Increased norovirus activity associated with the emergence of GII.17[P17] in season 2023/24 in Germany

Sandra Niendorf, Robert Koch Institute

Mirko Faber (Department of Infectious Disease Epidemiology, Robert Koch Institute, 13353 Berlin, Germany) , Sonja Jacobsen (Consultant Laboratory for Norovirus, Department of Infectious Diseases, Robert Koch-Institute, 13353 Berlin, Germany), Andi Krumbholz (Labor Dr. Krause & Kollegen MVZ GmbH, 24106 Kiel, Germany; Institute of Medical Microbiology, Kiel University, 24105 Kiel, Germany), Julia Enkelmann (Department of Infectious Disease Epidemiology, Robert Koch Institute, 13353 Berlin, Germany), Sandra Niendorf (Consultant Laboratory for Norovirus, Department of Infectious Diseases, Robert Koch-Institute, 13353 Berlin, Germany)

Molecular Epidemiology & Evolution

Human norovirus, a notifiable pathogen in Germany, is the leading cause for viral gastroenteritis worldwide. Over the past 20 years, the successive emergence of several variants has led to the predominance of GII.4 in Germany. During the 2023/24 season, we observed a steep increase in norovirus cases in parts of Germany as compared to the five seasons prior to the COVID-19 pandemic (2015/16 to 2019/20). In Berlin and Brandenburg (BBB), norovirus incidence remained well below pre-pandemic seasons (mean weekly incidence 0.93 vs. 2.32 per 100,000 population) during the second half of 2023. However, in January 2024 the incidence increased steeply (peak in week 5; 9.26 per 100,000) and remained elevated throughout the first half of 2024 (mean weekly incidence 3.89 vs. 2.34 in pre-pandemic seasons). A similar epidemiological pattern was observed in other federal states, but the incidence differences were less pronounced. Genotype distribution showed a sharp increase of genotype GII.17[P17] during the 2023/24 season. GII.17[P17] was detected in 78% in BBB and in 49% of samples in the remaining federal states, compared to only 3% and 5% in the pre-pandemic seasons, respectively. Interestingly, similar observations were reported from five other European countries and the USA. Phylogenetic analysis showed a distinct clustering of this predominant GII.17 as compared with previously circulating GII.17 linages. The new GII.17[P17] variant showed 14 amino acid changes in VP1 when compared with GII.17/Kawasaki323 strain and demonstrated a different antigenic profile. This study highlights the unpredictability of norovirus's evolution. Continuous monitoring is essential to inform the development of vaccines and therapeutics.