among hospitalized patients.

Prevalence peaked in the January to March period, but there was some variation among the common strains of coronavirus.  There was also with some strains significant variation from year to year.  There was evidence of co-infection with some other viruses and coronavirus infection tended to be high in high influenza years.  The authors suggested that the inverse patterns of serious infection between seasonal coronaviruses and COVID-19 in regard to children may be supportive of the cross-reactive immune defenses.  What remains unexplained is why the virus is so highly seasonal–is it something in the hosts (us) or something about the virus, or both.  In any event, keep seasonality in mind if infections remain at a low level.  And if they do, this is the opportunity to open things up so that even at that low level, some greater percent of the population has exposure.