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PubMed2026

Association of Baloxavir Treatment Timing with Serial Interval and Household Transmission of Influenza through a Likelihood-Based Analysis.

BACKGROUND: Baloxavir treatment is associated with reduced influenza transmission within households, and the serial interval varies by treatment status. However, it remains unclear how baloxavir-induced changes in the serial interval relate to household transmission. We aimed to quantify the model-based association between baloxavir treatment timing and the serial interval and household transmission risk. METHODS: We conducted a household survey of influenza cases in Japan between October 2018 and February 2019. We defined the likelihood-based model integrating the serial interval distribution by treatment status and the secondary attack rate (SAR) using individual-level data from index cases. Using this model, we estimated the reduction in the serial interval associated with baloxavir treatment. RESULTS: Compared with untreated index cases, baloxavir-treated cases were estimated to have a serial interval density reduced by 21.42% following treatment. Treatment within 24 hours was associated with a 0.1685 reduction in the area under the curve, with smaller reductions as treatment was delayed. Earlier treatment was associated with a shorter, more concentrated distribution, whereas treatment 72 hours after onset resembled untreated cases. CONCLUSIONS: Our findings highlight that baloxavir treatment is associated with a shorter serial interval and lower estimated secondary household transmission risk. We provide model-based estimates suggesting that earlier administration is associated with a greater reduction in serial interval density and estimated transmission risk, which may inform public health strategies for infection control.

Influenza↗
PubMed2026

Detection and characterization of antiviral-resistant viruses during the influenza season of 2024-25.

UNLABELLED: During the high severity season of 2024-25, CDC with public health partners sequenced and analyzed genomes of >10,000 influenza viruses for antiviral resistance markers. Available sequence-flagged and representative viruses were tested with antivirals using in vitro assays. In the US, three oseltamivir-resistant A(H3N2) viruses had treatment-emergent neuraminidase (NA) mutations, either E119V or R292K. Oseltamivir-resistant A(H1N1)pdm09 viruses with NA-H275Y were detected in 15 states, albeit at a low frequency (0.53%). They belonged to several phylogenetic groups, with hemagglutinin (HA) subclade D.3.1 combined with either NA subclade D.1 or D.2 being most common. Based on shared sequence data, nearly all H275Y viruses from Australia, Canada, and Chile also belonged to these HA and NA subclades. Conversely, most H275Y viruses (68/81) from China belonged to HA subclade C.1.9 and NA subclade D and shared the permissive mutation R257K. Influenza polymerase acidic (PA) mutations conferring 4- to 92-fold decreased baloxavir susceptibility were detected in nine influenza A viruses. Viruses with PA-I38T showed mild attenuation of replicative fitness in three cell lines. Based on available data, NA-H275Y and PA-I38T viruses were collected from patients with no exposure to antivirals. Baseline susceptibility to all US-approved influenza antivirals remained largely unchanged compared to previous seasons. All swine-origin viruses detected in the US had adamantane resistance-conferring marker, M2-S31N, but remained susceptible to other approved antivirals. Monitoring antiviral susceptibility has substantially improved with increased sequencing capacities and bioinformatic support at public health laboratories. Information gained through influenza surveillance has been used to guide recommendations on antiviral use. IMPORTANCE: Circulation of influenza viruses with reduced susceptibility to antivirals can diminish the usefulness of medications prescribed for influenza. This study informs on the prevalence of drug-resistant influenza viruses in the US during the high severity season of 2024-25. It provides information on susceptibility profile to all approved antiviral medications and on replicative fitness of representative drug-resistant viruses. Most drug-resistant viruses were collected from patients who were not exposed to antivirals indicating their ability to transmit from human to human. Whole-genome sequence (WGS)-based analysis is the cornerstone for surveillance, and numerous laboratories have been utilizing this approach. However, CDC laboratory is the only laboratory in the US conducting phenotypic testing of circulating viruses needed to confirm the outcomes of sequence-based analysis and to identify new molecular markers of resistance. Data gathered through virologic surveillance give much-needed information on drug susceptibility of influenza viruses which are used to guide recommendations on antiviral use.

Antiviral Agents↗
PubMed2025

Atypical Influenza A(H3N2) Activity Patterns in Germany, 2021-2023, and Characterization of Newly Emerged Virus Clades.

The first waves of the COVID-19 pandemic were accompanied by an unprecedented decrease of influenza activity which persisted throughout the 2020/21 and 2021/22 winter seasons. Here, we report on the unusual influenza circulation patterns that followed in the year 2022, which was dominated throughout by A(H3N2) influenza viruses. After a delayed spring wave in 2022, A(H3N2) influenza viruses circulated at low levels throughout the summer and rose to a prominent, prematurely-timed fall/winter wave peaking in December, with highest positivity rates observed in 10-12-years old children. This winter wave ended abruptly with the national school holidays, when positivity rates decreased sharply not only in children but also in other age groups. Genetic analysis of influenza virus hemagglutinin (HA) showed cocirculation of 10 A(H3N2) clades, of which three (2a.1b, 2a.3a.1, and 2b) became dominant in late 2022. All A(H3N2) viruses, including those assigned to the new clades, displayed high titers in HA inhibition tests with postinfection ferret antiserum raised against the A(H3N2) vaccine strains A/Cambodia/e0826360/2020 and A/Darwin/9/2021. All viruses were susceptible to neuraminidase inhibitors and the polymerase inhibitor baloxavir marboxil, but carried the M2-S31N substitution conferring adamantane resistance. Our findings shed light on disturbed seasonality of A(H3N2) circulation in the post-COVID-19 era.

Influenza A Virus, H3N2 Subtype↗
PubMed2026

Small molecule inhibition of the mitochondrial lipid transfer protein STARD7 attenuates influenza viral replication.

The increasing appearance of drug-resistant and zoonotic influenza strains highlights an urgent need for host-directed antivirals that offer broad-spectrum activity and a higher barrier to resistance. Here, we describe the characterization of M4, a small-molecule identified from a high-throughput screen that potently inhibits influenza A and B viruses. Mechanistic studies reveal that M4 suppresses influenza virus replication by preventing formation of export-competent viral ribonucleoprotein (vRNP) complexes in the nucleus. Chemoproteomic profiling identified the lipid transfer protein STARD7 as the primary cellular target, and genetic depletion of STARD7 phenocopies the antiviral effects of M4. Additional studies localized the M4 binding site to cysteine 302 within the lipid-binding domain of STARD7, supporting a model in which STARD7-dependent lipid transfer activity promotes efficient vRNP assembly and nuclear export. Combining M4 with baloxavir enhances antiviral efficacy in a murine infection model, providing in vivo support for a host-directed therapeutic strategy. Together, these results identify STARD7 as a metabolic checkpoint licensing vRNP nuclear export and they establish a proof of concept for therapeutic intervention with small molecule inhibitors.

Journal Article↗