“The truth is, we don’t know,” says Berit Lange. For the epidemiologist, the current wave of colds in children is no surprise, but why the infections are pushing clinics to their limits is still a mystery.

“Is this only happening because a lot of kids haven’t gone through the infection before? Or are there additional effects due to the dynamics of the wave?” asks the head of clinical epidemiology at the Helmholtz Center for Infection Research. “And for the single child born in 2020, is RSV worse or less bad now than it was in pre-pandemic years?”

RSV, the respiratory syncytial virus (RSV), currently causes the majority of all hospitalizations in children, says Johannes Hübner, deputy clinic director and pediatric infectiologist at Dr. from the Hauner Children’s Hospital in Munich. “The children who are causing us the most problems right now have RS viruses, which are increasing rapidly. And that also brings us to the limits of our capacities.”

However, it is by no means only these viruses that many more children are currently suffering from than is usual at this time. The doctor explains that some children are also seriously ill with rhinoviruses.

Other clinics confirm the picture of a severe, unusual wave of respiratory infections. According to Claus Peter Schmitt, the managing senior physician at the Center for Pediatric and Adolescent Medicine, “currently noticeably more children” are being treated at the Heidelberg University Hospital for severe respiratory infections. But there are “no signs that the courses are more difficult this year”

“We are also currently recording an increased number of small children with respiratory infections in the children’s UKE of the University Hospital Hamburg-Eppendorf (UKE),” writes Jun Oh, Deputy Director of the Clinic and Polyclinic for Pediatric and Adolescent Medicine of the UKE in Hamburg on request.

RSV is also the biggest problem in Hamburg. “Especially newborns, small children and children with previous lung diseases or other chronic diseases have an increased risk of falling ill.” At the moment, however, the wave is not quite as severe there as in other federal states.

Experts agree that the current wave of the disease is unusual – and unlike Sars-CoV-2, this is not related to new properties of the virus. “In the case of RSV, it is not the case that new variants have appeared here, but that a significant proportion of the children did not go through the initial infection with the virus at the normal time,” explains Lange.

The corona measures had meant that people were generally much less infected with respiratory pathogens. The normal rhythm of seasonal infections was disrupted, which may even have contributed to reduced mortality outside of the corona waves. The price for this is that the seasonal pathogens are currently coming back with much more momentum than in the pre-pandemic years. “RSV is also a very good example of what is currently happening with all respiratory pathogens,” says Lange.

Two factors influence when a child first becomes infected with RSV: whether the child is in kindergarten itself and whether they have siblings who are in kindergarten or elementary school. Both lead to a child getting the disease earlier. “Now, of course, you can correlate the RSV infections in a certain region with what measures were taken here before,” explains Lange.

The studies show a certain influence of the elementary schools. “However, we would expect that measures in kindergarten would have the greatest impact and are currently examining this hypothesis in epidemiological studies.”

The experience of the past year also indicates that the kindergarten closures were an important factor in the current wave. As early as August 2021, there was a very early wave of RSV infections in children. “And in the summer of 2021 we still had few measures in the schools, but no longer in the kindergartens,” says the researcher.

Hübner also sees the caught-up infections as an important factor. The term “immune debt” in the current discussion is unfortunate, he says, but there is a real effect behind it: Because of the failed infections, the children are now less protected from these pathogens.

It seems to him that the most likely explanation is that you are now seeing frequent initial infections. “On the other hand, there are also colleagues who think that Corona leads to an immune deficiency. But I don’t see any evidence of that,” says the doctor. “And it would also be atypical that it only affects respiratory viruses.” Ultimately, however, the cause of the current wave is still unclear.

In any case, it is clear that the annual cold waves after the pandemic will not simply resume their normal pattern. Even before that they were enigmatic in many aspects. Rhinoviruses, RSV, influenza, the endemic corona viruses, parainfluenza viruses – they all occur more frequently in the winter months. They cause the classic colds, the symptoms of which range from a mild cold to a serious flu-like infection with fever and headaches and body aches.

RSV and influenza, on the other hand, are more dangerous. The flu kills several thousand people each year, and RSV occasionally causes a serious form of pneumonia called bronchiolitis in very young children.

This also happens in normal years. “Children become particularly ill when they first get infected with RSV at a very young age,” explains Lange. “It’s the normal RSV wave that we see in hospitals and pediatricians every year.” RSV is part of the cold season, which begins in the fall with rhinovirus infections. In pre-pandemic years, the incidence of infection usually subsided temporarily over the autumn holidays, only to pick up again with a colorful mixture of pathogens. RSV and influenza finally follow after the turn of the year.

The interplay between viruses and population immunity means that the waves of colds occur every year on the one hand, but are usually limited by the waves of the previous year on the other. Depending on the virus, however, two different effects are decisive, explains Silke Buda, team leader influenza at the Robert Koch Institute (RKI).

In the case of influenza, the declining immunity of the entire population plays the decisive role. In addition to the waning of individual immune protection over time, the genetic drift of the virus also influences the severity of the wave. “This can also mean that the flu outbreak starts earlier if the virus has changed significantly.” So more likely in mid to late December than after the turn of the year.

It is different with rhinoviruses and RSV. “It’s just the way it is there that the susceptible group of immune-naïve children is infected first,” says Buda. And these infections are also what you perceive as an RSV wave, even if other parts of the population also become infected. Because once you have had the virus, further infections have almost no consequences thanks to the immune protection. “It is very difficult to understand the spread of the disease in older age groups because RSV is associated with a very mild disease.” That is why the RSV wave consists mainly of children born since the previous wave.

This difference between waves that arise from declining population-wide immunity and those that result primarily from initial infections of immune-naïve young children also applies to the current wave of the disease. Because while the RSV wave of 2021 was expected, the current situation is less clear. According to the latest reporting data, the infection appears to be causing more hospital admissions than the number of reported infections would suggest.

The fact that immunity has declined in adults could play an important role here. The result: They become infected more often than average and therefore often carry the virus to very young children, who then become seriously ill. “Of course, one explanation would be that the dynamics of the wave are so high and so fast that younger children who do not normally get infected are also being reached this year,” says Lange.

Another possibility is that missing infections in the mothers of infants now infected contribute to more serious illnesses. They give their offspring a set of antibodies against infections they have recently had, so that the children have partial protection. Due to the failed respiratory infections, this nest protection could have been missing. However, Johannes Hübner takes a critical view of this assumption.

“Of course, that’s a wild hypothesis, one has to say. Nobody knows exactly what the nest protection against respiratory viruses is like.” The role of the effect is unclear, but it is at least plausible. “That’s one of the reasons why we want to vaccinate pregnant women against influenza: so that they can transmit influenza antibodies to their children.”

However, it is by no means certain whether RSV is currently causing more hospital admissions than the number of cases explains. Above all, it is not as easy to draw conclusions from the surveillance data about seasonal infections as was done during the corona pandemic. Berit Lange also warns of this. “You have to make it clear once again that SARS-CoV-2 is special in terms of the infection-epidemiological data situation.” She points out that up to 80 percent of people infected with Sars-CoV-2 have been tested and identified, which is was never the case with the other respiratory infections.

“Therefore, the correlation between what we see in the surveillance system and what actually happens is not that easy,” says Lange. “In the data that we have, it doesn’t look like there have been significantly more hospitalizations per case in the post-2020 waves.”

Meanwhile, there are early signs that the RSV wave may have peaked. In the data from the National Reference Center for Influenza Viruses at the RKI, the number of reports and the positive rate for RSV no longer seem to be increasing. And with that, the number of seriously ill children could soon drop again. However, it is unclear whether this will relieve the clinics in the foreseeable future. Because the cold season is just beginning – and for months experts have been puzzling over how the most deadly of the seasonal viruses will come back from the pandemic break.

“I’m a bit worried about how things are developing with influenza,” says Silke Buda. “The flu wave started two weeks after RSV and is gaining momentum.” Most recently, the reported cases of flu had doubled from week to week – to around 13,000 evidence in calendar week 47. The flu was also almost completely gone during the pandemic for a long time disappeared from the scene. A variant of the influenza B virus – the Yamagata line, which was partly responsible for the very severe flu season 2017/2018 – may even have died out as a result of the measures.

However, at the same time, the immune protection in the population has decreased significantly, which plays a greater role in the wave in influenza than in RSV. Because the influenza does not only affect the youngest, explains Buda. All age groups are affected by a flu epidemic. “The flu now appears to be spreading from school children to the younger as well as the elderly.” Very young children are also particularly at risk from complications from the flu.

The previous season in the southern hemisphere gives the first indications of how the flu will develop this winter. Because countries like Australia had their first wave of flu after Corona six months ago. Historically, the waves there are considered an indication of what winter in Europe could be like. Reassuring: The Australian Department of Health rated the impact of the season as mild to moderate. However, the authority also notes that an above-average number of children were affected – after two years without flu, this may also be a catch-up effect that could be repeated in Europe. “I find it incredibly difficult to say in which direction this is going,” says Buda. “But that’s something we’re watching very closely.”

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The original of this article “Mystery of erratic wave of disease – more and more children are becoming infected” comes from Spektrum.de.