Detection of influenza A(H3N2) viruses with polymerase acidic subunit substitutions after and prior to baloxavir marboxil treatment during the 2022-2023 influenza season in Japan.

Baloxavir marboxil (baloxavir), approved as an anti-influenza drug in Japan in March 2018, can induce reduced therapeutic effectiveness due to PA protein substitutions. We assessed PA substitutions in clinical samples from influenza-infected children and adults pre- and post-baloxavir treatment, examining their impact on fever and symptom duration. During the 2022-2023 influenza season, the predominant circulating influenza subtype detected by cycling-probe RT-PCR was A(H3N2) (n = 234), with a minor circulation of A(H1N1)pdm09 (n = 10). Of the 234 influenza A(H3N2) viruses collected prior to baloxavir treatment, 2 (0.8%) viruses carry PA/I38T substitution. One virus was collected from a toddler and one from an adult, indicating the presence of viruses with reduced susceptibility to baloxavir, without prior exposure to the drug. Of the 54 paired influenza A(H3N2) viruses collected following baloxavir treatment, 8 (14.8%) viruses carried E23 K/G, or I38 M/T substitutions in PA. Variant calling through next-generation sequencing (NGS) showed varying proportions (6-100 %), a polymorphism and a mixture of PA/E23 K/G, and I38 M/T substitutions in the clinical samples. These eight viruses were obtained from children aged 7-14 years, with a median fever duration of 16.7 h and a median symptom duration of 93.7 h, which were similar to those of the wild type. However, the delayed viral clearance associated with the emergence of PA substitutions was observed. No substitutions conferring resistance to neuraminidase inhibitors were detected in 37 paired samples collected before and following oseltamivir treatment. These findings underscore the need for ongoing antiviral surveillance, informing public health strategies and clinical antiviral recommendations for seasonal influenza.

Duration of fever and symptoms in influenza-infected children treated with baloxavir marboxil during the 2019-2020 season in Japan and detection of influenza virus with the PA E23K substitution.

Data on the clinical effectiveness of the novel anti-influenza drug baloxavir marboxil (baloxavir) in children remain limited. We conducted an observational study to compare the duration of fever and symptoms between baloxavir- and oseltamivir-treated children infected with influenza A and B. In total, 159 outpatients with influenza A(H1N1)pdm09 or B/Victoria-lineage infections, aged <19 years, during the 2019-2020 influenza season in Japan were enrolled and assessed the duration of fever and symptoms using the Kaplan-Meier method and a multivariate Cox proportional hazard regression model. Polymerase acidic (PA) variants were examined before and after baloxavir treatment. In the multivariable analysis, the duration of fever and symptoms was unaltered between the A(H1N1)pdm09 (n = 116) and B/Victoria-lineage (n = 43) groups. Conversely, the fever duration was marginally longer in the oseltamivir-treated group (n = 59) than in the baloxavir group (n = 100) (hazard ratio (HR) = 0.67, p = 0.05); however, the duration of symptoms was unaltered between the two groups (HR = 0.74, p = 0.11). No patient presented PA reduced susceptibility marker(s) before baloxavir treatment in the analyzed groups. The PA/E23K variant was detected in one case (1.5%, 1/66) of A(H1N1)pdm09 after baloxavir treatment. One case (2.0%, 1/50) of A(H1N1)pdm09 with an N295S substitution in neuraminidase was detected following oseltamivir treatment. These results suggested that the duration of fever was likely to be shorter with baloxavir than with oseltamivir, but the difference between influenza A (H1N1)pdm09 and B/Victoria-lineage was unclear. It is important to continue evaluating the clinical effectiveness of baloxavir and monitoring its drug susceptibility to the influenza virus.

Duration of fever and symptoms in children after treatment with baloxavir marboxil and oseltamivir during the 2018-2019 season and detection of variant influenza a viruses with polymerase acidic subunit substitutions.

We conducted a prospective, multicenter, non-randomized observational study to assess the duration of fever and symptoms of influenza A/H1N1pdm09 and A/H3N2 infected children < 19 years old treated with either baloxavir or oseltamivir. Additionally, these symptoms were investigated in association with pre- and post-baloxavir treatment-emergent polymerase acidic unit (PA) variants as compared to non-substituted viruses. Following receipt of informed consent, baloxavir was administered to 102 influenza A patients, and oseltamivir to 52 patients during the 2018-2019 influenza season in Japan. The average age was higher in the baloxavir treatment group compared to the oseltamivir treatment group (10.6 ± 2.7 versus 6.9 ± 2.9 years old, p < 0.01). The duration of fever and symptoms in baloxavir-treated A/H1N1pdm09 and A/H3N2-infected children did not differ from those in oseltamivir-treated groups (median 22.0, 11.8, 23.0, and 21.0 h, and median 114.5, 121.0, 123.0, and 122.0 h, respectively). One (1.2%) of 83 A/H3N2 patients possessed a PA/I38T substituted virus prior to treatment. The frequency of PA variants in post-treatment samples obtained 2-11 days after beginning of baloxavir was 12.5% (4/32) for A/H1N1pdm09 and 14.1% (9/64) for A/H3N2 when the total number of cases was used as the denominator, however, were 57.1% (4/7) and 33.3% (9/27) when PCR-positive cases at the time of second sampling was used as the denominator. The most frequent PA substitution was I38T (9), with E23K (1), I38K (1), I38M (1), and PA/I38S (1) also observed. The duration of fever and overall symptoms did not differ significantly following baloxavir treatment in individuals with PA variant viruses, non-substituted virus, or in those that were PCR negative at the second sampling (median 20, 24 and 11 h, and median 121, 115 and 121 h, respectively). Rebound of viral RNA load was observed in 13.5% (2/13) of PA variants but it was not associated with recurrence of fever and symptoms. Hence, prolonged fever or symptoms were not observed in children treated with baloxavir following emergence of PA variants, however, further studies are needed to evaluate the clinical impact of PA variants.