According to a study published on the JAMA Network on 11 June, the novel coronavirus (SARS-CoV-2) might thrive in temperate climates in the Northern Hemisphere. This appears consistent with the behaviour of a seasonal respiratory virus, like the ones that cause the flu.
This study analysed data from 50 cities situated in the 30° N to 50° N latitude corridor, where, between January and mid-March 2020 the average temperature was between 5 – 11 °C, with low humidity. The cities examined included eight examples with substantial covid-19 community spread: Wuhan, China; Tokyo, Japan; Daegu, South Korea; Qom, Iran; Milan, Italy; Paris, France; Seattle, US; and Madrid, Spain. These were then compared with 42 cities without significant coronavirus outbreaks at the time.
Many infectious diseases have seasonal spreading patterns, but betacoronaviruses (such as SARS-CoV and the newest one, SARS-CoV-2) weren’t believed to display that. The flu, on the other hand, is the classic example of a seasonal disease. Influenza viruses thrive during periods of low temperatures (4 -11 °C) and low absolute humidity — between late autumn and early spring in the northern hemisphere.
If a similar seasonal pattern were observed with the novel coronavirus, it could help researchers create a reliable model to predict where it will spread next.
Why do climate conditions matter?
Climate conditions can increase transmission rates through a variety of mechanisms.
Firstly, low humidity means that the air is less saturated with moisture. This allows viral droplets, expelled by sneezing or coughing, to remain suspended for longer, increasing the chances that more people will breathe them in.
Secondly, our immune systems are naturally weaker during winter and early spring, which makes them more susceptible to infections.
Also, because of the inhospitable weather conditions, people tend to stay inside more, with the windows shut. All this extra time spent huddled together in unventilated offices or on public transport creates the perfect storm for viral transmission.
How do the recent findings hold up?
When the coronavirus outbreak first happened in Wuhan, China, in late 2019, local authorities expected nearby regions in Southeast Asia, such as Bangkok, Thailand, to be affected next. However, this wasn’t the case. Instead, the virus spread West, towards temperate parts of Asia, Europe and North America.
The virus seemed to be moving towards colder, drier climates than tropical Bangkok. Indeed, after the study’s results were written up, London and, later, Moscow experienced their own significant outbreaks, just as temperature and humidity in these cities were entering the favourable range.
Admittedly, the study has its limitations. Researchers are aware the data sample may have been too small (data was only collected between January and March) to safely deduce a pattern. Additionally, each country’s number of cases and mortality will inevitably differ due to testing availability and efficacy, and reporting. They also did not include other factors that influence viral transmission, such as population density, travel, air pollution, public health interventions and demographic characteristics.
Naturally, researchers are cautious at this stage. While this may not prove, conclusively, that we’ll be coronavirus-free during the summer months, it does bring us one step closer to understanding the disease that has turned our world upside down. The more we know, the better we can anticipate the pandemic’s course while the search for a vaccine goes on.
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