Introduction:
In May 2024, UKHSA was notified about an ongoing gastro-intestinal (GI) outbreak on a cruise ship over several weeks. The proportion of passengers and crew affected was 5-8% but quickly went over 10%. Norovirus was detected at point of care testing. UKHSA convened an outbreak control team to contain the infection and respond to public enquiries.

Methods:
Two OCT meetings were held to gather information, complete a risk assessment, and agree on measures. The OCT included UKHSA border health, international travel health, port health, public health and emergency preparedness, local infection prevention control, communications, harbour master, MCA, public health and medical officers from the cruise ship company.

Results:
Actions after initial meeting:

IPC measures at sea and in port:

• Improved hand hygiene, signage, stop alcohol hand gel
• Increased isolation of those affected to 48 hours after last symptom¹,
• Extended firebreaks between cruises,
• Improved cleaning agents, equipment, and methods

Boarding ship to review measures, training and sanitation schedules

Improved communication strategy to crew and passengers

Initial actions resulted in a drop to baseline levels of GI illness. Four weeks later, 7% of passengers and crew reported GI symptoms, with crew consistently being early cases. A second OCT meeting advised:

Cohort staff by duties on board including accommodation and eating arrangements

Review crew isolation measures

Reduce mixing of crew during firebreaks

Further measures were intended to create firebreaks at sea. A sustained drop to baseline levels of GI illness was reported after further measures were put in place.

Conclusions:
The low infectious dose of Norovirus, combined with its environmental stability afford it multiple routes for transmission. Within the context of maritime outbreaks, which are principally on enclosed platforms, the role of vomit-oral transmission in persistence and recurrence of outbreaks shouldn't be underestimated.