Update on influenza epidemiology and vaccines

Abstract

COVID-19 pandemic mitigation measures, including travel restrictions, effectively limited global circulation of influenza viruses for much of 2020 and 2021. Travel bans and quarantine requirements were lifted on 1 November 2021 enabling recirculation of influenza. In Victoria, follow up of early influenza cases detected Dec–Feb revealed a history of travel or contact with a recent traveller. Cases were few during the Omicron epidemic in January–February. However, in late March, there was a sharp increase in cases, which in Victoria was associated with an outbreak in university residential colleges. Genomic sequencing supported a likely point-source outbreak that spread throughout the state and to other jurisdictions. In the Northern Territory and New South Wales circulation of A(H1N1)pdm09 preceded circulation of A(H3N2) and was associated with a high rate of paediatric hospitalisations. Both the A(H1N1)pdm09 and A(H3N2) viruses that circulated were well matched to the vaccine, but the early onset of the epidemic meant that many cases were infected prior to availability of vaccines in April. Nationally, circulation declined sharply in June–July coincident with another COVID-19 wave. Circulation of influenza may continue to be disrupted by SARS-CoV-2 epidemics until it establishes a seasonal pattern, which may impact seasonal influenza preparedness.