Will heat, humidity slow down coronavirus outbreak? What experts say

Results from five early-stage studies by a diverse group of researchers suggest that the spread of COVID-19 may slow with higher average temperatures and higher humid conditions in countries approaching summers, such as India.

Epidemiologists, however, warn that establishing a causal link between disease progression and temperature is difficult because it does not take into account non-climatic factors such as human behavior, the capabilities of different systems of health and variable government / administrative response. They point out that COVID-19 currently transmits widely around the world, in widely varying local climates.

The five studies published between March 10 and March 23, 2020 analyzed the relationship of COVID-19 with temperature and humidity in China and other countries that are currently treating a large number of cases. These studies have been published in “SSRN” and “medRxiv”, two well-known global repositories of studies that have not yet been peer reviewed.

Some epidemic researchers that IndiaSpend spoke to have questioned the methodology of the studies and said their results were inconclusive. “We still do not know from empirical laboratory studies how temperature and humidity affect the persistence of COVID-19 and, therefore, whether cases are likely to decrease as temperatures rise in certain country, “Bethan Purse, head of the Disology Ecology Group at the Center for Ecology & Hydrology, UK, told IndiaSpend.

In such studies, it is important to take into account non-climatic factors that vary over time, said Purse, such as “the behavior of people – the average social distances at different stages of locking – that affects transmission and the amount of effort testing, which affects the number of cases seen. “

Although the contagiousness of COVID-19 may decrease slightly in warmer and wetter weather, “it is not reasonable to expect that these declines alone will slow transmission enough to make a big dent,” wrote Marc Lipsitch, director of the Center for Communicable Disease Dynamics at Harvard’s TH Chan School of Public Health in the United States, in an analysis of March 11, 2020.

In India, summer is fast approaching. The number of COVID-19 cases has increased steadily, as has the number of samples tested, and the states that tested more samples have detected more cases, we reported on March 25. COVID-19 has so far been detected in 27 states and Union territories. India had confirmed 724 cases of COVID-19 as of March 27.

“The way other coronaviruses have reacted to temperature and humidity in the past may not be true for COVID-19,” said an epidemiologist from the Indian Council for Medical Research, speaking on condition of anonymity. “We don’t know how the virus will react during the Indian summers, it’s too early to say anything.”

The five studies

In an article published for the first time on SSRN on March 19, 2020 (updated later), researchers from the Massachusetts Institute of Technology (MIT) stated that until March 21, 90% of COVID-19 transmissions were produced in regions where the temperature was between 3 and 17 degrees Celsius (° C) and the absolute humidity (the measurement of air humidity, whatever the temperature) between 4 and 9 grams per meter cube (g / m3) during the epidemic.

On the other hand, less than 6% of cases of transmission occurred in countries with average temperatures above 18 ° C and an absolute humidity above 9 g / m3.

Researchers analyzed COVID-19 cases and temperature and humidity data from all affected countries for the period between January 20 and March 21, 2020.

With the approaching summers, the temperature range may go higher, but any reduction in the impact of COVID-19, if any, can only occur at temperatures above 25 ° C, the authors wrote. .

Similar studies analyzing global trends

Other preprinted articles (not peer reviewed) have drawn similar conclusions, correlating hot and humid regions and COVID-19.

In an article published on SSRN on March 9, 2020, researchers from the University of Maryland’s Institute of Virology discovered that the COVID-19 virus is more efficiently transmitted between humans when humidity is low and temperatures are average are between 5 and 11 ° C.

“The distribution of large community epidemics along restricted latitude, temperature and humidity is consistent with the behavior of a seasonal respiratory virus,” according to the Maryland study.

Other coronaviruses, which usually cause cold symptoms in humans, have been shown to affect people most between December and April, and are undetectable during the summer months in temperate regions, added the study.

Another study, published on March 16, 2020 in medRxiv and written by two researchers from Spain and Finland, said that 95% of the positive cases in the world on March 10, 2020 occurred at temperatures between -2 and 10 ° C.

Studies analyzing trends in COVID-19 specifically in China

Two previous studies, one by Chinese researchers and the other by an independent Indian researcher, also make similar connections. The two studies published in SSRN analyzed the COVID-19 epidemic specifically in China for different periods and persistent weather conditions.

A team led by researchers from Beihang University in China studied the transmission of COVID-19 specifically in 100 Chinese cities with more than 40 cases, between January 21 and January 23, 2020, before the government announced interventions on January 24.

Hot, humid cities experienced a slower rate of spread than cold, dry cities, the study found. High temperature and high relative humidity significantly reduce transmission of COVID-19, he said – an increase of only 1 ° C in temperature and 1% relative humidity has significantly reduced transmission of the virus, said the study published on March 10, 2020.

“In July, the arrival of summer and the rainy season in the northern hemisphere can effectively reduce the transmission of COVID-19; however, risks remain in some countries in the southern hemisphere (eg Australia and South Africa), ”the authors wrote.

An Indian researcher then studied the relationship between the infectivity of COVID-19 and the meteorological data for 24 provinces of China (out of 26 – excluding Hubei and Tibet) for the cases occurring between January 21 and 16 February 2020. This study was published in March. 23, 2020.

On average, each additional centigrade above 5 ° C was found to be associated with a 1.2 percentage point decrease in the rate of increase in confirmed cases, according to the study. However, he found no substantial link between humidity and COVID-19 infectivity.

While Chinese researchers tried to understand the transmission of COVID-19 before government interventions to try to assess the true transmission rates, the Indian researcher and other studies mentioned above also took into account the interventions of the government.

“Our data sample was limited in average temperature from 6 to 20 degrees,” writes the author. “It is entirely possible that these results cannot be verified at more than 20 degrees.”

Do these studies change our understanding of COVID-19?

Experts say that several factors, in addition to temperature and humidity, will affect the way COVID-19 takes place in different countries, including their population density, health infrastructure and especially different policy interventions. All these factors make it difficult to predict the distribution of COVID-19 under different climatic conditions.

The fact that COVID-19 currently transmits so widely around the world, in widely varying local climates, suggests that transmission is not particularly limited by ambient conditions of temperature and relative humidity, but will rather be limited by local human behavior, health system factors and government policies, said Purse of Center for Ecology & Hydrology.

Many experts are drawing conclusions about COVID-19 based on the behavior of closely related viruses such as SARS (an old coronavirus) that appear to persist longer in cooler and drier conditions, she added.

The five studies are based on an understanding of how other types of coronaviruses, including SARS (see this and that) and common influenza coronaviruses (read this), have shown seasonality in the past.

Experts say, however, that drawing a parallel between a new coronavirus and past epidemics could lead to incorrect results. “SARS did not die from natural causes. He was killed by extremely intense public health interventions in cities of mainland China, Hong Kong, Vietnam, Thailand, Canada and elsewhere, “wrote Lipsitch of Harvard in his analysis.

With an example of the resurgence of SARS in Toronto after the initial wave ended, Lipsitch said that the resurgence was ultimately linked to a first wave case. He confirmed that the control measures had interrupted the transmission the first time, not a warmer weather.

“Predicting how a new virus will behave based on the behavior of others (influenza coronaviruses) is always speculative,” wrote Lipsitch, adding that “seasonal viruses that have been in the population for a long time behave differently from newly introduced viruses. in the population. “

Although COVID-19 is proven to be transmitted a little more efficiently in winter than in summer, the mechanisms responsible for this are unknown. The size of the change should be modest, and not enough to stop the transmission on its own, said Lipsitch. “We expect SARS-CoV-2 (COVID-19), as a new virus for humans, to be less immune and therefore more easily transmitted even outside of the winter season.”

Scientists should resist the urge to draw conclusions from analyzes that simply map patterns of disease to patterns of space and time, but should instead integrate a wider range of behavioral, political and of the state of health systems in their analyzes, said Purse.

(Tripathi is an IndiaSpend Fellow.)

Republished with permission from IndiaSpend. You can read the original article here


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