There is an interesting article about seasonality in diseases at Science by Jon Cohen. It’s a fairly lengthy, wide-ranging treatment of the subject. Here’s a snippet:
Will SARS-CoV-2, which has an envelope, prove fragile in spring and summer, when absolute and relative humidity climb? The most notorious of the other coronavirus diseases, SARS and Middle East respiratory syndrome (MERS), offer no clues. SARS emerged in late 2002 and was driven out of the human population in the summer of 2003 through intensive containment efforts. MERS sporadically jumps from camels to humans and has caused outbreaks in hospitals, but never widespread human-to-human transmission like COVID-19. Neither virus circulated for long enough, on a wide enough scale, for any seasonal cycle to emerge.
Four human coronaviruses that cause colds and other respiratory diseases are more revealing. Three have “marked winter seasonality,†with few or no detections in the summer, molecular biologist Kate Templeton, also at the University of Edinburgh, concluded in a 2010 analysis of 11,661 respiratory samples collected between 2006 and 2009. These three viruses essentially behave like the flu.
That does not mean COVID-19 will as well. The virus can clearly transmit in warm, humid climates: Singapore has more than 175 cases. Two new papers published on preprint servers this week come to opposite conclusions. One, co-authored by Lipsitch, looked at COVID-19 spread in 19 provinces across China, which ranged from cold and dry to tropical, and found sustained transmission everywhere. The second study concludes that sustained transmission appears to occur only in specific bands of the globe that have temperatures between 5°C and 11°C and 47% to 70% relative humidity.
Obviously, we don’t really have enough information yet to make any confident generalization but what we presently have in hand isn’t particularly favorable. There have been substantial outbreaks in places with high temperatures (Singapore) and low (South Korea), high humidity (Singapore) and low (Qatar), and all combinations and places in between.
Part of the decrease in infections from respiratory viruses during the summer probably comes from less closed in living. Instead of staying inside all day, people get out.
Steve
I suspect there are many reasons for the seasonality of some diseases including the viability of the organisms under different conditions of temperature, humidity, etc., immune systems performing differently in different seasons, and different behaviors in different seasons. I doubt we can disaggregate these.
There are studies that show SARS virus breaks down faster at higher temperatures and higher humidities – from 2011.
https://www.hindawi.com/journals/av/2011/734690/
Singaporean spend most of their day indoors with air conditioning, so what happens there neither validates or invalidate the hypothesis.
There are important cautions on relying on the summer to stop the pandemic. High temperature is 33+ and high humidity is 80%+. In air conditioned climates the virus can infect even after 5 days on a surface. That type of temperature does not occur until July — and if the outbreak is still ongoing then, catastrophe already occurred.
But it is a hopeful sign for low income countries in Sub-Sahara Africa.
“I doubt we can disaggregate these.â€
I’m sure that’s true, as it is in any dynamic and complex system not having 1 or two dominating variables.
I saw a map of incidence showing much lower rates in the Southern Hemisphere. I doubt that’s dispositive, but it may be indicative.