Could the new coronavirus outbreak be stopped by warmer weather?

16 March 2020

The simple answer to this is: we don’t know yet.

Not enough is known about the new coronavirus to say that it will disappear, or even have its spread reduced, in the UK as the temperature increases.

Evidence from similar viruses suggests that the virus may transmit less efficiently in the spring and summer months. Alongside changes in temperature, it is thought that humidity, differences in human behaviour and human immune system functioning also play a role in this pattern. 

However, even if it ultimately turns out to be a seasonal virus, it is unlikely to behave like similar viruses in the short term. This is because it is so new that very few people are immune from it.

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Why might we think that the virus will become less dangerous in spring?

The idea that warmer weather will help the fight against the disease is widespread. (The World Health Organisation (WHO) calls the disease itself Covid-19, and the virus that causes it SARS-CoV-2, and we will use that naming system here.)

For example in February, US President Donald Trump said on Fox News “You know in April, supposedly, it dies with the hotter weather.” He also tweeted that China will be successful in stopping SARS-CoV-2 “especially as the weather starts to warm & the virus hopefully becomes weaker”. However, following his tweet, experts interviewed by news outlets said that it was too early to say that warmer weather will weaken the virus. 

Last week, similar claims appeared in the Telegraph with an article called “coronavirus: why a warm spring could stop the virus in its tracks”. This article compares the new virus to the seasonal flu, which peaks during winter in the northern hemisphere. 

And the government’s Chief Scientific Adviser, Sir Patrick Vallance, also said at a press briefing that “there may be less transmission of [the] virus” in the summer months, as “respiratory infections in general tend to be a bit less common”.

Seasonal flu is caused by a group of influenza viruses, which are more common during winter in places with temperate climates, like the UK. Other coronaviruses generally show the same seasonal pattern.

Given our experience with these similar viruses, it’s not unreasonable to speculate that SARS-CoV-2 could peak during colder months and dissipate in the spring or summer. (That is if it lasts beyond the current outbreak. In 2003, the SARS coronavirus was contained—largely through public health interventions—and did not become a seasonal virus.)

But, as explained in an article written by Professor Marc Lipsitch from the Harvard T.H. Chan School of Public Health, new viruses behave differently to ones that have been in the population for a long time. 

To predict what might happen, we need to look at the reasons why viruses are seasonal

The reasons behind the seasonality of viruses are not fully known, however, there are several related factors that likely play a role. 

Temperature, moisture, dehydration, and UV light can impact on the effectiveness of viruses. A study of cases of Covid-19 in China and other affected countries found that absolute humidity had a positive, and temperature a slightly negative, association with the growth of Covid-19. However, the authors of this study also warn that “weather alone… will not necessarily lead to declines in case counts without the implementation of extensive public health interventions”.

There are also differences in human behaviour during colder months. People are more likely to spend time indoors and transmission of viruses is more likely in crowded indoor environments.

In addition, the human immune system becomes slightly depleted in colder months. Vitamin-D and melatonin, both related to daylight, are thought to contribute to this phenomenon.

A fourth factor, as explained by Professor Lipsitch, is the depletion of susceptible hosts. This means that, at a certain point, so many people are or have been infected that there are less susceptible people to infect.

Importantly, SARS-CoV-2 is so new that very few people are immune to it. This means that there is an abundance of susceptible hosts to infect and, therefore, this new coronavirus is unlikely to behave like other well established seasonal viruses.

Whatever the effect of temperature is on SARS-CoV-2, we will still need to take precautions

The US Centres for Disease Control and Prevention states that “at this time, it is not known whether the spread of COVID-19 will decrease when [the] weather becomes warmer”. It also says that, whilst seasonal viruses like the common cold and flu spread more in the winter, it is still possible to catch these illnesses in other months.

The WHO has stressed that cold weather and snow cannot kill SARS-CoV-2, reiterating the advice that “The most effective way to protect yourself against the new coronavirus is by frequently cleaning your hands with alcohol-based hand rub or washing them with soap and water.”

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