Seasonal flu: towards more accurate predictions of circulating viruses
11.03.2026
Every winter, there is talk of the "flu," but the viruses in circulation are not always the same. The main ones are three: type B and the A/H3N2 and A/H1N1 subtypes of type A influenza. Their spread varies from one season to another and from country to country, and in some cases, one of these can become predominant.
Predicting which subtype will dominate is important because different viruses do not have the same clinical impact and do not affect vulnerable groups, such as children and the elderly, in the same way.
These mechanisms are highlighted in the study "Characterization and Forecast of Global Influenza Subtype Dynamics," published in Nature Health and led by a team coordinated by Chiara Poletto from the Department of Molecular Medicine at the University of Padua and Francesco Bonacina, a researcher at Bocconi University.
The research shows that, in most flu seasons, the three subtypes circulate simultaneously, albeit in different proportions. However, there are exceptional seasons where almost exclusively one virus prevails, such as in 2009-2010 with the A/H1N1 pandemic and during the COVID-19 pandemic, when reduced travel and social contact limited the circulation of flu viruses.
One of the most significant findings concerns the role of international mobility. Countries where the subtypes circulate in similar proportions are also those most connected by air travel. This confirms that viruses spread following people's movements and suggests that observing what happens abroad can help predict the flu trend in the Italy as well.
For the study, data collected weekly by the World Health Organization in over 150 countries between 2000 and 2023 were analysed using a statistical approach called Compositional Data Analysis, suitable for comparing percentage data from different contexts.
Focusing on the period 2010-2019, the researchers identified areas of the world where flu subtypes follow similar patterns. Models considering the global geographical structure proved to be more accurate than those based only on local trends.
According to the authors, this discovery can provide health authorities with new tools to anticipate the evolution of seasonal flu and improve preparedness for future epidemics.
