Local and global positivity rates, presence of novel variants and local vaccination rates also important factors
Dry, cold conditions prevalent in most of India at present may be conducive for the transmission of the novel coronavirus (SARS-CoV-2) that causes COVID-19, according to the World Meteorological Organization (WMO).
The risk of transmission of SARS-CoV-2 is higher during the winter season in temperate climatic regions, WMO’s COVID-19 task team found on analysing available research on seasonality. “Many mid-latitude countries have witnessed spikes in virus transmission during winter and fall seasons.”
Better modelling of the spread of the disease that takes into account the weather and other environmental factors are needed to confirm the prediction, the global weather agency noted.
Xavier Rodó, a member of the task team and head of the The Climate and Health Program of the Barcelona Institute for Global Health in Spain, said:
At this point, we can detect and quantify the consistent and, at times, strong role of low temperatures and low absolute humidities in facilitating COVID-19 propagation in different regions, which adds on top of density-dependent factors.
There have also been spikes in transmission of the virus during warmer periods (spring and summer) in some countries, and the emergence of new variants of the virus have remained independent of seasons. The omicron variant, for instance, was isolated in South Africa during the country's spring season.
COVID-19 disease has seasonal characteristics but they need to be looked at in the context of other risk factors, such as local and global positivity rates, the presence of novel variants and local vaccination rates, according to the agency’s task team.
Human-to-human interactions play a great role in viral diffusion but temperature and humidity can still alter human behavioural patterns, immune system functioning / response and viral survivability in the external environment, according to a paper published in the journal Environmental Research May 2021. “These all ultimately contribute to high SARS-CoV-2 transmission.”
Cold season in the southern hemisphere countries caused an almost 60 per cent increase in the number of total infections, while the warm season in the northern hemisphere countries contributed to a close to 47 per cent reduction in the number of cases, showed another paper published February 2021 in the same journal.
The paper also concluded that COVID-19 seasonality was more pronounced at higher latitudes, where variations in environmental indicators were higher.
Role of other factors
There are other weather factors apart from temperature and humidity such as air-drying capacity (ADC) and ultraviolet radiation, which may have an impact on the transmission of the SARS-CoV-2 virus.
ADC is defined as the rate of decrease of droplet surface area with time, given ambient temperature and humidity. Droplets in the air from sneezing and cough are the primary media for the spread of the virus.
Essentially, ADC predicts the fate of these droplets under specific temperature and humidity conditions. Higher ADC leads to lower prevalence of the COVID-19. Similarly, higher UV radiation also leads to lower prevalence.
In a report published in the journal GeoHealth May 2021, scientists observed that ADC explained the spread of SARS-CoV-2 much better than if temperature and humidity were considered separately. They found that in temperate countries Canada, Germany and Chile, the virus spread more in winter months while in tropical countries India and Ethiopia, the spread was greater during monsoon months when humidity is higher.
“For meteorological and health services, evidence of COVID-19 seasonality has potential value for disease preparedness and response”, said Ben Zaitchik, co-chair of the task team.
It makes sense to prepare for rise in COVID-19 case numbers and severity in cold seasons in colder climate zones, he added. “Meteorological services can work with health services to communicate this message and to plan for distribution of adequate health resources and information.”
The impact of weather on the spread of COVID-19 disease needs to be integrated into modelling for spread of the disease, according to experts. The various national meteorological services with a strong scientific grounding can work closely with the health agencies to develop early warning systems using information technology, though caution should be maintained, they advised.
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