The transmission of the common flu or influenza virus is known to be affected by weather. A series of experiments conducted on guinea pigs showed that the virus was better transmitted at low temperatures and low humidity than at high temperatures and high humidity1. The study found that low relative humidities of 20%–35% were most favorable for viral transmission, while transmission was completely blocked at a high relative humidity of 80%. Additionally, when guinea pigs were kept at 5 °C, transmission occurred with greater frequency than at 20 °C, and at 30 °C no viral transmission was detected. However, it should be noted that these results are applicable only to temperate climates which have a winter season. In tropical areas flu infection rates are most closely correlated with the rainy season, when precipitation totals are greater than 150 mm per month2.

Many people are hoping that the coronavirus responsible for the current pandemic, known as COVID-19 will be similarly affected, meaning that as temperatures rise in the northern hemisphere in the approach to summer, the virus’ transmission will cease, or at least slow down. If this will be the case is still unknown. The pandemic outbreak is still in its early stages and therefore the amount of data collected is not yet sufficient to provide definitive conclusions. However, the first results of some preliminary studies are showing some encouraging results.

A paper published in February by the School of Public Health at the Sun Yat-sen University in China3 found that to a certain extent, temperature could significantly affect COVID-19 transmission, with low temperatures being beneficial to the virus’ transmission and that there may be an optimal temperature for the viral transmission.

In March, researchers from the Massachusets Institute for Technology (MIT) published a paper4 where they observed significant implications of weather-related behaviour in COVID-19. For example, the growth rate of cases is higher in colder countries than in warmer countries and the highest growth rates of the coronavirus in the US are in the northern states.

The most notable result they found is that, as of the 17th of March, more than 90% of the COVID-19 cases had occurred in places where the values of absolute humidity lay within the range 4 g/m3 and 10 g/m3. Less than 10% of the cases reported have been in areas where absolute humidity was greater than 10 g/m3 or less than 4 g/m3.

The following graph shows the temperature-relative humidity pair above which the absolute humidity is greater than 10 g/m3:

If new data and evidence confirm these results, it could be expected that as temperatures rise in the northern hemisphere, and as long as humidity levels remain at high enough levels, the transmission capability of the coronavirus could be at least partially reduced.

In the coming days WeatherTrend will publish a preliminary forecast for May where we will analyse the humidity – temperature expectations in Europe, and its possible implications for COVID-19 transmission.

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1. Influenza virus transmission is dependent on relative humidity and temperature. Lowen, 2. A.C. Mubareka S., Steel J., Palese P. (2007) PLoS Pathogens,  3  (10), pp. 1470-1476.
2. Environmental Predictors of Seasonal Influenza Epidemics across Temperate and Tropical Climates Tamerius J.D., Shaman J., Alonso W.J., Bloom-Feshbach K., Uejio C.K., Comrie A., Viboud C.(2013)  PLoS Pathogens,  9  (3) , art. no. e1003194
3. Temperature significant change COVID-19 Transmission in 429 c3.Temperature significant change COVID-19 Transmission in 429 cities Mao Wang, Aili Jiang, Lijuan Gong, WenLina Luo, Wenbin Guo, Chuyi Li,  Jing Zheng, Chaoyong Li, Bixing Yang,  Jietong Zeng, Youping Chen, Ke Zheng, Hongyan Li, medRxiv 2020.02.22.20025791
4. Will Coronavirus Pandemic Diminish by Summer? (March 17, 2020) Bukhari, Qasim and Jameel, Yusuf,. Available at SSRN: 

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