Clinical and Public Health Lessons from a Point-Source Polymicrobial Foodborne Outbreak: Implications for Vaccination of Vulnerable Populations.

BACKGROUND: Foodborne outbreaks remain a major public health concern, particularly for immunocompromised individuals who experience a disproportionate burden of severe enteric disease. Although environmental prevention represents the cornerstone of food safety, outbreak investigations may also provide valuable evidence for identifying populations that could benefit most from future vaccination strategies. This study investigated a point-source foodborne outbreak in Southern Italy to describe clinical outcomes among exposed individuals, including an immunocompromised oncology patient, and to explore the potential relevance of host vulnerability for future enteric vaccine research. METHODS: A retrospective cohort study combined with an environmental outbreak investigation was conducted following a restaurant-associated gastroenteritis cluster. Clinical data, epidemiological information, and laboratory findings were collected for all exposed individuals. Stool samples were analyzed using multiplex real-time polymerase chain reaction (RT-PCR), while microbiological and molecular analyses were performed on kitchen water samples collected during the official sanitary inspection. RESULTS: Four of six exposed individuals developed acute gastroenteritis (overall attack rate: 66.7%; food-specific attack rate: 100%) within 24-48 h after consuming the same meal. Three previously healthy adults experienced prolonged symptoms lasting 15-21 days and subsequently developed post-infectious irritable bowel syndrome (PI-IBS) during follow-up. In contrast, a 43-year-old woman receiving cisplatin chemotherapy for cervical cancer developed severe dehydration requiring emergency hospitalization. Multiplex RT-PCR identified polymicrobial infection with Campylobacter spp., norovirus, and three diarrheagenic Escherichia coli pathotypes (EAEC, EPEC, and ETEC). Environmental investigation demonstrated contamination of the restaurant's kitchen water supply with total coliforms (>80 CFU/100 mL), intestinal Enterococci (30 CFU/100 mL) and norovirus genogroups I and II, confirming the common source of exposure. CONCLUSIONS: This outbreak demonstrates how identical environmental exposure may result in markedly different clinical outcomes according to host immune status. While rigorous food safety measures remain essential, these findings may help develop hypotheses regarding populations that could be considered in future studies of enteric vaccination strategies. Outbreak investigations may therefore contribute not only to source identification and outbreak control but also to informing future vaccination strategies for vulnerable populations.