Epidemiologic, clinical, and genetic characteristics of influenza C virus infections among outpatients and inpatients in Sendai, Japan from 2006 to 2020.

BACKGROUND: Influenza C virus is a pathogen that causes acute respiratory illness in children. The clinical information about this virus is limited because of the small number of isolated viruses compared to influenza A or B viruses. METHODS: A total of 60 influenza C viruses were isolated by clinical tests using cell culture methods conducted in one hospital and one clinic during the 15 years from 2006 to 2020. These 60 cases were retrospectively analyzed by comparing outpatients and inpatients. Moreover, isolated viruses were analyzed for genomic changes during the study period. RESULTS: All were younger than 7 years, and 73% of inpatients (19 out of 26) were under 2 years of age. A significant difference was found in the frequency of pneumonia, accounting for 45% and 4% of inpatients and outpatients, respectively. Most of the viruses isolated from 2006 to 2012 belonged to the S/A sublineage of the C/Sao Paulo lineage, but three sublineage viruses, including the S/A sublineage with K190N mutation, S/V sublineage, and C/Kanagawa lineage, have cocirculated since 2014. Moreover, S/A sublineage viruses were undergoing reassortment since 2014, suggesting significant changes in the virus, both antigenically and genetically. Of the 10 strains from patients with pneumonia, 7 were in the S/A sublineage, which had circulated from 2006 to 2012. CONCLUSION: Infants under 2 years of age were more likely to be hospitalized with pneumonia. The genomic changes that occurred in 2014 were suggested to affect the ability of the virus to spread.

Antiviral Susceptibilities of Distinct Lineages of Influenza C and D Viruses.

The emergence and spread of antiviral-resistant influenza viruses are of great concern. To minimize the public health risk, it is important to monitor antiviral susceptibilities of influenza viruses. Analyses of the antiviral susceptibilities of influenza A and B viruses have been conducted globally; however, those of influenza C and D viruses are limited. Here, we determined the susceptibilities of influenza C viruses representing all six lineages (C/Taylor, C/Yamagata, C/Sao Paulo, C/Aichi, C/Kanagawa, and C/Mississippi) and influenza D viruses representing four lineages (D/OK, D/660, D/Yama2016, and D/Yama2019) to RNA polymerase inhibitors (baloxavir and favipiravir) by using a focus reduction assay. All viruses tested were susceptible to both drugs. We then performed a genetic analysis to check for amino acid substitutions associated with baloxavir and favipiravir resistance and found that none of the viruses tested possessed these substitutions. Use of the focus reduction assay with the genotypic assay has proven valuable for monitoring the antiviral susceptibilities of influenza C and D viruses as well as influenza A and B viruses. Antiviral susceptibility monitoring of all influenza virus types should continue in order to assess the public health risks posed by these viruses.

Longitudinal course of influenza C virus antibody titers of healthy adults in Sendai, Japan.

BACKGROUND: Influenza C virus causes mild respiratory diseases in humans. Previous studies suggested that the predominant hemagglutinin-esterase gene lineage circulating in children might be selected among the adult population, yet the prevalence of influenza C virus in adults has not been described. OBJECTIVES: To evaluate the frequency of influenza C virus infection in adults. STUDY DESIGN: We performed hemagglutination inhibition assays of serum samples collected at periodic occupational medical checkups from employees of a hospital. A total of 679 serum samples were collected from 57 subjects who participated in biannual medical checkups between 2011 and 2016 as part of a longitudinal series. Titers of antibodies against the C/Kanagawa and C/Sao Paulo lineage viruses were detected. RESULTS: Ten serum sample pairs from among the 57 subjects showed at least a four-fold increase in influenza C antibody titers. Samples from three subjects exhibited antibody titer increases for both the C/Kanagawa and C/Sao Paulo lineages, four subjects showed an increased titer against the C/Sao Paulo lineage, and three subjects showed an increased titer against the C/Kanagawa lineage. Half of the antibody titer increases for the C/Kanagawa lineage were detected in May 2014, while the increases for the C/Sao Paulo lineage were detected from 2011 to 2016. CONCLUSION: The 5-year influenza C virus infection rate was estimated at 17.5 %. There were antibodies that cross-reacted with the C/Sao Paulo and C/Kanagawa lineages. The results suggest that C/Sao Paulo was the main lineage in the adult population of this area, with cocirculation of the C/Kanagawa lineage.