Humid climates, summer weather may not limit pandemic growth: Study

New York, May 19

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Since the COVID-19 pandemic can easily spread among a vast pool of highly susceptible people, without control measures, seasonal and geographic climate variations alone may not slow down the pandemic, a new study says.

The findings of the research, published in the journal ‘Science’, noted that strong outbreaks were likely in more humid climates, and summer weather would not substantially limit the growth of the pandemic.

According to the researchers, including those from Princeton University in the US, a large number of people are still vulnerable to the novel coronavirus behind the COVID-19 pandemic.

Based on the speed at which the pathogen spreads, they said climate conditions were only likely to “make a dent” in the current rate of infection.

“We project that warmer or more humid climates will not slow the virus at the early stage of the pandemic,” said study first author Rachel Baker from Princeton University.

While climate has some influence on the size and the timing of the pandemic in general, Baker said, there was enough number of susceptible people in the population for the virus to spread quickly, no matter the atmospheric conditions.

She said the rapid spread of the virus in Brazil, Australia, and other nations in the tropics and the Southern Hemisphere — where the virus began during the summer season — provides indication that warmer conditions might not halt the pandemic.

“It doesn’t seem that climate is regulating spread right now,” Baker said.

Scientists, however, were unsure how temperature and humidity influenced the virus’ transmission, she said.

Based on studies of other similar viruses, the researchers said, COVID-19 might become responsive to seasonal changes only after the supply of unexposed hosts was reduced if vaccines or other control measures were not present.

“Previously circulating human coronaviruses, such as the common cold, depend strongly on seasonal factors, peaking in the winter outside of the tropics,” said Bryan Grenfell, another co-author of the study from Princeton University.

If the novel coronavirus could be similarly seasonal, Grenfell said, it might settle down to become a winter virus as it becomes endemic in the population.

Exactly how this might happen, depends on a lot of complex factors for a given location, he added.

Grenfell explained that the pandemic’s trajectory over the next several months would be influenced by “both human-induced factors such as non-pharmaceutical interventions to reduce contact, and fundamental biological uncertainties.

These, he said, included factors such as the strength and duration of immunity following infection.

“As knowledge of the immune response develops, we hope to be able to project its interaction with seasonality more accurately,” Grenfell added.

In the study, the researchers ran simulations on how the pandemic would respond to various climates across the globe.

Since COVID-19’s response to warmer weather is not well known, the researchers ran three scenarios based on the behaviour of similar viruses under varying seasons.

In the first scenario, they assumed that the novel coronavirus had the same climate sensitivity as influenza, based on earlier models from laboratory studies that highlighted the importance of low humidity to promote spread.

In the second and third cases, they gave the virus in the model the same climate dependence and length of immunity as human coronaviruses OC43 and HKU1, which are two causes of the common cold.

According to their analysis, climate only became a mitigating factor when large portions of the human population were immune or resistant to the virus.

“The more that immunity builds up in the population, the more we expect the sensitivity to climate to increase,” Baker said.

If the model was run long enough, they said the big pandemic settled into a seasonal infection.

“We’re assuming that if the same climate drivers apply to COVID-19, this would be the outcome,” Baker added.

When the scientists ran a simulation accounting for the impact of control measures like physical distancing, the results suggested that the longer these measures are in place, and slow the transmission of COVID-19, the more sensitive the virus becomes to warmer weather.

“The next step is to test our model by comparing future changes in the pandemic curve with detailed measurements of local climate, control measures, and other local variables in different climatic regions,” said study co-author Jessica Metcalf from Princeton University.

According to the scientists, the study has broader implications for refining the integration of weather information with disease outbreaks models.

They said a deeper, interdisciplinary understanding of the interplay of multiple factors that impact disease evolution, such as disease dynamics, weather, and mitigation measures undertaken by society, is needed. PTI