Strong Evidence That COVID-19 Is a Seasonal Infection – And We Need “Air Hygiene”

New research provides strong evidence that COVID-19 is a seasonal infection linked to low temperatures and humidity, much like seasonal flu.

A new study led by the Barcelona Institute for Global Health (ISGlobal), an institution supported by the “la Caixa” Foundation, provides robust evidence that COVID-19 is a seasonal infection linked to low temperatures and humidity, just as seasonal flu. The results, published in Nature Computational Science, also support the significant contribution of airborne SARS-CoV-2 transmission and the need to switch to measures that promote “air hygiene”.

An important question regarding SARS-CoV-2 is whether it behaves, or will behave, like a seasonal virus such as the flu, or whether it will be transmitted at any time of the year. An initial theoretical modeling study suggested that climate was not a driver of COVID-19 transmission, given the large number of susceptible individuals without immunity to the virus. However, some observations suggested that the initial spread of COVID-19 in China occurred at latitudes between 30 and 50o N, with low humidity levels and low temperatures (between 5o and 11o C).

“The question of whether COVID-19 is a true seasonal disease is becoming increasingly important, with implications for determining effective intervention measures,” explains Xavier Rodó, director of the Climate and Health program at ISGlobal and coordinator of the study. To answer this question, Rodó and his team first analyzed the association of temperature and humidity in the early stages of SARS-CoV-2 spread in 162 countries on five continents, before implementing changes in human behavior and public health policies. The results show a negative relationship between the transmission rate (R0) and both temperature and humidity on a global scale: higher transmission rates were associated with lower temperatures and humidity.

The team then analyzed how this association between climate and disease evolved over time and whether it was consistent across geographic scales. For this, they used a statistical method specifically developed to identify similar patterns of variation (i.e., a pattern recognition tool) in different time periods. Again, they found a strong negative association for short time windows between disease (number of cases) and climate (temperature and humidity), with consistent patterns during the first, second and third wave of the pandemic at different spatial scales: global, countries, to individual regions in hard-hit countries (Lombardy, Thuringia and Catalonia) and even at city level (Barcelona).

The first epidemic waves decreased as the temperature and humidity increased, and the second wave increased as the temperature and humidity decreased. However, this pattern was broken in the summer on all continents. “This can be explained by several factors, including mass gatherings of young people, tourism and air conditioning,” explains Alejandro Fontal, researcher at ISGlobal and lead author of the study.

When adapting the model to analyze temporal correlations at all scales in countries in the Southern Hemisphere, where the virus arrived later, the same negative correlation was observed. Climate effects were most apparent at temperatures between 12o and 18oC and humidity levels between 4 and 12 g/m3, although the authors caution that these ranges are still indicative given the short data available.

Finally, using an epidemiological model, the research team showed that including temperature in the transmission rate works better for predicting the rise and fall of the different waves, especially the first and third waves in Europe. “Overall, our findings support the picture of COVID-19 as a true seasonal, low-temperature infection, similar to flu and the more benign circulating coronaviruses,” said Rodó.

This seasonality could be a major contributor to SARS-CoV-2 transmission, as low humidity has been shown to reduce the size of aerosols and thereby increase airborne transmission of seasonal viruses such as flu. “This link justifies the emphasis on ‘air hygiene’ through improved indoor ventilation, as aerosols can linger for longer periods of time,” says Rodó, highlighting the need to include meteorological parameters in the evaluation and planning of control measures.

Reference: “Climatic features in the different COVID-19 pandemic waves across both hemispheres” by Fontal A, Bouma MJ, San José A, Lopez L, Pascual M, Rodó X, October 21, 2021, Nature Computational Science.
DOI: 10.1038/s43588-021-00136-6