Links between COVID-19 & temperature sending mixed messages

Source: Jeremy Segrott (via Flickr)
With many conflicting studies it can be confusing to understand the links between COVID-19 and temperature. With autumn quickly approaching Gair Rhydd decided to look into the current research and conclude what we know so far.

By Holly Giles | Deputy Editor

COVID-19 has dominated nearly every title and newspaper in the last six months since the UK went into lockdown. With so many articles being published, it is easy to become confused with conflicting headlines about the effect of temperature on COVID; some researchers have claimed the virus favours warm climates and others that it favours cold. With autumn quickly approaching Gair Rhydd decided to look into the current research to conclude the effects of temperature on COVID-19.

The original assumption was that COVID would behave similarly to flu. Influenza rates often spike as we move into winter, as a result of three factors. The first is that the virus itself is more stable in cold conditions with less exposure to UV. Secondly, in winter people spend more time inside and in close proximity to one another, which boosts transmission of the virus. Finally, the lack of sunlight weakens our immune system meaning we have fewer defences against the virus. COVID-19 has a similar biological structure to influenza, so it was previously widely assumed it would work in the same way. However, some studies have shown conflicting data.

The first study, specifically looking at COVID-19 and its relationship with temperature, was published by Harvard University back in February. They concluded that there was no significant difference in transmission rates between different provinces of China, meaning temperature and humidity were non-significant. The next day, another study emerged which concluded the virus would spread better in summer, with an optimum temperature of 19˚C, humidity of 75 per cent and less than 30 millimetres of monthly rain. They concluded that the virus worked better in warm temperatures and that temperature was highly significant. When comparing these two studies, published only a day apart, it is easy to see why we, as members of the public, are confused about the effect of temperature on the virus.

Unfortunately, over the last few months, things have not become much clearer. There have been many studies into the correlation between temperature, humidity and COVID transmission, and the findings have varied from every extreme.

On June 1, Mount Auburn Hospital published a study looking at the transmission rates in the United States during Spring 2020. They concluded that there is a strong correlation between transmission of COVID and temperature up to a maximum of 11˚C, after which the temperature has no further effect. This may be a surprisingly low figure for many readers, with the average temperature in Cardiff remaining below this level from November to April last year.

Many researchers have been using statistical models with current data to predict transmission rates for the future. Some unpublished analyses looking at the transmission rates so far have concluded that temperate warm and cold climates are the most vulnerable to the virus, with more extreme regions being better protected. It has also been suggested that the virus does not respond well to humidity, as a result of tropical parts of the world being the least affected. 

However, Richard Gray from BBC Future, states: 

“without real data over a number of seasons, researchers are relying upon computer modelling to predict what might happen over the course of the year”

The scientific community is learning more about the virus every day and it is only when we understand it fully and, when we have seen its transmission rates across all seasons, that we will be able to successfully conclude the effect temperature, humidity and climate have on the virus. Until then we are guessing – educated and informed guessing, but guessing nonetheless.

In his article Gray also discussed the behaviour of pandemics: “pandemics often don’t behave in the same way as seasonal outbreaks” so it’s natural preference for temperature may not be relevant until levels are contained. If the infection rate is high, the virus will be able to spread in any temperature, even if it is not its environment of choice.

Jan Albert, professor of Infectious Disease Control at Karolinska Institute Stockholm explained this further: 

“Eventually we would expect to see Covid-19 becoming endemic … And it would be really surprising if it didn’t show seasonality then. The big question is whether the sensitivity of this virus to [the seasons] will influence its capacity to spread in a pandemic situation. We don’t know for sure, but it should be in the back of our heads that it is possible”

Due to this unknown, scientists conclude that we should be wary to lift lockdown, as a direct result of the decreasing temperature. Many of the studies suggest that temperature has a role but it is widely accepted that the role of human behaviour and human contact is much larger in changing virus transmission. Whilst we may think a colder climate hinders the virus, if we have gatherings or lift lockdown too early, the virus will be all too able to replicate and spread. 

The biggest fear, as we enter into winter, is that the transmission of COVID may be able to piggyback on the transmission of other viruses. If a person comes down with the flu, they are then less able to fight off other infections, meaning this could be a gateway for COVID to enter. Owing to this it is especially important that people maintain good hygiene practices regardless of lockdown rules; protecting yourself against the common cold can greatly decrease your susceptibility to COVID. 

Professor Julian Hiscox, from the University of Liverpool, echoed this statement in an interview with the BBC: “I think we should be prepared for a second wave and not let up in terms of our social distancing and hand hygiene because it will come if we don’t.”

This was seconded by Dr Rachel Lowe from London School of Hygiene and Tropical Medicine, in her statement: 

“I think [a winter surge] is something we absolutely have to prepare for, the biggest danger is people becoming complacent about protecting themselves”

The overriding message throughout the studies and reviews, is that human behaviour is a far bigger driver of transmission than temperature or humidity will ever be. I cannot conclude definitively whether the virus favours warm or cold environments at this time, but I know it is greatly hindered by mask wearing, social distancing and hand hygiene. That is how the virus will be overcome, not by waiting for the Ice Age or the next heatwave.


Science and Technology Holly Giles

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