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An update on Covid-19 seasonality and warmer weather

Don Paul

President Trump wasn’t alone among nonexperts who were optimistic about April warming and reduced contagion, but he was the most prominent.

Earlier this week, I observed several physicians express some real confidence on a prime-time network coronavirus special about reduced contagion being likely during warmer months. Their reasoning seemed to center on the behavior of a number of viruses that become less active toward summer and a possible relationship between that and increased exposure to ultraviolet radiation during the longer days. These less-active viruses refer to older coronaviruses with more defined seasonality patterns. They include many strains of the flu as well as coronaviruses. On the surface, it sounds like it makes sense, but upon closer examination the operative phrase is “we (epidemiologists and virologists) don’t know.”

There are some scientific agencies that are rather pessimistic on chances for warmer weather having a major impact on contagion, as summarized in this Brazilian news report outlining the view of the European Union Disease Control Center.

The evidence-based European report does have its limitations, though, because there is much evidence yet to be gathered in the Northern Hemisphere. After all, it’s only early April. However the early signs in warmer regions of the U.S. are not yet showing much reason for optimism.

Cases are on the rise in South Florida, east Texas and New Orleans, to name just a few warm-weather locations of many showing increases. These locales are not yet at late spring levels of heat and humidity, which may be significant. There have been a handful of studies that suggest the virus does poorly in muggy air masses, with droplets from sneezes and coughs sinking more quickly to the ground under those conditions. But it is already warm with moderate humidity in these locations and, thus far, there are no signs reduced contagion is in progress over the southern U.S.

Where curves are being flattened overseas, the evidence remains overwhelming the lessened number of new cases is due to the effects of extreme social distancing, not the weather, as a primary factor. There is also the presence of flattening and lowering that was always modeled following huge spikes in incidence. All in all, however, no one can yet discern early signs of seasonality with Covid-19, which is evident in the European Center graph:

None of this is to suggest some temporary seasonal weather impacts may not yet appear. Yes, the influenza virus is usually seasonal as are many older cold coronaviruses. But this brand-new coronavirus is not related to the flu virus, and is in its first trip around the globe, infecting populations who have zero immunity to it. New viruses can behave very differently from older viruses to which there is already some immunity in the population. Dr. Marvin Lipsitch, director of the Harvard School of Public Health Center for Communicable Disease Dynamics, has expressed his reasoned scientific skepticism on this topic.

The epidemiologist states: “The short answer is that while we may expect modest declines in the contagiousness of SARS-CoV-2 in warmer, wetter weather and perhaps with the closing of schools in temperate regions of the Northern Hemisphere, it is not reasonable to expect these declines alone to slow transmission enough to make a big dent.”

He reminds his colleagues and the public the control of the 2003 SARS epidemic in Asian countries was accomplished by faster applications of strict social controls and quicker diagnoses of the patients. And, Lipsitch points out, when SARS was initially controlled in Toronto, premature relaxing of the social controls and widespread testing led to a second surge there in warmer, late spring weather. In other words, he found no evidence weather played a role in the Asian controls.

Other epidemiologists have pointed out similar trends in examining the data. Dr. Mauricio Santillana, also at Harvard, told “As of now, we haven’t seen evidence that higher temperatures (and higher humidity), such as those being experienced in the Southern Hemisphere, will lead to decreases in Covid-19 transmission,” he told Healthline.

“This is in contrast to what one would expect for seasonal influenza … we expect that the Southern Hemisphere will be as vulnerable to the Covid-19 outbreak as the Northern Hemisphere is,” Santillana said.

If you look at Singapore, for example, the country is experiencing summerlike temperatures and humidity comparable to many locations in the Northern Hemisphere.

Lipsitch also notes we simply don’t know how much school closures will help in total. Children have low infection rates, though it’s very possible or even likely they can be carriers to adult staff and parents. The reduced rates of flu infection during school closings may not apply as much with this virus.

For now, the majority of epidemiologists and infectious disease specialists are taking a conservative approach. A Rutgers Medical School professor kept it simple for “Dr. David Cennimo, an infectious disease expert at the Rutgers New Jersey Medical School, says it's too early to know for certain whether the warm spring and summer temperatures will help knock down the coronavirus.

The bad news about weather-related seasonality has already been posed in speculation by Dr. Anthony Fauci. He believes any seasonal reduction in the summer would be followed by a resurgence in the fall. Even so, if that happens, there still could be a benefit during any speculative temporary notable reduction: “It’s good news in the sense that it gives the country a break,” Dr. Peter Hotez, the dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, told the Daily Caller News Foundation on the possibility of a seasonal pattern.

“If we have to live like this the rest of the year you can imagine what that’s going to do to the economy,” he continued. “If it gets people back into the workplace even for a period of time until we see it start to go up again that would obviously be a benefit.”

I will continue to monitor the related research as spring progresses, and update you as new studies emerge.

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