SARS-CoV-2 May Confound Seasons, Persist in Warmer Months, Report Shows

Alicia Ault

April 09, 2020

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Although conflicting, the available data indicate that SARS-CoV-2 could continue to spread in warmer spring and summer months in the US, according to a new report from the National Academies of Science, Engineering, and Medicine (NAS).

Current data suggest that the novel coronavirus may be transmitted less efficiently in higher temperatures and humidity, but the studies are not conclusive because of poor data quality, confounding factors, and the relatively short existence of the pandemic, which makes it difficult to determine its true course, writes David A. Relman, MD, a member of the NAS' Standing Committee on Emerging Infectious Diseases and 21st Century Health Threats, in a rapid expert consultation letter to the White House Office of Science and Technology Policy on April 7.

A number of factors could influence whether SARS-CoV-2 follows the same seasonal pattern as the influenza virus and other seasonal coronaviruses, which wane during warmer months, writes Relman, a professor of microbiology and immunology at Stanford University in California. 

But he pointed out that previous coronavirus strains that have caused serious illness — SARS-CoV and MERS-CoV — "have not demonstrated any evidence of seasonality following their emergence." 

Relman cites an example from the current outbreak: "Given that countries currently in 'summer' climates, such as Australia and Iran, are experiencing rapid virus spread, a decrease in cases with increases in humidity and temperature elsewhere should not be assumed…Additional studies as the pandemic unfolds could shed more light on the effects of climate on transmission," he writes.

And even if SARS-CoV-2 turns out to be less infectious in warmer months, "given the lack of host immunity globally, this reduction in transmission efficiency may not lead to a significant reduction in disease spread without the concomitant adoption of major public health interventions," writes Relman.

Conflicting Data

Relman cites a handful of studies indicating that, on the one hand, SARS-CoV-2 has declined with increasing humidity and temperatures, but that conversely, infectivity has increased in warmer, more humid climates.

A recent study in China, published on the repository and international journal site SSRN, found that while increased temperatures and humidity decreased the infectivity, "the average R0 (R naught) was still close to 2 at maximum temperatures and humidity in their data set, suggesting that the virus will still spread exponentially at higher temperatures and humidity," said Relman.

Several other studies found higher growth rates in temperate regions. One study, still in preprint on MedRxiv, looked at 310 geographic regions across 116 countries, and shows an inverse relationship between temperature and humidity and the incidence of COVID-19.

All the available studies so far have significant limitations, including limitation in time and location, confounding factors having to do with geography, access to and the quality of public health and health care systems, human behavior, and the availability of testing, said Relman.

However, he said, "it is useful to note that pandemic influenza strains have not exhibited the typical seasonal pattern of endemic/epidemic strains," and, regardless of whether they started in a warmer or a cooler month, "all had a peak second wave approximately six months after the emergence in the human population."

Worrisome Persistence on Masks

Seasonality can also be potentially gauged in the laboratory. Most of the studies on environmental persistence of SARS-CoV-2 have been conducted using virus grown in tissue culture. But that, too, is an imperfect method.

Virus disseminated into the environment from naturally infected humans likely has different survival properties than virus grown in culture, said Relman.

In addition, many labs cannot, or fail to, control and vary relative humidity, the committee letter noted. The aerosol studies so far have used humidity levels of 50% to 65%, which is more favorable to decay, while respiratory fluid is more likely to protect against infectivity, and the 20%-to-40% wintertime indoor humidity in temperate regions is more favorable for virus survival.

Even with these caveats, the committee cited worrisome studies on
SARS-CoV-2 survival.

In a study study published April 2 online in The Lancet, Hong Kong researchers reported significant reductions in virus in culture starting with temperatures at 37°C (98.6°F) or above.

On surfaces at a room temperature of 22°C (71.6°F) with a relative humidity of 65%, there was no infectious virus on printing paper or tissue papers after just 3 hours. It took 4 days for an infectious level to break down on glass and money, and 7 days for stainless steel and plastic. But after 7 days, investigators found 0.1% of the original inoculum on the outside of a surgical mask.

"The persistence of infectious virus on PPE is concerning," writes Relman, noting that more studies are needed to guide healthcare workers, especially on what might be used to disinfect personal protective equipment "when they cannot be discarded after single use."

Chad Roy, PhD, a researcher from Tulane University National Primate Research Center in New Orleans, Louisiana, told Relman by phone that in experiments where the virus was suspended as an aerosol at a temperature of 23°C (73.4°F) and about 50% humidity, SARS-CoV-2 had a longer half-life than the influenza virus, SARS-CoV-1, monkeypox virus, and Mycobacterium tuberculosis.

"This result is also concerning, but quite preliminary," writes Relman.

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