Risk of COVID-19 infection higher this fall/winter than summer

Nov 9, 2020

By "risk of infection" I mean catching COVID-19 from any of your contacts (who catch it from their contacts, etc... in your community).

Depending on where you are, there might already be higher transmission now than this summer, or even spring. If this hasn't happened already, prepare yourself for it. There is strong evidence that COVID-19 (SARS-CoV-2) transmission is under seasonal influence in non-tropical climates. If your locale does not have a fall/winter wave count yourself lucky.

For guidance on Thanksgiving plans, I defer to CDC recommendations.

Seasonal Effect on Coronavirus Transmission

There is a seasonal effect on the transmission of the already endemic coronaviruses (these strains are not lethal). Figure 1 from the study Potential impact of seasonal forcing on a SARS-CoV-2 pandemic sums it up very well:

Positive tests for already endemic coronaviruses ("CoVs") in Stockholm, Sweden.

"All CoVs show a marked decline in summer and autumn, with HKU1/OC43 peaking January–December, and NL63 and 229E peaking in February–March."

Same results are found in the US (Human coronavirus circulation in the United States 2014–2017 and Coronavirus occurrence and transmission over 8 years in the HIVE cohort of households in Michigan).

Southern Hemisphere

We can already see fall/winter waves in the non-tropical countries of the southern hemisphere (where the seasons are shifted six months). Though it's not a perfect fit considering Argentina.

Herd Immunity

It's unlikely your locale has built up enough population immunity to avoid a fall/winter wave. Few locales have built up as much population immunity as New York City, Lombardy (Italy), Madrid and Belgium. These locales have had some of the highest levels of population infection. Yet the case data rolling in for the past few weeks shows cases growing exponentially. The growth rate estimates at broadly predict exponential growth this late fall in the US.

My Change In Plans

I can share that my household opted out of our usual travel to Pennsylvania/DC for Thanksgiving. It is sad, but I should point out that during our year in Spain, we did not eat a traditional Thanksgiving dinner with extended family nor friends. And we survived. It was quite easy to do in fact.


The 1918 pandemic is similar, but also different, to our 2020 pandemic. Keeping that in mind, it is sobering to remember the 2nd larger wave of fall 1918.

by Erik Castedo Ellerman