The effect of weather on COVID-19 infection rate

Viral epidemics are nothing new in human history. Several viruses circulate freely every year among human beings, causing seasonal outbreaks of disease and death, such as the seasonal flu. A new study published on the preprint server medRxiv* reports that temperature plays a significant role in the growth of the ongoing COVID-19 pandemic.

About the novel coronavirus

The COVID-19 outbreak is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), of the family of coronaviruses, which also originated the SARS and the MERS outbreaks over the last two decades. The reproductive number R0 of SARS-CoV-2 IS 2.2, which means each infected individual infects 2.2 other people.

Novel Coronavirus SARS-CoV-2 Transmission electron micrograph of a SARS-CoV-2 virus particle, isolated from a patient. Image captured and color-enhanced at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID
Novel Coronavirus SARS-CoV-2 Transmission electron micrograph of a SARS-CoV-2 virus particle, isolated from a patient. Image captured and color-enhanced at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID

The incubation period is currently suggested to be approximately five days. The viral load increases after every seven days, on average, allowing infected individuals to spread the virus asymptomatically. This has led to a high exponential chain transmission, overwhelming health systems across the globe.

The challenge that is currently being faced by public health authorities is how best to bring down the R0 to less than 1, so that the pandemic will slow its rate of growth, and eventually die down.

At present, this is being attempted by mitigation strategies centered on physical distancing, wearing masks, and cleaning hands regularly with alcohol-based sanitizers, if not soap and water.

How temperature constrains COVID-19 spread

Although COVID-19 reports a lower mortality rate than its predecessors SARS and MERS, researchers say that its exponential growth rate is cause for concern.

There has been limited evidence that SARS-CoV-1, the virus responsible for the SARS epidemic of 2003, loses its stability and high contagion in tropical areas, which have higher temperatures and humidity. The researchers investigated the role of several environmental conditions - temperature, precipitation, wind speed, and cloud cover.

Temperature changes have an effect on the COVID-19 infection rate, influencing it at the rate of 13-16 cases a day per 1°C rise in temperature, the study said.

An earlier study first reported that the SARS virus becomes inactive at higher temperatures and in high humidity. Similarly, recent studies report that the virus spreads in cold climates within 30-50o N at temperatures of 5 oC to 11 oC, and not in tropical or arid zones.

Another study observed a decline in transmission by 10% with each 1oC increase in temperature above 5oC. Based on these, the authors state, "these reported studies 88 provide substantial evidence that the climatic conditions can regulate the transmission of the 89 rapidly spreading novel coronavirus SARS-CoV-2."

At a temperature below zero Celsius, the epidemic growth rate is constant but begins to decrease at temperatures above this.

The researchers postulate that all the countries which suffered substantial infection rates, in particular the USA, Italy, Spain, and China, exhibited a temperature gradient which was less than 15oC. On the other hand, countries that reported lower infection and death tolls like India, Africa, and Australia, had temperatures higher than 15oC.

Malaysia, Indonesia, and Australia seemed to be exceptions. The study hypothesized that this could be because of their difference in elevation, or seasonal location changes.

Rain and wind speed

The study also stated that countries next to regions that receive low precipitation (less than 400 mm) and high wind speed (10-12 m/s) were more vulnerable to the virus. Finally, the affected countries had 70% to 80% cloud cover in the period of the study.

"The statistical estimation suggests -6.28oC and +14.51oC, as the most the favorable temperature range for the growth of COVID-19," the study says. Wind speed at below 2 m/s could also help predict the growth of the pandemic.

What does the study imply?

If these findings are validated, researchers predict that the intensity of the pandemic in the USA, China, and Europe will "start decreasing from April and substantially disappear by October, while it may intensify in Canada between May to September-October."

Africa was not considered likely to be significantly affected by the pandemic. They predict that the total number of infections due to COVID-19 will decrease from 13-16 cases per day, due to 1oC rise or change in temperature. The study concludes that this "is a significant sign or impact as a result of the increase in temperatures."

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.

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