Objective To introduce a retrospective tracing investigation of a norovirus outbreak at Factory A using consumption records, providing a reference for similar outbreak investigations.

Methods Following the Technical Guidelines for norovirus Outbreak Investigation and Prevention and Control (2015 Edition), a field epidemiological investigation and sample testing were conducted. Consumption records from the Factory A canteen were collected and used for a retrospective cohort tracing investigation.

Results A total of 78 cases were reported in this outbreak, with an attack rate of 1.32% (78/5890). Among them, 8 asymptomatic carriers were canteen staff, while the remaining 70 cases were factory workers. The 70 worker cases were distributed across 11 departments in 2 factory zones. The epidemic curve exhibited an intermittent common-source exposure pattern, with potential exposure occurring between the evening of the 15^th and midday on the 17^th, during a total of 7 meal periods. None of the cases had a habit of drinking raw water. The retrospective cohort study showed that dining at the canteen was a risk factor (RR=8.35, 95%CI: 2.63−26.49). Among the 7 meal periods, dining at Catering X was the only risk factor (RR=13.12, 95%CI: 5.22−32.93). Of the 7 suspected meal periods, lunch on the 16^th (RR=3.07, 95%CI: 1.64−5.73) and lunch on the 17^th (RR=2.21, 95%CI: 1.26−3.87) were identified as suspected meal periods. Further stratified analysis showed that both lunch on the 16^th and lunch on the 17^th were risk meals. Laboratory tests revealed that 8 cases, 3 chefs, and 5 vegetable cutters from Catering X were positive for norovirus GⅡ. The dishwashing of Catering X utensils was managed by 11 workers, including vegetable cutters, vegetable washers, steamers, and general workers.

Conclusion Lunch services on the 16^th and 17^th were risk factors, and contamination of utensils by multiple Catering X kitchen staff carrying norovirus was likely the main cause of the outbreak. Consumption records can be effectively used to trace the source of norovirus outbreaks.