Follow-up evaluation for norovirus asymptomatic infection among healthy adults: a prospective matched cohort study.

Objectives: No study has evaluated the follow-up of asymptomatic norovirus infection among healthy adults. This study compared norovirus detection rates between previously known norovirus-positive and norovirus-negative adults without diarrheal symptoms, and evaluated reinfection risk among the previously-norovirus-positive group, and new asymptomatic norovirus infection risk among the previously-norovirus-negative group after 600 days.

Methods: This prospective age- and sex-matched cohort study, conducted in 2019 at a Japan teaching hospital, included apparently healthy asymptomatic adults with a positive norovirus result and those with a negative result (ratio 1:3) during the 2016-2017 screening. The primary outcome was real-time reverse-transcription polymerase chain reaction-confirmed norovirus in stool specimens. We evaluated descriptive statistics and associated factors, including demographics, social habits, and clinical parameters.

Results: Of 288 participants [mean age, 59.9 (standard deviation: 12.6) years; male, 143 (49.7%)], 73 [genogroup (G) I 35; GII 37; both included 1 each] were positive for norovirus previously, while 215 were negative. After a median of 599 (interquartile range 515-799) days between baseline screening and follow-up, 14 (4.9%; GI 0; GII 14) tested positive for norovirus (2.7% and 5.6% among positive- and negative-norovirus groups at baseline, respectively). Among previously norovirus-negative participants, being older, having elevated blood pressure and haemoglobin A1c level, and drinking Japanese sake at baseline were associated with positive results at follow-up.

Conclusions: Genogroup homotypic protective effect may exist for subsequent asymptomatic infection. There may be higher risks of future asymptomatic norovirus infection in previously no-norovirus asymptomatic infection people with specific lifestyles or medical histories. SUMMARYThe detection follow-up rates of norovirus were 2.7% and 5.6% among asymptomatic adults with positive- and negative-norovirus status at baseline, respectively. Specific lifestyles or medical histories may confer higher risk of norovirus detection.