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Explained: One year on, many unknowns in the role of weather in Covid-19 spread

Right from the beginning after the first cases were reported from Wuhan in China in December 2019, the popular scientific speculation was that the respiratory viral infection could show some seasonality.

Written by Anjali Marar , Edited by Explained Desk | Pune |
Updated: March 23, 2021 8:19:49 am
A busy market in New Delhi (File Photo)

One full year into the Covid-19 pandemic, when the coronavirus SARS-CoV-2 has gone through all the seasons, it is not yet fully understood how the virus behaves in and responds to the changing weather.

Although this remains an area of research globally, a 16-member Covid-19 Task Team constituted by the World Meteorological Organization (WMO) has found no concrete answers on the influence of several meteorological parameters on coronavirus cases and related mortality, the world over.

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When the pandemic broke, what role was weather expected to play?

Right from the beginning after the first cases were reported from Wuhan in China in December 2019, the popular scientific speculation was that the respiratory viral infection could show some seasonality.

Based on past experience with influenza, many medical and public health experts expected that cases of respiratory viral infections would spike during the autumn and winter months. And, fewer such infections would emerge in temperate climates and during summer months. Besides, variations were predicted based on geographical location, altitude and inter-annual variabilities.

However, in early efforts to document Covid-19 trends viz-a-viz weather conditions and air quality, the results were not always as predicted. A number of studies threw up uncertain and contradictory results.

So, did the weather not have any influence on Covid-19 cases?

What can be said with a certain degree of surety, as of now, is that the immunology of various populations outweighed environmental factors to a large extent, with seasonality playing a relatively smaller, but inevitable, role.

The extent to which seasonality drove Covid-19 cases and led to mortality globally is yet to be ascertained, according to the first report of the WMO Covid-19 Task Team, ‘Review on Meteorological and Air Quality Factors Affecting the Covid-19 Pandemic’, released last week.

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What do studies suggest?

The WMO Task Team focused on three meteorological parameters — ultraviolet radiation, air temperature and humidity. Based on several studies, what is now known —

UV RADIATION: Under controlled laboratory settings, the impact of UV on virus survival is clear, but its importance for the transmission of SARS-Cov-2 is yet to be proven. Many studies drew negative correlations or non-linear associations between UV radiations and indicators of SARS-Cov-2 transmission, which included the case growth rate. But the possibility of the influence of UV radiation and its implications on seasons, latitude and altitude remain unestablished.

TEMPERATURE & HUMIDITY: Environments with cool temperatures (5°C) and relatively low humidity (20-35%) positively influence virus transmission through aerosols, especially in mid-latitude winter settings. In warm and humid settings, small respiratory droplets are capable of absorbing water; when a larger droplet settles on a surface it leads to transmission via contact rather than airborne, especially in tropical climates.

Under large temperature variations, respiratory mortality increases. Exposure to high ambient temperatures, particularly for the elderly, children, those who have pre-existing conditions and are vulnerable to heat, puts them at a higher risk of respiratory and cardiovascular diseases.

AIR POLLUTION: Covid-19 infection can deteriorate upon prolonged exposure to polluted air, especially high levels of PM2.5, ozone and nitrogen oxide, all of which can trigger immune system dysregulation. This in turn increases the chances of developing co-morbidities and of requiring hospitalisation. But, the evidence on impact of air quality on virus transmission remains limited.

HUMAN BEHAVIOUR: This is being seen as the most consistent factor with a role in SARS-Cov-2 transmission. It is now established that the transmission commonly occurs when people congregate within closed spaces with poor ventilation. Extreme heat could force people to avoid masks, thereby aiding greater transmission. Warm conditions would also mean an increase in use of air-conditioners, adding to the risk of indoor transmission.

What are the key conclusions of the WMO Task Team, then?

SARS-Cov2 survives longer in cold, dry and low UV radiation conditions, it said. But Covid-19 transmission dynamics in 2020 and 2021 (until January first week) appear to have been influenced primarily by government interventions like lockdowns, travel restrictions and use of masks. Other transmission drivers include human behaviour, demographics, immunity of the population and virus mutations.

Meteorological factors and air quality must not be the basis on which governments relax intervention measures to curb the virus transmission, the task team has recommended.

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