An overview of the 2024-25 flu season in the Northern Hemisphere

7 February 2025

As winter firmly grips the Northern Hemisphere, the annual influenza season seems to have reached its peak levels, with influenza A viruses taking the lead in spreading across multiple regions. The season has seen a significant circulation of both A/H3N2 and A/H1N1 strains, with A/H1N1 being the predominant strain in the United States in recent weeks, while A/H3N2 has been more prevalent in Northern Asia. The current level of influenza activity is similar to what was seen before the COVID-19 pandemic, suggesting a return to traditional seasonal patterns.

Hospitals across several countries have reported a surge in influenza-related hospitalizations, with data indicating a peak around late December, early January. This spike in hospital admissions echoes the severity of some of the past severe flu seasons. Influenza activity remains relevant, with stable number in many countries, or increasing numbers reported in some countries following a temporary light decrease.

The World Health Organization (WHO) recommends trivalent vaccines containing specific strains of influenza A/H1N1, A/H3N2, and B/Victoria lineage viruses. This recommendation reflects the likely extinction of the B/Yamagata lineage, which has not been detected in circulation since March 2020, leading to its exclusion from vaccine formulations.

Looking southward for insights, the Southern Hemisphere's last flu season provided some data on vaccine effectiveness. In countries such as Argentina, Brazil, Chile, Paraguay, and Uruguay, the 2024 influenza vaccine showed about 35% effectiveness in preventing hospitalizations among high-risk groups. This interim data suggests that if the same strains dominate in the Northern Hemisphere, the updated vaccines could offer comparable protection. However, the effectiveness of vaccines can vary based on how well the vaccine strains match with the circulating viruses, the health status of the population, and vaccination coverage rates.

There are strong indications that we have passed the peak of the seasonal flu, particularly with Influenza A. However, there's a watchful eye on the B type, specifically the B/Victoria lineage, since the B/Yamagata lineage appears to be extinct. Currently, the B type is on the rise around the Northern Hemisphere, and there might be a late surge, as influenza B can sometimes cause a second wave later in the flu season.