In Germany, a second wave of influenza has started this season. According to the definition of the Robert Koch Institute (RKI), the beginning of the second wave is dated the week ending March 5th, as per Wednesday night’s weekly acute respiratory disease report. The second flu wave was triggered by the circulation of influenza B viruses. This means that the flu season in Germany is continuing unusually. At the end of February, the institute had already pointed out that influenza numbers were increasing again.
This season there was already an exceptionally early flu epidemic before the turn of the year. This was caused by influenza viruses of the A(H3N2) subtype and slowed down relatively quickly by the Christmas holidays, so that the official criteria for the end of the wave were already met in the first calendar week of 2023. Compared to the beginning of the first flu wave, influenza activity is currently increasing more slowly and less, according to the RKI.
In the previous seasonal flu waves, after the circulation of influenza A viruses at the beginning of the wave, there was often an increase in the proportion of influenza B viruses in the further course. “But this went smoothly into one another,” the RKI recently explained. Influenza B then led to an extension of the flu wave. An outage with such a sharp drop in influenza activity as this season is unusual.
The results from a monitoring system in which samples from people with acute respiratory diseases are examined are decisive for the assessment. Various pathogens are routinely checked, such as rhinoviruses, Sars-CoV-2 and influenza. The proportion of Sars-CoV-2 infections in the past few weeks has remained consistently low under ten percent in the sample examined.
According to the RKI, the annual wave of flu usually began in January in the years before Corona and lasted three to four months. In the past two seasons, however, the pandemic and the measures taken to counteract it have changed the usual course significantly: In 2020/21 there was no flu epidemic worldwide. And in 2021/22 there was not a wave on the usual scale in Germany either, the number of registrations only rose after the Easter holidays and therefore very late.