Do High Temperatures Weaken the Coronavirus?

On Thursday, April 23, the U.S. government published a study suggesting the new coronavirus could weaken when the temperature rises, following the example of the flu. Even if this possibility is considered “encouraging,” it doesn’t mean we’ll see the end of the pandemic here this summer.

A glimmer of hope for the months ahead? At a press conference on Thursday, President Donald Trump presented information on how heat influences the coronavirus. According to a study, the new virus, whose global impact continues to grow, could weaken in a hot and humid atmosphere, as well as under the sun’s rays. “Our most striking observation to date is the powerful effect that solar light appears to have on killing the virus, both on surfaces and in the air,” announced Bill Bryan, a senior official at the Department of Homeland Security.

According to this information, the half-life of the virus, that is, the time taken to cut its strength by half, is 18 hours at a temperature of 21 to 24 degrees Celsius, with 20% humidity on a nonporous surface. But this half-life is reduced to six hours when the humidity rises to 80%, and only two minutes when sunlight is added to the equation.

Since the outbreak, scientists have talked of the possibility that the coronavirus will grow weaker as the temperature rises, particularly as certain hot countries like Australia have only recorded around 6,600 known cases with 75 deaths, figures far below countries in the Northern Hemisphere. Vice President Mike Pence described these observations as “encouraging.” Trump remained evasive but noted that the United States could be in a better position at the start of the summer.

Weakened Does Not Mean Eradicated

In reality, this study only further confirms what most experts already knew: that heat is a favorable environment for a reduction in the transmission of a virus.

But in the case of COVID-19, things are a little more complicated because of its highly contagious nature. In order to consider an epidemic to be over or less dangerous, its basic reproduction number or R0, the expected number of cases directly generated by a single case, needs to be less than one.

“Without lockdown measures, the R0 of the coronavirus is between 4 and 5,” points out Etienne Decroly, virologist and director of research at the French National Center for Scientific Research in Marseille. “Historically, we already know that viruses diminish in the heat, but what we don’t know here is at what point.”

In the case of the flu, this famed R0 “ranges from 1.3 to 1.5,” says the researcher. A drop of 0.5 in the heat is therefore enough for it to disappear. With the coronavirus having an R0 of 4 outside of lockdown measures, this single climatic factor would need to considerably lower the figure. “So while with flu, the R0 is lowered by 0.5, will high temperatures lower the R0 of the coronavirus by nearly 4? Personally, I doubt it,” confesses Decroly. In France, according to the Pasteur Institute, the R0 has dropped from 3.3 to 0.5, but this has been during the lockdown period and, a priori, largely because of it.