Epidemiological Characteristics of Influenza A Pandemic
10.14192/kjicp.2025.30.2.96
- Author:
Sung-Woon KANG
;
Sung-Han KIM
- Publication Type:Review Article
- From:
Korean Journal of healthcare-associated Infection Control and Prevention
2025;30(2):96-104
- CountryRepublic of Korea
- Language:English
-
Abstract:
Influenza viruses cause annual seasonal epidemics with a substantial healthcare burden, and global pandemics at multidecadal intervals. Pandemic emergence is driven by the capacity of the segmented RNA genome for antigenic drift and, critically, reassortment-mediated antigenic shift. Reassortment among avian, swine, and human influenza A lineages, facilitated by pigs that express both α2,3- and α2,6-linked sialic acid receptors, can yield viruses with efficient replication in mammalian cells and affinity for the human upper respiratory tract. Historically, the 1918 H1N1 and 1957 H2N2 pandemics likely arose from avian–human reassortants, whereas the 2009 H1N1pdm09 virus represented a quadruple reassortant involving swine, avian, and human progenitors. Age-specific mortality patterns may deviate from the U-shaped seasonal profile; the 1918 and 2009 pandemics disproportionately affected younger adults, plausibly reflecting antigenic recycling and partial immunity in older cohorts. The diagnosis of novel influenza relies on RT-PCR with subtype-specific confirmation in reference laboratories, as rapid antigen assays calibrated to seasonal antigens may fail to detect shifted viruses. Antivirals include neuraminidase inhibitors and the cap-dependent endonuclease inhibitor, baloxavir.Early administration of antivirals reduces progression to severe disease, and current H5N1 viruses remain broadly susceptible. Contemporary infection-prevention frameworks emphasize airborne transmission, across a spectrum from direct deposition of large droplets to aerosol inhalation, modulated by ventilation and procedure-related aerosol generation—implications that extend beyond traditional droplet–airborne dichotomies. Since March 2024, the highly pathogenic avian influenza H5N1 has spread across North America with sporadic human infections—mostly linked to exposure to infected dairy cattle or contaminated raw milk—typically manifesting as mild conjunctivitis. Although sustained human-to-human transmission has not been demonstrated, the virus exhibits an unusually broad mammalian host range with severe neurotropism in animal models. Wastewater detection suggests persistent animal circulation independent of human influenza activity. These features, coupled with the seasonal co-circulation of the human influenza virus, increase the risk of viral genomic reassortment. Multiple H5N1 vaccine candidates, including mRNA platforms, have been licensed or are advancing, supporting targeted pre-exposure strategies for the high-risk population. Sustained surveillance, rapid characterization and vaccination capacity, ventilation-aware infection control, and transparent risk communication, especially amid the post-COVID skepticism, remain central to mitigating the next influenza pandemic.
