This year’s influenza wave is continuing, even if the high values from the end of 2022 are no longer reached. A recent US study using data from 2021/2022 could explain why influenza is so common this season: previously lower disease rates and fewer vaccinations during the Covid 19 wave.
Melissa Rolfes of the US Centers for Disease Control and Infectious Diseases Division of Influenza and her co-authors analyzed past influenza infections with strain A(H3N2) in US households in five US states. The data from households in the period before Covid-19 (2017 to 2020) and during the 2021/2022 influenza season were compared.
Significantly more infections
In their study published on Thursday on the JAMA Network, the scientists found clear differences between the time before and during Covid-19: “During the prepandemic influenza seasons, 152 cases were registered among 353 household contacts. During the 2021/2022 influenza season there were 84 cases among 186 household contacts.” The infection rate before Covid-19 in the closest private environment was 20.1 percent, but in 2021/2022 it was 50 percent or 2.31 times higher.
The investigation only concerned confirmed infections with the influenza virus of subtype A(H3N2), which has been dominant in seasonal “real flu” in recent years. A(H1N1)pdm09 followed in second place as a result of the “swine flu” pandemic of 2009. In Austria, A(H3N2) has been dominant in the current flu wave so far.
The US scientists have several possible explanations for the phenomenon of increased infections in households: In the years before 2021/2022 there would have been only relatively weak waves of influenza. This could have led to less immunity from surviving diseases. A drop in influenza vaccinations during the Covid-19 pandemic might also have contributed to this.
Particularly strong wave
What the US scientists found out for the winter of 2021/2022 is likely to have been repeated in recent weeks – with some variation in terms of timing. As in Europe, the United States experienced an “abnormally” early and severe influenza outbreak in late 2022/early 2023. In the first few days of this year, 79 influenza deaths were recorded among children in the USA. (apa)