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What might warmer weather do for Covid-19 pandemic?

Don Paul

With sharply colder temperatures moving in at the start of the weekend, this may seem like an inopportune time to write about warm weather. The weekend will not bring any evidence of it. These are forecast afternoon temperatures for Saturday.

Average highs this time of the year slowly rise through the 40s, and by Tuesday we will be back above 50 degrees. The general probability of above average temperatures is fairly high by late next week into the following 10 days.

President Trump and some others have previously suggested the rate of contagion may or should begin to tail off as we head through April, owing to the fact some other viruses diminish with warm weather, including influenza. Dr. Marc Lipsitch, professor of epidemiology and director of the Center for Communicable Disease Dynamics at the Harvard School of Public Health, has examined the evidence, and is decidedly pessimistic on warmer weather slowing the rate of infection for covid-19.

Some optimism has existed because it has been noted the SARS outbreak of 2003 began to fade with the advent of consistently warmer weather. Lipsitch states the SARS reduction, which would have been important because of the time of year, was not tied to warming: “SARS did not die of natural causes. It was killed by extremely intense public health interventions in mainland Chinese cities, Hong Kong, Vietnam, Thailand, Canada and elsewhere. These involved isolating cases, quarantining their contacts, a measure of “social distancing” and other intensive efforts. These worked well for SARS because those who were most infectious were also quite ill in a distinctive way – the sick cases were the transmitters, so isolating the sick curbed transmission. In Toronto, SARS resurged after the initial wave was controlled and precautions were discontinued. This resurgence was eventually linked to a case from the first wave. The resurgence confirms that it was control measures that stopped transmission the first time.”

In the U.S., the lengthy delays in testing followed by the now gradual increase in testing have already taken us to the expected rapid increase in the rate of known infection, and this trend will inevitably continue into April. Even if there were some virological benefit to warming, it would be overwhelmed in the near future by the fast increase in identified infected people.

There are quite a number of viruses, including some coronaviruses, which do show a lessening in rates of infection with warmer weather as well as with increased humidity. Good epidemiological data exists demonstrating rates of flu and some cold virus infections improve with the presence of warmer, more humid air. Lipsitch states older viruses that have been around a long time are probably not good models for a brand new virus with very little data and no immunity in the human population. There had been considerable transmission earlier in Singapore, since reduced by measures taken more rapidly to prevent spread. The infection rate is beginning to move rapidly upward in warm Florida, though some of that is related to more testing in Palm Beach and Dade counties. Again, there is that resurgence that occurred in Toronto in the 2003 SARS outbreak during warm weather when social controls were relaxed. There are other factors about which nothing is known, such as the effect on viral transmission of longer days with more ultraviolet radiation. As Lipsitch sums up, “we simply don’t know.”

Lipsitch notes the known lessening of viral transmission with more personal space and isolation, and the recognized improvements that occur even in normal times in school populations when there is better ventilation and more humidity. On this brand new virus: “The relevance of school terms is important but unknown for the SARS-CoV-2 (another name for COVID-19). Few children have been identified as cases. This may mean they do not get easily infected and don’t do much transmitting. Or it may mean only that they don’t get severe symptoms when they are infected, and transmit nonetheless. Or something in between. Understanding this is key if we want to know whether school closures can help control COVID-19 spread, as well as to anticipate how much summer vacation may help reduce spread.”

The professor does offer a sliver of hope on the entirely inconclusive evidence that increased production of melatonin with more vitamin D absorption accompanying longer days may play some role in reducing respiratory infections. Most in his field feel the evidence is inadequate and too randomized to be reliable. But he agrees more research is needed, rather than declaring this to be a dead end.

Lipsitch, with other epidemiologists and virologists, emphasize new viruses operate with the advantage of no immunity in the host population, and can often produce high rates of contagion even “out of season.” This has happened with some flu strains when they were new, with since-reduced infection rates in warmer months following in years later as immunity grew. He sums up: “For the novel coronavirus SARS-CoV-2, we have reason to expect that like other betacoronaviruses, it may transmit somewhat more efficiently in winter than summer, though we don’t know the mechanism(s) responsible. The size of the change is expected to be modest, and not enough to stop transmission on its own. Based on the analogy of pandemic flu, we expect that SARS-CoV-2, as a virus new to humans, will face less immunity and thus transmit more readily even outside of the winter season.”

Even with this evidence-based pessimism, the warmer weather that is coming can bring great advantages to our psychological and physical conditions. If you haven’t read it yet, take a look at this very useful and uplifting article by The News' Scott Scanlon:

Build your immunity and take some control with tips from wellness experts

After almost four years of regular gym attendance now cut off, I felt better just reading these fine ideas. Now I have to get my butt moving, which is a considerable step beyond reading. Sometimes, there’s a lot of Homer Simpson in me.

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