RSV severity in New Zealand 2021 and 2022: applying the WHO severity assessment framework

Respiratory Syncytial Virus (RSV) causes a large burden of severe respiratory disease in the first two years of life and is increasingly recognised as an important contributor to deterioration of respiratory health in elderly populations.1 Following the 2009 influenza pandemic, minor delays in the onset of peak RSV activity in temperate regions were identified, which persisted for three years.2 Implementation of measures including border-closures, social distancing and facemask use, was much greater following the SARS-CoV19 pandemic in 2020. In Aotearoa New Zealand (NZ), RSV transmission was eliminated for 12 months.3 In June 2021, following brief border-opening limited to Australia, RSV with an identical genotype to the circulating Australian strain, emerged in NZ with rapid onset of very high peak incidence.3 Intense, out-of-season RSV transmission in the immediate post-COVID period has been observed in a wide range of countries, with hypotheses for this including immunity debt due to absent RSV transmission, interaction between SARS-CoV-2 and RSV and increased RSV virulence.4 The NZ situation of RSV resurgence prior to community transmission of SARS-CoV-2 in 20223 can shed light on these competing hypotheses.