A Decade of Endemic Human Coronaviruses: Epidemiological Patterns and Clinical Significance in an Adult Cohort at a Tertiary Care Hospital in Germany

While SARS-CoV-2 dominated headlines for years, four other human coronaviruses have been quietly circulating in the background, causing seasonal respiratory illness around the globe. HCoV-OC43, HCoV-NL63, HCoV-HKU1, and HCoV-229E are endemic pathogens that most people encounter repeatedly throughout their lives, typically producing mild cold-like symptoms. Yet despite their ubiquity, detailed epidemiological data on these viruses remain scarce, particularly in Germany, where systematic surveillance of endemic coronaviruses has been limited. A new retrospective study published in BMC Infectious Diseases by researchers at the University of Leipzig Medical Center now provides one of the most comprehensive pictures to date of how these viruses behave in an adult hospital population, drawing on more than a decade of diagnostic data.

The research team, led by Christian Propach and Mario Hönemann of the Virology Department at Leipzig University Medical Center, analyzed over 29,000 respiratory specimens collected between 2012 and 2022. Using nucleic acid amplification assays capable of detecting multiple respiratory pathogens, the investigators identified 469 unique adult cases of endemic human coronavirus infection across 11 seasons. This unusually long observation window allowed the team to capture year-to-year variation in viral circulation that shorter studies inevitably miss, revealing patterns that carry important implications for understanding coronavirus biology more broadly.

Among the four endemic species, HCoV-OC43 emerged as the most frequently detected, accounting for 38.6 percent of cases, followed by HCoV-NL63 at 25.6 percent, HCoV-229E at 22.4 percent, and HCoV-HKU1 at 13.4 percent. All four viruses exhibited pronounced seasonality, with the vast majority of detections occurring during winter months and activity peaking between January and March. This winter concentration mirrors the behavior of other respiratory viruses such as influenza and respiratory syncytial virus, though the researchers also documented sporadic summer detections of HCoV-OC43 and HCoV-NL63, indicating that these pathogens never entirely disappear from the population.

Perhaps the most intriguing finding concerns the circulation dynamics of individual species. While HCoV-NL63 was detected year-round, HCoV-OC43 and HCoV-HKU1 displayed a striking biennial pattern, alternating between years of high and low activity. No consistent cyclic pattern was observed for HCoV-229E. The authors suggest that this alternating behavior may reflect immunological interactions within the coronavirus family, a phenomenon in which infection with one species generates cross-reactive immunity that temporarily suppresses circulation of another. Similar mechanisms have been proposed for other virus groups, and the biennial rhythm observed here provides some of the clearest real-world evidence that such interference may operate among endemic human coronaviruses.

Beyond epidemiology, the study yielded clinically significant findings about disease severity. HCoV-OC43 infections were more frequently associated with dyspnea, occurring in 48.1 percent of cases, lower respiratory tract infection at 45.3 percent, exacerbation of obstructive lung disease at 17.6 percent, and systemic prednisolone administration at 16 percent. These associations were statistically significant, suggesting that OC43 may be particularly prone to driving lower airway disease in vulnerable adults. The finding that corticosteroid use was more common in OC43 cases underscores the clinical burden this virus imposes on patients with pre-existing respiratory conditions.

HCoV-HKU1 told a different story. Although it was the least frequently detected species, HKU1 infection was significantly more common among patients with lymphoma, appearing in 35.6 percent of HKU1 cases, and was associated with higher in-hospital mortality at 18.6 percent. This finding raises questions about whether HKU1 disproportionately affects immunocompromised individuals or whether the association reflects underlying differences in patient populations. Either way, the result highlights that endemic coronaviruses are not uniformly benign and that certain species may pose heightened risks to specific patient groups.

Co-infections emerged as another critical determinant of outcome. The researchers identified 119 co-infections among the 469 cases, representing 26.0 percent of the cohort. These included viral pathogens in 10.9 percent of cases, bacterial pathogens in 10.5 percent, and fungal pathogens in 1.7 percent. Co-infections were significantly associated with lower respiratory tract infection, need for mechanical ventilation, and ICU admission, indicating that the presence of additional pathogens substantially worsens clinical course. This observation aligns with growing recognition that respiratory viral infections rarely occur in isolation and that polymicrobial disease carries disproportionate severity.

The study’s conclusion emphasizes that the clinical presentation and severity of endemic coronavirus infections depend heavily on patient age, comorbidities, and the presence of co-infecting pathogens. This nuanced picture challenges the common perception of these viruses as mere cold-causing nuisances. In hospitalized adults, particularly those with underlying conditions such as obstructive lung disease or hematologic malignancies, endemic coronaviruses can trigger serious respiratory illness requiring intensive care. The findings carry practical implications for diagnostic testing, infection control, and clinical management in tertiary care settings.

From a broader scientific perspective, the decade-long dataset offers valuable context for understanding coronavirus behavior in the post-pandemic era. The biennial cycling of OC43 and HKU1, the year-round presence of NL63, and the irregular pattern of 229E suggest that these viruses occupy distinct ecological niches shaped by population immunity, viral interference, and possibly environmental factors. As researchers continue to investigate whether endemic coronaviruses influence SARS-CoV-2 transmission dynamics, longitudinal datasets like the Leipzig cohort become increasingly important for disentangling these complex interactions.

The research, conducted in accordance with the Declaration of Helsinki and approved by the Ethics Committee of Leipzig University, relied on anonymized biological material obtained for standard viral diagnosis with written informed consent from all participants. The team acknowledges that retrospective single-center designs have limitations, including potential referral bias inherent in a tertiary care population. Nevertheless, the consistency of the seasonal and species-specific patterns across 11 seasons lends considerable weight to the findings. As climate patterns shift and respiratory virus epidemiology evolves, continued surveillance of these endemic pathogens will be essential for anticipating future circulation trends and preparing healthcare systems for the clinical burden they impose.

Subject of Research: Epidemiology and clinical significance of endemic human coronaviruses in adult patients at a German tertiary care hospital

Article Title: A decade of endemic human coronaviruses: epidemiological patterns and clinical significance in an adult cohort at a tertiary care hospital in Germany

Article References: Propach, C., Hönemann, M., Vučinić, V., Frille, A., Berthold, T., Bergs, S., Maier, M., & Pietsch, C. (2026). A decade of endemic human coronaviruses: epidemiological patterns and clinical significance in an adult cohort at a tertiary care hospital in Germany. BMC Infectious Diseases, 26(1), Article 1706. https://doi.org/10.1186/s12879-026-14377-1

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14377-1

Keywords: endemic coronaviruses, HCoV-OC43, HCoV-NL63, HCoV-HKU1, HCoV-229E, respiratory infections, epidemiology, seasonality, co-infection, tertiary care hospital, Germany, viral interference