Sars-CoV-2: What is summer doing with the virus?

Christiane Fux studied journalism and psychology in Hamburg. The experienced medical editor has been writing magazine articles, news and factual texts on all conceivable health topics since 2001. In addition to her work for, Christiane Fux is also active in prose. Her first crime novel was published in 2012, and she also writes, designs and publishes her own crime plays.

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Some viruses are "weather sensitive": flu viruses, for example, are mainly transmitted in the winter months. In the warmer seasons, however, they hardly spread. Does that also apply to Sars-CoV-2? Studies show that a special climatic factor is of particular importance for the spread.

Despite relaxed contact restrictions, the infection rates - apart from local outbreaks - remain stable. Temperature, humidity and UV radiation could all contribute to this. So researchers at Mount Auburn Hospital have examined the impact of these factors on COVID-19 numbers. To do this, they used data that had been collected in the USA in spring 2020.

Cold favors, heat slows you down - a little

In fact, the coronavirus spreads faster when it is cold than when it is warm. At outside temperatures of one degree or lower, new infections occurred comparatively often, the researchers found.

The number of cases decreased with increasing temperatures. However, this only applied up to temperatures of up to 11 degrees Celsius. If the temperatures continued to rise, however, they hardly dampened the infection rate any more. The expected summer heat will therefore not have a serious impact on the pandemic. A higher UV index can also reduce the spread - but the effect is quite small.

"It is unlikely that the temperature will reduce the spread of the disease dramatically in the summer," says Shiv Sehra, who is the director of studies at Mount Auburn Hospital, who also teaches at Harvard Medical School.

Why humidity is crucial

However, another climatic factor could have a significantly greater influence: the humidity. "When this is lower, the aerosols shrink," explains Prof. Michael Ward, epidemiologist at the University of Sydney. These are tiny droplets that are expelled when speaking, sneezing or coughing - but also when breathing, and contain coronaviruses in infected people.

The shrunken infectious droplets are lighter and could stay in the air longer. This increases the risk of infecting other people. “When the air is more humid, the droplets stay bigger. They are more likely to fall to the ground, ”says the researcher.

In fact, the researcher and his team had found that a one percent decrease in humidity increased the number of Covid 19 cases by six percent.

More infections again in winter?

Covid -19 could therefore be a seasonal disease that returns increasingly in times of lower humidity. Since the air can absorb less moisture at colder temperatures, the effectiveness of the viruses could be significantly higher in winter. "Moisture could be the main driver here - even more so than cold temperatures"

In regions with low humidity or during periods when the humidity is decreasing, there could also be a higher risk of the virus spreading more successfully during the summer months. "So we have to stay vigilant," says Ward.

The researcher and his team had evaluated around 750 cases of Covid-19 that occurred between February 26 and March 31, 2020 - most of them in the Sydney and New South Wales area. In doing so, they linked the postcodes of the patients with the data from the nearest weather stations with regard to precipitation, temperatures and humidity.

Staying outdoors protects

The fact that Sars-CoV-2 spreads less on warm days than on cooler days could be influenced by another factor: In summer, people spend much more time outdoors. There, however, the air movements quickly blow out virus-containing aerosols. According to studies, the risk of being infected outside is 18 times lower than in closed rooms.

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