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Background: The association between inactivated influenza vaccination and viral load in young children remains unclear. Methods: During the 2013/2014 to 2017/2018 influenza seasons in Japan, children under 6 years of age with pre‐defined influenza‐like illness and influenza‐positive status by real‐time RT‐PCR were recruited at pediatric clinics for this observational study. Influenza viral load was measured for the most predominant subtype/lineage in each season. Using median dichotomized viral load as an outcome, a multilevel logistic regression model was applied to estimate the multivariable adjusted odds ratio (MOR) and 95% confidence interval (CI) for higher viral load. Results: A total of 1,185 influenza‐positive children were analyzed. The median log10 viral load copy number (copies per milliliter) was 5.5 (interquartile range, 4.6 to 6.1) and did not differ by vaccination status: 5.5 for unvaccinated, 5.7 for one dose, and 5.5 for two doses (p = 0.67). The MOR of vaccinated (one or two doses) versus unvaccinated children was 1.19 (95% CI: 0.86–1.64). Other factors showing significant associations with higher viral load were positive results for A(H1N1)pdm09 and A(H3N2) in comparison with B/Yamagata. The respective MORs were 3.25 (95% CI: 2.28–4.64) and 1.81 (95% CI: 1.32–2.49). Significantly elevated MORs against higher viral load were also observed for higher body temperature at influenza diagnosis and shorter duration from fever onset to specimen collection. Conclusion: No association was observed between inactivated‐influenza vaccination and viral load at influenza‐positive diagnosis. Influenza subtype/lineage, body temperature, and time elapsed since fever onset were significantly associated with viral load. [ABSTRACT FROM AUTHOR]

Large scale investigation of the clinical effectiveness of neuraminidase inhibitors for circulating influenza viruses are important along with the surveillance of virus susceptibility in vitro. The duration of fever and other influenza symptoms as markers of the clinical effectiveness of laninamivir octanoate hydrate (laninamivir) were investigated in the Japanese 2017/18 and 2018/19 influenza seasons and compared with the results of the previous six seasons. Influenza A(H1N1)pdm09, A(H3N2), and B were found in 14, 45, and 52 patients in the 2017/18 season and in 22, 62, and 0 in the 2018/19 season, respectively. The median duration of fever for B was significantly longer than for A(H1N1)pdm09 and A(H3N2) in the 2017/18 season (p = 0.0461) and for A(H3N2) than for A(H1N1)pdm09 in the 2018/19 season (p = 0.0290). However, the differences were subtle in both seasons for other symptoms, with no significant differences in their median duration in comparison of the circulating types/subtypes. Over the eight seasons with the previous six seasons added, the median durations of fever were consistently longer for B than for A, but the relation between the A subtypes was inconsistent. The median durations of fever were comparable over the eight seasons for the virus types/subtypes, as were the median durations of other symptoms. The percentage of febrile patients decreased in a similar pattern over the eight seasons for each type/subtype. The results confirmed that laninamivir has continued to be clinically effective against all types/subtypes of influenza viruses, with no safety issues. [ABSTRACT FROM AUTHOR]

While clinical trials have illuminated both the virological and clinical efficacy of baloxavir for influenza and post-treatment viral resistance, these aspects warrant further study in real-world settings. In response, we executed a prospective, observational study of the Japanese 2022–2023 influenza season. A cohort of 73 A(H3N2)-diagnosed outpatients—36 treated with baloxavir, 20 with oseltamivir, and 17 with other neuraminidase inhibitors (NAIs)—were analyzed. Viral samples were collected before and after administering an antiviral on days 1, 5, and 10, respectively. Cultured viruses were amplified using RT-PCR and sequenced to detect mutations. Fever and other symptoms were tracked via self-reporting diaries. In the baloxavir cohort, viral detection was 11.1% (4/36) and 0% (0/36) on day 5 and day 10, respectively. Two isolates from day 5 (5.6%, 2/36) manifested I38T/M-substitutions in the polymerase acidic protein (PA). For oseltamivir and other NAIs, viral detection rates were 60.0% (12/20) and 52.9% (9/17) on day 5, and 16.7% (3/18) and 6.3% (1/16) on day 10, respectively. No oseltamivir-resistant neuraminidase mutations were identified after treatment. Median fever durations for the baloxavir, oseltamivir, and other NAI cohorts were 27.0, 38.0, and 36.0 h, respectively, with no significant difference. Two patients harboring PA I38T/M-substitutions did not exhibit prolonged fever or other symptoms. These findings affirm baloxavir's virological and clinical effectiveness against A(H3N2) in the 2022–2023 season and suggest limited clinical influence of post-treatment resistance emergence. • Analyzed 73 influenza A(H3N2) outpatients with longitudinal viral and clinical data. • Baloxavir cohort showed lower viral isolation rate than those of oseltamivir/other NAIs on both days 5 and 10. • Baloxavir-resistant viruses detected in 2 patients on day 5, but disappeared by day 10; with no prolonged fever/symptoms. • No significant differences in fever and symptom durations between baloxavir and other treatment groups. [ABSTRACT FROM AUTHOR]