A significant decrease in norovirus prevalence and concentration was observed in oyster production areas in Ireland during winter 2020/21. Oyster production areas impacted by human wastewater discharges that were under-going norovirus surveillance since 2018 were investigated. Samples collected in the winter seasons of 2018/19 and 2019/20, prior to when the COVID-19 pandemic interventions were applied, showed a prevalence of 94.3% and 96.6% respectively and geometric mean concentrations of 533 and 323 genome copies per gram (gcg -1 ) respectively. These values decreased significantly during the winter of 2020/21 (prevalence of 63.2% and geometric concentration of <LOQ) coinciding with the control measures to mitigate the transmission of SARS-CoV-2. Divergence between norovirus GI and GII prevalence and concentrations was observed over the three year monitoring period. Norovirus GII was the dominant genogroup detected in winter 2020/21 with over half of samples positive although concentrations detected were significantly lower than pre-pandemic winters with a geometric mean concentration of <LOQ.
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