Because of the corona pandemic: some flu variants have disappeared

There was no flu epidemic last year. Experts fear that it will be particularly violent this year. However, some flu variants seem to have disappeared.

Distance and hygiene measures, contact restrictions and the requirement to wear a mask not only curb the spread of the coronavirus. Other viruses and bacteria also find it difficult to spread. The flu epidemic almost stopped last winter. In the 2019/20 flu season, laboratories in Germany reported 186,919 cases of influenza. In 2020/21 there were only 564.

This winter, however, the flu could return with even greater power – especially if many measures against the corona pandemic are dropped. This is already becoming apparent with the other colds. The number of cases is significantly higher than in previous years, especially among children, says Ulrike Protzer, Professor of Virology at the Technical University of Munich. The children lack the immunity that they would otherwise have built up in recent years, and “if we leave the masks off now, all these cold viruses and also the flu will come back from December.”

A new flu vaccine every year

The immune system is busy fighting off pathogens every day. During the pandemic, however, it was less of a challenge and is now less ready for defense. This also applies to the causative agent of the flu, the influenza virus. A flu shot is therefore particularly important this year. The influenza viruses circulate in Germany from around the beginning of October. The flu season extends into May of the following year. That is why the flu vaccinations are started in October so that as many people as possible are protected by the peak of the wave – usually in January.

The flu vaccine has a different composition every year. There are three different types of influenza viruses with different subtypes. Mutation always creates new strains. Reference laboratories around the world monitor which variants of the influenza virus are currently circulating in certain areas. When it is summer in the northern hemisphere, the researchers particularly observe the infection process in the southern hemisphere. The viruses that spread there are very likely to reach the north six months later. The World Health Organization (WHO) announced in February which four virus strains are likely to spread the most. They are then the basis of the vaccine for the following flu season.

Corona measures also stop the flu

However, the measures against the corona pandemic this year have apparently ensured that the current flu vaccine is effective against a virus strain that probably no longer exists. A research group of doctors at the University of Hong Kong reports that a line of the influenza B virus called Yamagata has been extinct since April 2020. Individual cases have still been registered, but these could be false-positive test results. The H3N2 subtype of the influenza A virus has also often disappeared. The pandemic fight has apparently broken so many chains of infection and nipped smaller outbreaks in the bud that these virus lines have completely or partially disappeared.

However, this is not good news: a virus line that has so far been rare or only locally occurring could at some point trigger a severe flu epidemic, as vaccines provide poor or no protection against it. In addition, the immune system cannot build up any immune protection against influenza viruses that do not circulate.

The effectiveness of the vaccine cannot be predicted

If the current flu vaccine works against a strain of virus that is already extinct, it still covers sufficient virus variants. How effective the vaccine actually is can only be determined after the flu season has ended. If a flu vaccine matches the influenza viruses currently circulating very well, the protective effect can be up to 80 percent. However, if there is little agreement between the vaccine and virus and in older people, it can be significantly lower.

Nevertheless, a vaccination is the most effective remedy for an impending flu epidemic. Its severity cannot be predicted any more than the effectiveness of the vaccine.