The re-emergence of influenza following the COVID-19 pandemic in Victoria, Australia, 2021 to 2022

Abstract

Background COVID-19 pandemic mitigation measures, including travel restrictions, limited global circulation of influenza viruses. In Australia, travel bans for non-residents and quarantine requirements for returned travellers were eased in November 2021, providing pathways for influenza viruses to be re-introduced. Aim We aimed to describe the epidemiological and virological characteristics of the re-emergence of influenza in Victoria, Australia to inform public health interventions. Methods From 1 November 2021 to 30 April 2022, we conducted an epidemiological study analysing case notification data from the Victorian Department of Health to describe case demographics, interviewed the first 200 cases to establish probable routes of virus reintroduction and examined phylogenetic and antigenic data to understand virus diversity and susceptibility to current vaccines. Results Overall, 1,598 notifications and 1,064 positive specimens were analysed. The majority of cases (61.4%) occurred in the 15–34 years age group. Interviews revealed a higher incidence of international travel exposure during the first month of case detections, and high levels of transmission in university residential colleges were associated with return to campus. Influenza A(H3N2) was the predominant subtype, with a single lineage predominating despite multiple importations. Conclusion Enhanced testing for respiratory viruses during the COVID-19 pandemic provided a more complete picture of influenza virus transmission compared with previous seasons. Returned international travellers were important drivers of influenza reemergence, as were young adults, a group whose role has previously been under-recognised in the establishment of seasonal influenza epidemics. Targeting interventions, including vaccination, to these groups could reduce future influenza transmission.

Genomic Analyses Uncover Evolutionary Features of Influenza A/H3N2 Viruses in Yunnan Province, China, from 2017 to 2022

“…However, from 2019 to 2022, the evolution of the Yunnan strains showed relatively stability with a high nucleotide sequence homology, predominantly clustering in the 3C.2a1b.1b clade for 2019–2021 and the 3C.2a1b.2a.1a.1 clade for 2022. This pattern diverges markedly from Myanmar circulating strains in 2019 (3C.2a1b) and 2021 (3C.2a1b.2a.2a.3) and Italy/Australia circulating strains in 2021 and 2022 (3C.2a1b.2a.2) [ 16 , 43 , 44 , 45 ].…”

Quantifying the impact of pre-vaccination titre and vaccination history on influenza vaccine immunogenicity

“…There were 4,958 sera samples collected from 2020-2022 used for this analysis, representing a period during which influenza did not circulate in Australia (2020-2021) [21] and resumed ciruclation (2022). [22] These 4958 serum samples came from 1,646 HCW, of which the majority of samples (3460/4958) were from frequently vaccinated HCWs (vaccinated in 5/5 seasons before enrolment) and 237/4958 samples were from infrequently vaccinated individuals (not vaccinated in 5/5 seasons before enrolment). An overview of the characteristics of this cohort is given in Table 1 .…”

Resurgence of human metapneumovirus infection and influenza after three seasons of inactivity in the post‐COVID‐19 era in Hokkaido, Japan, 2022–2023

“…However, since 2021, there have been reports of a resurgence of various infectious diseases worldwide 16–24 . In a previous study, we reported the resurgence of respiratory syncytial virus (RSV) infections in Hokkaido Prefecture in August 2021 based on our surveillance of hospitalized children with various infectious diseases from July 2019 to June 2022 25 .…”

“…[7][8][9][10][11][12][13][14][15] However, since 2021, there have been reports of a resurgence of various infectious diseases worldwide. [16][17][18][19][20][21][22][23][24] In a previous study, we reported the resurgence of respiratory syncytial virus (RSV) infections in Hokkaido Prefecture in August 2021 based on our surveillance of hospitalized children with various infectious diseases from July 2019 to June 2022. 25 Before the COVID-19 pandemic, RSV infections had been observed in all seasons since 2017 in Hokkaido Prefecture, but the significant outbreak in August 2020 was unexpected.…”