Risk factors for sporadic norovirus infection: A systematic review and meta-analysis

Norovirus is responsible for 20% of acute gastroenteritis worldwide. The fecal-oral route of transmission is known, but we proposed a first attempt to identify the relative importance of different sources and vehicles for sporadic cases using meta-analysis models. Case-control and cohort/cross-sectional studies were systematically reviewed and analyzed to assess the main risk factors associated with sporadic norovirus infections. Suitable scientific articles were identified through systematic literature search and subjected to a methodological quality assessment. Mixed-effects meta-analyses models were adjusted by population type to appropriate risk factor categories. The quality assessment stage led to include 14 primary studies conducted between 1993 and 2014. From these, eight studies investigated exposures in children/infants, and eight concerned the mixed population.

The meta-analysis confirmed the oro-fecal route for norovirus infections, with the person-to-person transmission (pooled OR=3.002; 95% CI: [2.502-3.062] in mixed population), and the lack of personal hygiene (pooled OR=2.329; 95% CI: [1.049-5.169]). The meta-analysis also enlightened the role of indirect transmission through the environment with pathways like untreated drinking water (mixed population), with a pooled OR=2.680 (95% CI: [1.081-6.643]) and farm environment (children population). Indirect transmission also involved the food pathway, which was finally found significant with consumption of seafood (mixed population) (pooled OR=2.270; 95% CI: [1.299-3.968]) and composite food (eating outside/uncooked mixed and young population) (pooled OR=4.541; 95% CI: [3.461-5.958]).

These results are coherent with the findings from studies on outbreaks. However, a too broad definition of exposure factors limited the interpretation of results, as occurred with the seafood pathways that combined fish and shellfish. Other factors such as consumption of Food-handled products or the type of drinking water deserveE to be better investigated. Furthermore, better harmonization in case definition and appropriate case-control or cross-sectional studies would allow better addressing sporadic cases risk factors, especially for susceptible populations, such as children, elderly or immunosuppressed persons.