Covid behaves like the flu

Study suggests that the virus is best transmitted at low temperatures and low humidity

3 min
A woman looks at her phone in the Barcelona subway

BarcelonaCovid-19 transmission is seasonal: it spreads better in winter. This is what a study by the Institut de Salut Global de Barcelona (ISGlobal), published this Thursday in the Nature Computational Science journal which argues that SARS-CoV-2 behaves like the flu and colds caused by other coronaviruses. After collecting data from 162 countries around the world - including regions heavily affected by the virus as Lombardy (Italy), Thuringia (Germany) or Catalonia -, the investigation finds that, in addition to the restrictions adopted, the contagions of the first wave decreased when both the temperature and humidity increased, while in the second wave the positive cases multiplied again when the temperature and humidity had fallen. Under these conditions, they stress, the size of the aerosols is reduced and airborne transmission of the virus is accelerated. The researchers therefore stress the "considerable" contribution of aerosol transmission and the need for measures that promote "air hygiene" (such as better ventilation of rooms or installation of air meters) rather than hand hygiene.

Researchers have analysed the association of temperature and humidity in the early phase of virus spread in 162 countries, correlated it with the rate of SARS-CoV-2 transmission, and found that higher transmission rates are associated with lower temperatures and humidity. This theory was initially dismissed by the scientific community because it appeared that infections occurred with the same intensity throughout the year, and an early study with theoretical models supported the thesis that climate did not appear to be an important factor. Early observations already suggested that the initial spread of the virus in China occurred at a latitude between 30 and 50°N, with low humidity and temperature levels (between 5 and 11°C).

Human behaviour was decisive in summer

The first wave was the most aggressive, partly because almost no one had immunity to the virus, and until the fifth wave, researchers argue that the pattern of seasonality played a very relevant role. This summer, however, human behavior was decisive. Despite high temperatures and vaccination, restrictions were lifted all at once, the vast majority of the younger population (the main focus of spread) had not been able to access the vaccine and mass gatherings were held. "If the use of protective measures is more relaxed, the virus spreads even if conditions are not optimal", argued the director of ISGlobal's climate and health program and coordinator of the study, Xavier Rodó, in statements to RAC1.

"The question of whether covid is really a seasonal disease is becoming increasingly important to implement effective interventions", said Rodó, who, in statements to Efe, recalled that this study has developed "one of only two predictive computational models that have been applied in many countries, with very positive results in most states". The team of scientists has also analysed the evolution of this association between climate and disease over time, and whether it was consistent at different geographical scales using a statistical method specifically designed to identify patterns of similar variation.

When they studied the correlations with climate in southern hemisphere countries, where the virus arrived later, they observed the same pattern of behaviour. The effects of climate were most evident at temperatures between 12°C and 18°C and humidity levels between 4 and 12 g/m3, although the researchers cautioned that these are indicative ranges due to the limited time of the records. Finally, using an epidemiological model, the study shows that incorporating temperature into the transmission rate works best for predicting the rise and fall of different waves, particularly in Europe during the first and third waves. "Our results as a whole support the idea that covid is a truly seasonal infection, similar to influenza and the other common cold coronaviruses", concluded Rodó.