Covid-19 is a seasonal disease that is associated with low temperatures and humidy, like the flu, concludes a ISGlobal study and published this Thursday (21 October) in the scientific journal 'Nature Computational Science'.
The research, developed by Spanish scientists, also highlighted the important role that aerosols play in the transmission of coronavirus infections and the need to adopt measures that promote air hygiene.
By studying what happened around the world at the beginning of the pandemic, before changes in the public’s behaviour and public health policies were implemented, ISGlobal scientists found a negative correlation between the transmission rate and temperature and humidity.
In this way, they discovered that there were higher transmission rates in places with low temperatures and low humidity, something similar to what happens with the rest of the respiratory viruses.
Subsequently, the team analysed the evolution of this association between climate and disease over time.
Experts determined that on a global scale the first pandemic waves decreased with increasing temperature and humidity and vice versa; the waves grew as the temperature and humidity decreased. Specifically, the virus spread more in places with temperatures between 12 and 18 degrees and absolute humidity levels of the air between 4 and 12 g/m3, for example, such as the one Madrid has throughout the year, one of the places of Spain and the world most affected by the virus in the spring of 2020, although scientists warn that these are indicative ranges.
But then they found a paradox: in the summer, on all continents, there was an upturn in infections. Why did the positives go up if it was hotter? "This could be due to several factors, including large concentrations of young people, tourism and air conditioning, among others," said Alejandro Fontal, ISGlobal researcher and the principal author of the study. The authors also began to think that meeting in closed and poorly ventilated places could also be a decisive factor in the increase in cases.
Because at this point another element comes into play. Until Covid-19 came along, scientists believed that most respiratory virus infections occurred when someone expelled infected droplets of saliva that reached another person. But pandemic investigations have turned this theory upside down and experts now agree that most infections occur through aerosols, tiny particles that accumulate in the air indoors and remain suspended even for several hours.
When the humidity drops, the size of the aerosols that carry any respiratory virus is reduced, which can be in the air for longer and thus increases the transmission not only of Covid-19, but also of flu and colds.
"This association justifies placing the emphasis on air hygiene through better ventilation of interior spaces, since aerosols can remain suspended for longer," says Rodó, who underlines the need to include meteorological parameters in the planning and evaluation of control measures.
To better understand the behaviour of the virus and predict the rise and fall of waves, ISGlobal scientists ask that temperature be incorporated as another indicator. "All of our results support the idea that Covid-19 is a truly seasonal infection, similar to the flu and other common cold viruses," reiterates Rodó.