Question: Clearly, summertime conditions (heat, humidity, people spending more time outdoors) do not reduce coronavirus spread in any meaningful way (in contrast to seasonal flu, and to the other coronaviruses that cause common colds). Could this mean, conversely, that winter conditions won’t make Covid much worse? Many folks had been expecting Covid to “lay low” over the summer and explode in fall and winter, but maybe that’s not how this disease works? Maybe it has almost no seasonal component?

Answer: While I hope that the pandemic won’t be worse in the fall and winter, I don’t think that will be the case. It’s bad now and it’s likely to be worse then.

There has been a lot of debate, both political and scientific, about whether COVID would lessen or even “burn off” in the summer. We last talked about this in our Q&A of 3/22 and I still find informative this Harvard professor’s blog on the topic, which I cited back then. Because the virus is relatively easy to transmit and because we have no/limited immune protection, experts generally believe that the virus will overcome any effect warm/humid weather may have (Vox summarizes here). As experts pulled together by the National Academies for a rapid consultation on the matter wrote in their report of 4/7, “In summary, although experimental studies show a relationship between higher temperatures and humidity levels, and reduced survival of SARS-CoV-2 in the laboratory, there are many other factors besides environmental temperature, humidity, and survival of the virus outside of the host, that influence and determine transmission rates among humans in the “real world.”” This brings us to the issue of fall and winter transmission. Unfortunately, fall and winter bring conditions besides environmental temperature and humidity that make SARS-CoV-2 easier to spread — including more time indoors, more time in close quarters, and more illness (like flu and the common cold) that will both tax our health systems and our immune systems.

Finally, here’s where I could be wrong with my assessment up top. JAMA published an article earlier this month, “Temperature, Humidity, and Latitude Analysis to Estimate Potential Spread and Seasonality of COVID-19,” which concluded, “Using weather modeling, it may be possible to estimate the regions most likely to be at higher risk of substantial community spread of COVID-19 in the upcoming weeks and months, allowing for a concentration of public health efforts on surveillance and containment.” These findings indicate that environmental factors make some geographies more/less likely to be hit hard by the pandemic at different points in the year. But these findings do not prove a direct causation and have big limitations — including no incorporation of myriad factors into the model, like public health interventions or population density. So at this stage, I still wouldn’t put much weight on them and I stand by my assessment. The pandemic is likely to be worse in the fall and winter.