Mitigating norovirus spread on cruise ships: a model-based assessment of diagnostic timing and isolation

Background: Acute gastroenteritis outbreaks caused by noroviruses are a common public health issue on cruise ships. Understanding the main drivers of sustained outbreaks is critical for evaluating the effectiveness of preventive interventions such as the isolation of infected individuals. Methods: We analysed a line-list of 121 cases from a norovirus outbreak on a cruise visiting Mediterranean ports (cumulative incidence among passengers 9.7%). We used a Bayesian inference model to reconstruct likely transmission chains, taking into account different transmission settings and the isolation of cases after diagnosis. We then calibrated a branching process model to simulate alternative isolation scenarios and estimate their effectiveness in reducing transmission. Results: Reconstructed transmission chains revealed a high heterogeneity in individual transmission, with 57% (95% CrI: 48%-65%) of secondary cases caused by 10% of infected individuals (here termed 'superspreaders'). Superspreaders exhibited longer diagnostic delays (mean 83 hours, 95% CrI: 70-96 hours) compared to other infectors (mean 47 hours, 95% CrI: 44-50 hours) and a halved frequency of vomiting and diarrhoea episodes. The 72-hour isolation protocol implemented during the outbreak averted 71% of potential cases compared to a no-intervention scenario, halving the effective reproduction number from 9.8 (95%CrI of the mean: 7.1-12.7) to 4.9 (95%CrI: 3.0-7.1). Reducing diagnostic delays further reduced the effective reproduction number, resulting in lower case numbers and probability of sustained outbreaks. Conclusions: Timely diagnosis and isolation have a remarkable impact on norovirus containment on cruise ship outbreaks. Targeted information campaigns encouraging passengers to seek immediate medical assistance upon gastrointestinal symptoms can significantly improve outbreak management.