Antiviral susceptibility monitoring: testing algorithm, methods, and findings for influenza season, 2023-2024.

Antiviral susceptibility monitoring is integral to influenza surveillance conducted by CDC in collaboration with partners. Here, we outlined the algorithm and methods used for assessing antiviral susceptibility of viruses collected during 2023-2024 season. Virus specimens were provided by public health laboratories in the United States (US) and by laboratories in other countries that belong to the Pan American Health Organization. In the US, antiviral susceptibility surveillance conducted nationally is strengthened by sequence-only analysis of additional viruses collected at a state level. Viral genome sequence analysis was the primary approach to assess susceptibility to M2 blockers (n = 5123), neuraminidase (NA) inhibitors (n = 6874), and a polymerase acidic protein (PA) inhibitor (baloxavir, n = 6567). Over 99 % of type A viruses had M2-S31N that confers resistance to M2 blockers. Although oseltamivir-resistant viruses carrying NA-H275Y (N1 numbering) were rare (0.35 %), a cluster of four such viruses was identified in Haiti. Viruses with other NA mutations conferring reduced inhibition by NA inhibitor(s) were also detected sporadically. This includes a cluster of three influenza B viruses in Texas that shared a new mutation, NA-A245G conferring reduced inhibition by peramivir. Three viruses with reduced baloxavir susceptibility were identified, which had PA-I38T, PA-Y24C or PA-V122A; the latter two new mutations identified through augmented approach to sequence analysis. To monitor baseline susceptibility, supplementary in vitro testing was conducted on approximately 7 % of viruses using NA inhibition assay and cell culture-based assay IRINA. Implementation of Sequence First approach provided comprehensive and high throughput methodology for antiviral susceptibility assessment and reduced redundant phenotypic testing.