Influenza C virus was first isolated in 1947 [1]. Subsequent seroepidemiological studies revealed that this virus is distributed worldwide [2], [3], [4], [5], [6], [7] and recurrent infection occurs in both children and adults [8], [9], [10]. Influenza C virus is a member of the Orthomyxoviridae family together with the influenza A, B, and D viruses. The genome of the influenza C virus consists of seven RNA segments, each with PB2, PB1, P3, hemagglutinin esterase (HE), NP, M, and NS genes. The fourth segment encodes the hemagglutinin esterase (HE) glycoprotein, which similarly acts to both hemagglutinin (HA) and neuraminidase in influenza A and B viruses and carries major antigenicity determinants. The HE glycoprotein forms trimers. Additionally, Rosenthal et al. [11] determined three functional domains, including a receptor-binding (R), an esterase (E), and a fusion (F) domain.
The influenza C virus was divided into six lineages based on the HE gene: C/Taylor, C/Kanagawa, C/Mississippi, C/Yamagata, C/Aichi, and C/Sao Paulo [12]. The HE gene of the influenza C virus evolves slowly compared with the HA genes of influenza A and influenza B [12,13], and several HE lineages can cocirculate within a community [12,14]. Our previous work revealed that epidemics of influenza C virus infection periodically occur, and the predominant HE lineage might be selected among the adult population [14,15]. Additionally, some outbreaks were caused by the reassortant virus [12], suggesting that reassortment is an important means of increasing the ability to predominate in humans, as well as antigenic variation [12,16,17].
Usually, the influenza C virus causes mild upper respiratory diseases in humans but severe lower respiratory illnesses in children aged <2 years [16,18,19] or in cases with comorbidities [20]. Febrile convulsions due to high fever and a case of encephalopathy have been reported [18,21]. Coinfection with other viruses was reported as a factor in the severity of hospitalized children [19,20,22]. Therefore, further research is necessary to clarify the epidemiological and clinical significance of the influenza C virus. We conducted virus testing in one hospital and one clinic for 15 years from 2006 to 2020, isolated the virus by cell culture, and obtained 60 influenza C virus-positive children. This study performed clinical and epidemiological analyses on these 60 patients, between outpatients and inpatients.
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