Norovirus is a highly infectious virus from the Caliciviridae family which causes ~700 million acute gastroenteritis cases annually, leading to more than 210,000 global deaths, primarily in children in developing countries. The aim of the study was to conduct a surveillance of circulating norovirus strains in NSW between January 2021 and December 2023. We monitored the changing patterns of viral diversity and levels in New South Wales (NSW) and deciphered the trends influenced by the SARS-CoV-2 pandemic and lockdowns.
Viral RNA was extracted from clinical samples and also monthly wastewater samples collected from sewage treatment plants in Sydney and Melbourne covering a population of 6.4 million. Norovirus RNA was amplified from clinical samples using RT-PCR and amplicons were genotyped by phylogenetic analysis. Wastewater samples were sequenced using Illumina MiSeq and mapped to reference sequences to investigate population norovirus diversity.
Over the three-year period, genogroup II (GII) noroviruses caused 91.7% (n=711/775) of clinical cases and outbreaks, with the remainder comprised of GI noroviruses. On average, patients <5 years account for 47.2% (n=366/775) of samples. The dominant strain in 2021 was the pandemic strain GII.4 Sydney 2012 [P31] accounting for 78.9% (n=146/185) of cases, followed by GII.2 [P16] (15.1%; n=28/185) which was prevalent in 2020. Following the lockdown in 2021, six GII strains associated with past epidemics and childhood infections within the last decade were equally prevalent in 2022. Finally in 2023, GII.4 Sydney 2012 [P16] re-emerged as the dominant strain, accounting for 67.4% (n=196/289) of cases in that year.
Through clinical and wastewater sampling, we have demonstrated that lockdowns can cause sudden shifts in the distribution of norovirus strains. Wastewater analysis provided insight into the diversity and levels of norovirus strains circulating in a population, providing a fuller picture of norovirus molecular epidemiological trends when used in conjunction with clinical analyses.