Could Warmer Weather Slow the Spread of Coronavirus?

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The idea that warmer weather will help us in the fight against COVID-19 is widespread, including amongst government officials responsible for the world’s response to the disease. Donald Trump, for instance, said earlier in March that the crisis would be over by April, claiming that ‘The heat, generally speaking, kills this kind of virus’.  Meanwhile UK Health Secretary Matt Hancock told ITV that the hope was to slow the transmission of the virus so if it does cause an epidemic here, it will happen in the spring and summer months when other coronaviruses are, generally speaking, less transmissible. But how much hope should we actually place on the weather slowing the spread of COVID-19?

When Trump said ‘this kind of virus’, he was probably referring to how the group of influenza viruses that cause the seasonal flu are more common in the winter months, a pattern that other coronaviruses also share. The reason for this seasonality is not fully known, but there other multiple factors that likely play a role.

This includes elements like temperature, moisture and dehydration, with studies demonstrating that cold and dry environments tend to favour the growth and spread of flu transmission. However, a study investigating the role of humidity on the spread of COVID-19 in China point out that the relationship between the two has not yet been established and concludes that ‘changes in weather alone will not necessarily lead to declines in COVID-19 case counts’.

The colder weather of winter also tends to weaken our immune system, possibly because of the lack of sunlight and lower vitamin D levels that we get. We also tend to spend more time indoors during winter, often in crowded areas. Schools are particularly a hotbed for the spread of diseases; in 2009, the swine flu pandemic dipped significantly in the US during the school summer holidays but re-surged when pupils returned in September.

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With this, it’s clear where our political leaders are coming from, but there is no guarantee that COVID-19 will act the same as other similar diseases because this virus is a new one. Because of this, there are far fewer people immune to it. This lack of immunity will speed up the spread of COVID-19 faster than any slowing power that factors like heat and humidity could slow it. As Havard’s Marc Lipsitch writes,

New viruses have a temporary but important advantage – few or no individuals in the population are immune to them. In simple terms, viruses that have been around for a long time [unlike COVID-19]can make a living — spread through the population — only when the conditions are the most favourable, in this case in winter

However, SARS, another influenza virus that rocked the world in 2002-2003, did ease over the summer, raising hopes surrounding the same for the pandemic that we’re currently facing. But, Lipsitch stresses, ‘SARS did not die of natural causes’ and it was not the heat alone that stopped it. Instead, it was the ‘extremely intense public health interventions’ of governments that ended the crisis.

Thus, even if the warmer weather of spring and summer could end the spread of COVID-19 in the UK, it’s important not to rely on it and act as if it won’t. The WHO has stressed the importance of protective measures, like regularly cleaning your hands and avoiding touching your face ‘Regardless of climate’.

Looking at what has happened so far, if the spread of COVID-19 in Australia is anything to go by, there’s not much evidence to suggest that summer in the northern hemisphere will slow its spread.

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