Nosocomial norovirus (NoV) infection is common and may lead to complications in vulnerable hospitalized patients. Understanding sources and modes of transmission of noroviruses within health care settings will support the design of evidence-based strategies for reducing introduction and further spread. We sequenced a highly variable segment of the genome to identify possible clusters in patients with and without acute gastroenteritis who were hospitalized in the period 2002-2007. Admission and sampling dates were used to separate patients with nosocomial infection from those without nosocomial infection. Epidemiological clustering retrieved 22 clusters, defined as >2 patients with nosocomial infection on the same ward within 5 days. In total, 264 patients (of 2,458 tested) were diagnosed with NoV infection, and 61% of the patient strains could be genotyped. Of those, 51% (n ‫؍‬ 82) belonged to GII.4, 34% (n ‫؍‬ 54) belonged to GII.3, and 15% (n ‫؍‬ 24) belonged to other genotypes (GI.6B, GII.17, GII.7, and GII.2). In children's wards, GII.3 strains were associated with nosocomial spread more often than other viruses were, whereas in adults this was the case for GII.4 strains. Sequence alignment recognized 11 new clusters based on identical P2 domains (4 GII.3 and 7 GII.4 clusters), involving patients in different wards. This increased the total number of recognized clusters by 50%. Five of these clusters involved at least one outpatient, providing a possible target for improvement of infection control. We concluded that the use of sequence-based typing should be considered for identifying hidden nosocomial clusters of NoV infections within health care settings.

S ince the end of 2015, invasive meningococcal disease (IMD) caused by Neisseria meningitidis strain W135 has emerged as a severe threat to public health in the Netherlands (1). Before 2015, IMD W135 cases occurred sporadically, averaging 4 cases per year. From 2015 on, the number of cases increased rapidly, to 103 patients in 2018 alone. Cases were reported among persons in all age groups, but the largest numbers of cases were among children <5 years of age, teenagers, and elderly persons. The case-fatality

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