influenza viruses are often transmitted to humans mainly in two main groups: influenza A and influenza B. influenza A is the most severe bienator, responsible for most seasonal flu epidemics and all flu epidemics recorded. Influenza B is mainly concentrated in humans.
The influenza A virus has many different subgroups, depending on the combination of two surface proteins, haemagglutinin (H) and neuraminidase (N). The group that is commonly circulating in humans is H1N1 (including the vaccine that caused the pandemic in 2009, now becoming seasonal flu) and H3N2. These clusters may change seasonally due to small changes.
The influenza B virus has two main lines: B/Victoria and B/Yamagata. Of which, the B/Yamagata line has been almost unknown since 2020.
The high diversity of the influenza virus is the reason why influenza vaccines must be produced annually, with a combination of 3 or 4 strains of viruses that were circulating in the previous season.
This year's flu season starts a few weeks earlier than previous years. In Japan, the number of cases has increased abnormally since October.
In the European Union, the flu also appeared earlier than expected. In Spain, the epidemic threshold was broken a few weeks earlier than usual.
Another notable development is the overwhelming dominance of the A/H3N2 influenza virus, especially the K sub-group. Although this year, the A/H1N1 virus has dominated the male hemisphere and the A/H3N2 K sub-group has only broken out in the final stages, in the UK and Japan, A/H3N2 K has become the dominant group. Nearly 90% of flu virus samples in these two countries are currently in sub-group A/H3N2 K. The US and Canada are also recording a similar trend.
Classification K of A/H3N2, formerly known as J 2.4.1, has now been detected on all continents. In the period from May to November 2025, this group accounts for about 1/3 of the total number of A/H3N2 viruses analyzed globally and nearly half in the EU.
Genetic changes of sub-group K of the A/H3N2 virus do not come from the recomboration or mixing between different viruses and are not considered a major change. In fact, this is still the H3N2 virus that has been circulating in the human community since 1968 but has had some additional genetic changes.
This is a normal development, because the flu virus develops through turns every season to avoid the human immune system. In some years, this process is more effective than in other years.
In fact, East Asian countries are currently recording a decrease in the number of K infections and no cases of unusually severe illness have occurred. The analysis also shows that the K sub-group species circulating in these countries are no different from those present in the EU. This gives a positive signal, showing that group K does not tend to cause more serious illness.
In recent flu seasons, A/H3N2 is not the dominant virus, but this position often belongs to A/H1N1. That could reduce the community's immunity level for A/H3N2 due to recent limited exposure.
In addition, the influenza seasons dominated by A/H3N2 tend to be more serious, with lower vaccine effectiveness and higher number of severe cases in the elderly than the influenza seasons dominated by A/H1N1.
