What does … the flu actually do? After two years in which the infectious disease seemed to have almost disappeared, more and more experts are asking themselves this question. And it looks like the influenza virus will return in full force that year.

In combination with Corona, this could create a dangerous double wave. Such fears had already existed in 2021. But now there is a crucial difference: the flu is actually back.

As early as spring 2022, Germany experienced a kind of mini flu wave. And throughout the summer, the influenza virus kept appearing in the samples of the sentinel system, which the Robert Koch Institute uses to monitor respiratory infections – in normal summers, no flu has been seen in the data so far.

In the southern hemisphere, especially in Australia, there was a significant number of people suffering from the flu again this year.

In normal years, what happens in the winter months there is an indication of how the northern flu wave will turn out six months later. This time, given the unusual situation, the attention is even greater.

In any case, the fact that the population’s immunity to influenza has decreased due to the lack of contact with the virus speaks in favor of a strong flu epidemic. Children in particular could be particularly badly affected if two cohorts catch up on the missed infections.

In Australia this year, more than half of those hospitalized with influenza were children under the age of 16 – even though children and adolescents rarely become seriously ill.

Last but not least, an influenza epidemic would intensify the consequences of the corona epidemic, which is also expected this winter, for the healthcare system, the economy and the population. “The simultaneous or rapid occurrence of infection peaks can lead to a strain on the healthcare system, due to more patients on the one hand and staff shortages on the other,” Anke Huckriede, professor of vaccinology at the University of Groningen, told the Science Media Center.

The researcher does not necessarily see a severe flu epidemic imminent. “Data from Australia, where the flu season is now ending, does not support this fear,” she said.

While the number of infections detected in the laboratory was very high compared to the last five years – which caused a series of rather pessimistic media reports in English-speaking countries – other key figures, such as doctor visits and days absent, were in the range of inconspicuous pre-pandemic flu outbreaks. The Australian Department of Health rated the impact of the season as mild to moderate. In South Africa and South America, the authorities did not record a particularly strong wave of influenza either.

However, an evaluation of the flu season in the southern hemisphere offers no guarantee that the wave in the northern hemisphere will be similar. Surprises are possible, both ways. “The course of a flu season depends on many different factors and cannot generally be predicted,” says Ralf Dürrwald, head of the National Reference Center for Influenza at the Robert Koch Institute.

However, the data from the RKI showed that respiratory pathogens are currently spreading almost unhindered in the population, and thus also influenza viruses, explains the researcher. “Since the end of September, the number of influenza cases reported to the RKI has increased significantly.”

The physician Markus Rose, medical director of the pediatric pneumology department at the Stuttgart Clinic, is already seeing the first signs of a potentially serious flu epidemic. “In the Stuttgart Olgahospital alone, the children’s hospital of the Klinikum Stuttgart, there are currently seven children with lung infections caused by influenza viruses.”

The fact that the number of cases is already increasing in October instead of around the turn of the year as in normal years, in turn fits in with the experience with the flu epidemic in the southern hemisphere. It also started there about two months earlier than usual. Rose also refers to the experiences with the respiratory syncytial virus (RSV), which caused many infections in children, some of them serious, in 2021. “The high number of infections from Australia was also a harbinger of what awaited us.”

In addition to the unknown reduction in immunity in the population, the genetic changes in the influenza viruses also cause additional uncertainty. The corona pandemic had a significant impact on this. Various previously widespread, known virus lines died out, the flu fragmented into several regional hotspots in which different subtypes dominated.

The consequences are unclear, because how strong the flu epidemic will also depend on which viruses prevail and what properties they have. The genetic impoverishment caused by the corona pause could lead to reduced fitness and thus weaken the coming wave.

However, it would also be possible for previously rare variants to dominate the outbreak, for which both vaccination and previous infections prepare poorly – a particularly unpleasant scenario. After all, initial data from the southern hemisphere make such fears seem unlikely. More than 90 percent of all virus samples examined had antigens similar to the viruses depicted in this year’s flu vaccine, reports the Australian Department of Health.

The viruses used in each case are based on the flu epidemic of the previous year, so it was questionable whether there was any chance at all of selecting the right virus variants. Preliminary data showed that the effectiveness of the vaccines was around the lower end of the average range. That means the vaccine reduces the chance of getting seriously ill by only around 40 percent.

Nevertheless, experts strongly recommend vaccination. “An influenza vaccination is particularly recommended for people over the age of 60, the chronically ill and pregnant women, but everyone else can also be vaccinated, which should be discussed individually in the doctor’s office,” says Ralf Dürrwald from the RKI.

In view of the signs that children and young people could be particularly affected, the doctor Markus Rose also advises going beyond the recommendations of the Stiko: “Since children are a risk group for the real virus flu, it would be even better to basically all children from six months to vaccinate against influenza every five years, as recommended by the World Health Organization,” he says.

In addition to the uncertainties surrounding the flu itself, it is unclear what exactly will happen if the flu and corona rise in parallel. They could spread together and cause a massive double wave – but it is also possible that they compete with each other. In fact, some respiratory viruses, such as rhinovirus and influenza, do not cause simultaneous waves, and there is evidence that the same may be true for influenza and Covid-19. In January 2022, for example, flu numbers rose significantly in the USA and seemed to herald a difficult season. But then came the omicron wave – and the flu numbers plummeted again. In Germany, too, the mini wave only began in the spring when the number of corona infections declined.

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The original of this article “The return of the flu wave” comes from Spektrum.de.