Why is the flu wave sometimes worse and sometimes less bad ?

The fact that waves of influenza can vary in severity from year to year is due to the constant interplay between genetic changes in viruses and the adaptation of the human immune system to these changes. One example: In one winter there is a particularly severe wave of influenza and a high percentage of the population is infected during the winter. All infected people are now immune to the responsible virus strain.

If the strain does not undergo any serious genetic changes in the next few months, it will not be able to trigger a particularly severe wave of influenza in the following winter, as the majority of people are still immune to it. The opposite example: The winter is mild and the annual flu epidemic is very weak, but in the following months until the next winter the responsible virus strain changes considerably due to gene drift and gene shift. Now everyone, including those who were infected with the strain last winter, is once again at the mercy of the flu and the flu wave hits all the harder.

Flu virus types

Within the group of influenza viruses there are three types that can be considered as the cause of a “real” flu: A, B and C. While type C plays only a very minor role, type B is mainly found in children and adolescents, but usually causes only relatively mild flu illnesses. Type A, on the other hand, is to a certain extent the prototype of the flu virus: it is responsible for the majority of real flu illnesses and can sometimes provoke particularly complicated disease progressions. The pathogens of the Spanish flu, which killed millions of people worldwide in a pandemic around 100 years ago, are also type A, as are the H5N1 avian flu virus and the H1N1 swine flu virus.

Here, a central distinguishing feature of the virus types becomes clear: only type A viruses can also infect other mammals, while humans are the only hosts for types B and C. The RNA of influenza viruses consists of eight segments of a long strand, which in turn contains four different bases that alternate in a fixed pattern – the same construction principle as in human DNA. When the viruses multiply, their genetic material stored in the RNA must also be multiplied. During the copying and assembly processes for the new RNA, errors occasionally occur, usually in the form of point mutations.

This term describes the insertion of a single incorrect base into the base sequence of the newly assembled RNA strand. However, unlike human cells, viruses do not have the appropriate repair mechanisms to correct the errors. The fact that this is not an after effect but rather an advantage for the viruses can be explained as follows: The altered RNA sequence is reflected in a change in the proteins present on the surface of the viruses, to which the human immune cells first have to readjust.

However, this takes some time. In this way, Gendrift contributes to the ability of the flu virus to stay one step ahead of the human defense system, thus preventing the development of immunity to the flu. When two influenza viruses of different strains infect a human cell, one or more RNA segments may be exchanged during viral replication.

This genetic recombination can also change the structure of the viruses’ antigens, i.e. proteins on the surface of the viruses that serve as recognition features for human defense cells. For a certain period of time, the viruses are, so to speak, “undercover” by this modification of their surface proteins and cannot be recognized by the immune system and thus cannot be eliminated. A particularly impressive form of gene shift is the development of completely new subtypes of the influenza virus. Thus, worldwide influenza pandemics are mostly caused by the gene shift-driven exchange of genes between human and avian (bird) influenza viruses.