COVID-19 appears to spread less in warm and wet climates, studies suggest

TORONTO — Warm and wet climates appear to reduce the spread of COVID-19 while cold and dry weather conditions seem to facilitate virus transmission, according to a systemic review of multiple studies.

The review looked at 17 studies regarding the effect of temperature and humidity on the seasonal viability and transmission rate of the novel coronavirus.

Researchers reported that warm and wet climates seem to reduce the spread of COVID-19, however the review acknowledged that these variables alone could not explain most of the variability in disease transmission.

“Public isolation policies, herd immunity, migration patterns, population density, and cultural aspects might directly influence how the spread of this disease occurs. Thus, weather conditions associated with the health policies is a knowledge of great value for the benefit of the humanity in this critical period,” the review read.

The review also found that the virus appears to facilitate transmission best in cold and dry conditions.

However, the review reported that the certainty of evidence was “graded as low” for both climate outcomes evaluated as some of the studies presented a considerable risk of bias.

The findings were published Friday in the PROSPERO database.

Despite the risk of bias, researchers found “homogeneity between the results” reported by the included studies.

“Therefore, the countries most affected by the disease should focus on health policies, even with climates less favourable to the virus,” the review read.

In 15 included studies, researchers considered the association between temperature and humidity in the transmission rate of COVID-19. In the other two articles, the association was made only with temperature. Although humidity has been measured in different ways, the majority of the included studies indicated that there is a relationship between this variable and the spread of the virus.

While the study found that warm weather may reduce the spread of COVID-19, reports on seasonal variation are not new.

The common cold, the flu and other coronaviruses are typically moderate during warmer months. Health experts say this could be due to the direct effects of heat on viruses and because fewer people congregate indoors when the weather is warm.

Researchers noted that the transmission of both SARS and MERS was also decreased in hot and humid climates.

“The favourable cold and dry weather conditions facilitates the spread of the coronaviruses and seems to be the same for SARS-CoV-2 virus,” the review read.

According to the review, cold air may contribute to the spread of viruses for various reasons.

Researchers reported that low temperature provides suitable survival and reproduction conditions for viruses, cold air causes vasoconstriction of the respiratory tract, which contributes to weakening of the immune system, and dry, cold air makes the nasal passage prone to small ruptures that can create opportunities for virus invasion.

Other recent studies from the University of Maryland, Harvard and Princeton have found at least some impact from temperature changes on COVID-19.

A paper published in the Journal of the American Medical Association in June by Maryland researchers found that the virus acts in a way that is “consistent with the behavior of a seasonal respiratory virus,” spreading along with temperature and humidity levels.

Researchers at Harvard recently noted that it would be possible to develop a weather model to predict places most likely at higher risk for COVID-19 spread.

Princeton researchers highlighted that winter conditions could increase transmission of the virus. However, if counties maintain effective control measures, the study reported that they will be able to largely limit outbreaks regardless of weather.

Canada’s Chief Public Health Officer Dr. Theresa Tam previously warned Canadians that the fall will bring new risks in the COVID-19 pandemic along with colder weather and indoor family holiday gatherings.

Fears are mounting among health experts that Canada may experience a so-called “twindemic” consisting of duelling flu and coronavirus outbreaks when cold weather sets in for most of the country.

Doctors are concerned they won’t be able to administer enough flu vaccinations and ease the burden on the health-care system.

While both illnesses share similar symptoms, such as cough and fever, and appear to spread more readily in cold temperatures, the flu is largely preventable thanks to annual vaccines.

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