In 2020 and 2021, 56.2% of hospitalized COVID-19 cases in Germany were categorized with the principal diagnosis of an acute lower respiratory tract infection [1]. In the meantime, however, approximately 95% of the German population have IgG antibodies against the S-antigen [2]. At the same time, the omicron variant of SARS-CoV-2 became dominant with a lower pathogenicity resulting in a reduced risk for intensive care treatment and mechanical ventilation [3]. Both developments may result in a lower burden of acute respiratory tract infection among hospitalized COVID-19 cases. That is why we analyzed the renumeration data of the national health insurances in Germany (January 2020–September 2022) provided by the Institute for the Hospital Renumeration System [4]. Any patient admitted and treated in a hospital irrespective of the duration of stay was regarded as a hospitalized COVID-19 case when the following criteria were fulfilled: a person with a laboratory confirmed detection of SARS-CoV-2 by PCR or antigen test irrespective of the type and presence of symptoms. A COVID-19 pneumonia was assumed whenever the ICD-10 code for another viral pneumonia was found (J12.8) in combination with detection of SARS-CoV-2 (U07.1). Between January and September 2022, 561,802 COVID-19 cases were treated in German hospitals, which is more in comparison to all cases in 2021 (n = 353,828) and 2020 (n = 176,574). During the same time, the proportion of other viral pneumonia among all hospitalized COVID-19 cases was much lower between January and September 2022 (63,476 of 561,802 cases; 11.3%) compared to 2021 (181,387 of 353,828 cases; 51.3%) and 2020 (85,165 of 176,574 cases; 48.2%). In contrast, the proportions of acute upper respiratory tract infections (J00–J06) increased slightly from 2.1% (2020) and 1.5% (2021) to 3.4% (2022). All other respiratory tract diseases accounted for 11.0% (2020), 7.6% (2021) and 9.4% (2022) of all COVID-19 cases. The proportion of other principal diagnoses in COVID-19 cases has increased from 2020/2021 until 2022 (Fig. 1). They were in 2022 mainly diseases of the circulatory system (I00–I99) with 12.2% (+ 6.2%), injury, poisoning and certain other consequences of external causes (S00-T98) with 8.6% (+ 5.0%), symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R99) with 8.2% (+ 4.5%), diseases of the digestive system (K00–K93) with 5.8% (+ 3.5%) and neoplasms (C00-D48) with 4.7% (+ 3.2%). The proportions of these principal diagnoses among COVID-19 cases are now mostly in the range of the commonly treated principal diagnoses in German hospitals (data not shown) irrespective of the diagnosis of COVID-19. This finding suggests that in 2022, COVID-19 is a secondary diagnosis in the majority of hospitalized COVID-19 cases in Germany, and that a relevant proportion of these patients primarily have a mild course of COVID-19 so that other diseases are the principal diagnosis or some patients had a positive PCR test without any respiratory tract symptoms while being treated for other major diseases. The most likely explanation for our finding is the dominance of the SARS-CoV-2 omicron variant in Germany since January 2022 which is associated with a lower pathogenicity [3]. Our finding may have implications for public health measures.

Fig. 1
figure 1

Source: InEK Datenportal. [4]

Overview on the number of hospitalized COVID-19 cases in Germany between January 2020 and September 2022 and their principal diagnoses according the ICD 10 disease codes.

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