Let’s talk about how the weather affects epidemics and about how, relatedly, there are waves of deaths across time during epidemics. It may seem early to think about this, but we are just at the beginning of the first wave of Covid-19, but likely not the last.
Epidemics have always been with us. Here is a long and sad well-curated list. Why epidemics end altogether is itself an interesting question (for another day, related to the onset of herd immunity and the common evolution of pathogens and other factors). (The image below is a plague panel with the triumph of death from 1607–35.)
We have survived flu epidemics before, though many people die. We will survive this one, though very many will be afflicted in the USA and worldwide. In the end, Covid-19 will become “endemic,” which means that it will be one of those pathogens (like the regular flu, or the viruses that cause the common cold, that circulate in our species). Now that Covid-19 is here, it will remain with us. SARS-CoV-2 resembles other coronaviruses that cause the common cold, in fact.
However, at present, we are just facing our first wave of Covid-19. It may be premature to think of subsequent waves peaking, but there almost surely will be such waves. We must plan for the long haul.
Peaks in epidemics have to do with pathogen flows across social networks; other social factors (like changes in population mixing across time, or the occurrence of major congregations, like wars or elections); and the weather. For instance, flu has a baseline seasonality.
This seasonality of influenza has been known for a long time. And newly emergent pandemics, like existing diseases, often obey such seasonality too.
During the 1918 pandemic flu, there were three different waves of illness, starting in March 1918 through the summer of 1919. The pandemic actually reached its zenith in the U.S. during the highly fatal second wave, in the fall of 1918.
How might weather play a role in Covid-19? Preliminary studies by Chinese scientists regarding the transmission of SARS-CoV-2 (the virus that causes Covid-19), exploited the geographic variation in weather conditions across China, and suggested that the illness peaks with an average temp of 40°F.
Another early study from China suggested that changes in weather alone (i.e., an increase of temperature and humidity as the spring and summer months arrive in the Northern Hemisphere) will not necessarily lead to declines in Covid-19 case counts.
Here is the relationship, for instance, between absolute humidity and the SARS-CoV-2 (Covid-19) transmission rate across Chinese provinces. Except for very low humidity, there is not much variation.
Another paper used a sophisticated model to suggest that recurrent wintertime outbreaks of SARS-CoV-2 will probably occur after the initial pandemic wave.
And other analyses based on diverse country data, as of March 28, suggest that it is unlikely that the spread of SARS-CoV-2 would slow in the U.S.A. or Europe due to temperature or humidity in the summer in the Northern hemipshere.
Hence, research so far suggests that we cannot expect a “weather cure” for Covid-19, whereby it just disappears. There will likely be waves of SARS-CoV-2. It will come back again as it moves from the Northern to Southern hemispheres with the seasons. This is still an open question, however. We don’t yet know for sure exactly how much relief we might get from the pandemic with the arrival of the summer; but it is likely to be limited.
Regardless, Covid-19 will likely be with us forever, becoming “endemic,” and we will get used to it.