Respiratory viruses no longer ‘seasonal’

Viruses that cause the common cold are now emerging in the summer because of the changes in human behaviour during and after the COVID-19 pandemic, a Portuguese study has revealed.

The analysis of national epidemiological data from the USA and Canada, collected between 2016 and 2023, on influenza viruses (influenza A and B) and other respiratory viruses, such as respiratory syncytial virus and metapneumovirus.

First author Irma Varela-Lasheras, of the LIP-Laboratory of Instrumentation and Particle Physics, Lisbon, said because weather conditions and mobility were decoupled during the pandemic, it was an ideal time to investigate the importance of these two factors individually.

Writing in PLoS Digital Health, the team says its findings reinforce the idea that the dynamics of respiratory viruses involve a complex system of interactions between different factors, such as weather conditions, human behaviour, viral interaction and intrinsic viral characteristics, but that COVID-19 has disrupted the seasonal equilibrium and may never be the same again.

Varela-Lasheras said: “The results show that, before the pandemic, atmospheric conditions (seasonal variation in temperature and humidity) played the predominant role in explaining the dynamics of all these viruses, [while] in the pandemic period, it was human mobility (measured using different metrics derived from anonymised, aggregated mobile device data, such as the number of trips away from residential areas or the proportion of visitors to transit stations) that played the main role in explaining viral dynamics, prevailing over the effects of meteorological conditions.”

Although influenza has a seasonal capacity, this study reinforces that winter is neither a necessary nor a sufficient condition for respiratory epidemics that were previously seasonal. if enough people are susceptible and there are no measures in place to prevent contagion, there can be outbreaks in summer.

However, the team also found that several respiratory viruses were associated with cold and high humidity, but now the equilibrium has been broken, they no longer are seasonal.

Study author Joana Gonçalves de Sá, principal investigator of the SPAC-Social Physics and Complexity group at LIP-Laboratory of Instrumentation and Particle Physics, said: “This work raises three important questions. The first is that we clearly need to study more about the biology and dynamics of these viruses, including the potential interactions between them.

“The second is that infections previously thought to be winter seasonal can appear all year round, and this has important consequences for the surveillance of respiratory viruses and for public health in general. The third is that this ability to infect even when the temperature outside is high has to be framed in a scenario of climate change, which could further disrupt the known viral dynamics.”

The authors add the study results reinforce the importance of public health organisations expanding the monitoring and surveillance periods for respiratory infectious diseases and including new indicators in their surveillance models and systems, particularly taking into account population mobility.

The effects of weather and mobility on respiratory viruses dynamics before and during the COVID-19 pandemic in the USA and Canada. PLoS 21 December 2023


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