The Science on Covid-19 and the Weather

A large meta-analysis was published last week [17 July] by researchers at King’s College London on the relationship between Covid-19 and weather conditions. Researchers found that with every one degree increase in temperature, hospitalised Covid-19 patients were 14.6% less likely to die in hospital.

Of course, correlation does not equal causation. Let’s look at how the study was done and what the researchers made of the medical plausibility of the results.

Does Temperature Affect Covid-19?

The meta-analysis aggregated the results of studies that charted disease progression of 6,914 hospitalised Covid-19 patients in Europe and China. As a comparator, they also considered the 37,187 individuals reporting their symptoms via the UK’s app-based COVID Symptom Study.

Results varied from place to place, but when taken overall, the research found an association between higher humidity and warmer temperatures, on the one hand, and better survival rates, on the other. Similarly, the UK study showed “decrease in symptom duration and disease severity with time” as Northern spring turned to summer.

How does temperature affect coronavirus?

The King’s College research paper noted, first of all, that all known coronaviruses are highly seasonal. So the findings are not necessarily surprising.

Why is temperature important to Covid-19? The researchers’ suspect the cause lies with mucosal immunity. “Mucosal barrier and mucociliary clearance can significantly decrease viral load and disease progression,” researchers noted.

It is a common misperception that mucus is merely a physical barrier against infection; it is in fact a biological barrier. Mucus contains compounds that mimic the targets of pathogenic viruses, trapping them and enabling them to be removed. 

Unfortunately, mucous doesn’t function properly when it is dehydrated by dry air. This explains, according to the study authors, why mortality from influenza is reduced significantly by increasing humidity from 20% to 50%. For this reason, the study team recommended humidification of public indoor spaces, and lamented the dry-air climate-controlled nature of hospital settings.

All of that said, researchers noted that age remains the most significant risk factor for Covid-19 disease progression and severity.