Germany heads into the flu season

The incidence of infections appears to change in winter: Data from the last week of December indicates a decline in visits to the doctor and respiratory illnesses. How strong are the effects of the holidays on the figures? The reports from clinics give us hope.

Four years after the start of the global coronavirus outbreak, infection risks and sickness-related staff absences continue to accompany public life in Germany: In winter 2023/24, coronavirus, RSV and increasingly influenza viruses are increasingly circulating in the population in addition to the usual seasonal cold and cold pathogens, as current data from the Robert Koch Institute (RKI) show.

Since mid-December, Germany has been facing the onset of a flu epidemic. "Influenza cases have so far mainly affected school-age children and young adults," says the current RKI weekly report. "RSV activity and Covid-19 activity remain high." Children under the age of two are particularly affected by more severe courses, including hospitalization with RSV infection, emphasizes the RKI. In older people, on the other hand, corona is currently the most common cause of severe illnesses. Experts urgently recommend vaccination. In the years before corona, the flu epidemic often lasted well into March.

In the virological surveillance samples, the proportion of coronaviruses detected fell to 16.3 percent towards the end of the year. RSV viruses also appear to have passed the peak of the wave that began in the fall. At the same time, however, the proportion of real flu pathogens increased significantly. Since the 51st calendar week, the proportion of influenza in the analyzed samples has exceeded 20 percent.

This means that the guideline has been exceeded: as soon as more than one in five samples tests positive for influenza, the RKI officially declares the start of a flu epidemic.

At the same time, the number of reported visits to the doctor with a Covid diagnosis and the number of respiratory illnesses are falling significantly. So is the worst already over when it comes to waves of infection? The figures are not yet fully reliable, as the RKI points out: "The data from the 52nd calendar week can only be interpreted with restrictions," warns the institute. "In this period, there were also deviations in previous years during the nationwide vacations and public holidays, as well as major subsequent changes to the results."

How many people in Germany are currently infected with which pathogen and how many people have contracted Covid? Answers to these questions can only be determined indirectly since the abolition of mandatory coronavirus testing. The number of coronavirus infections in Germany has no longer been recorded comprehensively since spring 2023.

Since the discontinuation of citizen testing, only those cases that become known to the health authorities at all - for example through systematic reports from clinics and doctors' surgeries - appear in the RKI's case reporting system. The vast majority of infections presumably remain undetected. However, the data from the clinics indicate that the number of severe Covid cases does not increase further during the winter. In most federal states, intensive care units even reported a significant decline in the number of Covid intensive care patients at the beginning of January 2024.

The RKI bases its assessment of the current infection situation on a bundle of other, much softer indicators. One method of recording the general burden of disease in Germany is the GrippeWeb project. Based on voluntary reports from the public, the RKI calculates the so-called ARE rate, i.e. the estimated proportion of the population with an acute respiratory disease (ARE), on a weekly basis.

With an infographic on the ARE rate, ntv.de shows the course of the infection waves in 2023 compared to previous years, with the disease rate given here as a percentage. The result: At the end of December 2023, this key figure remains at the level of the previous winter of 2022/23. Brief explanation: On the RKI website and in the ARE weekly report, the incidence of acute respiratory infections in the population has been given as incidence per 100,000 inhabitants since the beginning of September 2023.

The RKI justifies the change from percentage to incidence with "better comparability of data". The conversion is very simple: "An ARE incidence of 3,000 ARE cases per 100,000 inhabitants corresponds to an ARE rate of 3.0 percent," explains the RKI. The current ARE rate is 7.3 percent. In its projections, the RKI therefore assumes 7300 respiratory diseases per 100,000 inhabitants, with a downward trend.

Based on the ARE rate and incidence, experts can normally reliably identify the start and progression of waves of infection and assess their extent. In the simplified visualization, larger movements are easily recognizable even for laypersons. However, the extent to which current outbreaks are due to the coronavirus cannot be deduced from this information. After all, ARE illnesses can be triggered by various pathogens, from harmless infections with cold viruses (e.g. rhinoviruses) and common colds to genuine influenza, RSV or Sars-CoV-2.

The RKI is attempting to close this knowledge gap through sample surveys. For the"virological surveillance" of the Influenza Working Group (AGI), individual throat swabs from all over Germany are analyzed on a weekly basis and searched for various pathogens in the laboratory. The result is a reasonably reliable picture: the development of the current viral load in Germany can be seen with the proportions of the individual pathogen categories.

However, the sample on which the data is based is comparatively small and its significance is limited. An average of around 240 samples per week were recently analyzed nationwide. Nevertheless, the wide spread of the samples submitted in the time series allows conclusions to be drawn about the development in the population. As a rule, the spread of certain viruses such as influenza, RSV and corona can be easily recognized in these data sets, as can the start or slowdown of any waves of infection.

The Divi Intensive Care Register of the German Interdisciplinary Association for Intensive Care and Emergency Medicine (Divi) therefore remains the most reliable and best source of data on the Covid situation. The data used there comes directly from the hospitals and provides a direct, independent and comprehensive overview of the incidence of infection in Germany. The case numbers collected there form the basis for the most reliable indicator currently available on the coronavirus situation.

The reason is simple: Covid infections are quickly noticed in everyday hospital life. In hospitals, patients are routinely tested for Sars-CoV-2 in suspected cases, as infected patients require special treatment - also to avoid infection among particularly vulnerable patients and hospital staff.

The detection network is therefore comparatively narrow. Laboratory-confirmed cases are routinely reported to the public health department and also to the intensive care register, which can therefore provide one of the few remaining useful key figures for assessing the pandemic situation. The nursing staff and medical staff treating patients are still at the forefront of the corona crisis.

The Divi data can also be used to derive information on regional infection rates in the federal states with little delay. If the number of infections in the population increases, this does not go unnoticed in the hospitals - the Divi data is not unnoticed.

The development in intensive care units also allows conclusions to be drawn about possible changes in the clinical picture. If one of the new coronavirus variants is actually more contagious or causes more severe courses of the disease, this would quickly be reflected in the Divi data. At the beginning of January 2024, the case numbers of Covid intensive care patients are moving downwards in the majority of federal states and therefore still do not signal any alarming developments overall.

Additional data for assessing the pandemic situation is provided by doctors in private practice: According to the RKI, up to 350 medical practices take part in the RKI's nationwide corona monitoring every week. The total number of medically certified Covid-19 infections is also used to calculate an incidence value that reveals nationwide trends.

However, as with most new indicators, the RKI does not carry out regional evaluations - partly because the available case numbers are far too small for a breakdown by federal state. With an extrapolated 23 Covid cases per 100,000 inhabitants, Germany is currently at the level of the corona wave from last spring.

After a long preparation period, information from the so-called wastewater surveillance has also been available since fall 2023. For the project, regular wastewater samples are taken from sewage treatment plants to search for traces of coronavirus genes. The connection is proven: The more Sars-CoV-2 genetic material is found in wastewater, the more the virus circulates in the population.

The nationwide average viral load rises sharply in December 2023. Since mid-November, the RKI has been publishing an independent weekly report with wastewater data from more than 100 locations across Germany. For the RKI and Federal Ministry of Health's pandemic radar, a nationwide cross-section of the viral load in wastewater is calculated every week from the available data.

The advantage of this method is that the key figure is independent of reporting events. Undiagnosed infections are also recorded. The disadvantage is that the procedure is quite complex and not free of methodological limitations. Various influencing factors, such as the "dilution" of the sample due to heavy rain, must be taken into account and factored out during the evaluation. It can take a good two weeks from sampling to publication of the data.

In the nationwide evaluation, the number of reporting locations therefore fluctuates from week to week. In particular, the values for the last reporting week are generally based on incomplete reports and are corrected in the following week on the basis of late reports. The RKI emphasizes that the significance of the results is still limited. The selection is not representative and measurement at all locations is impossible.