Characterization of a human H3N8 influenza virus

Background In 2022 and 2023, novel reassortant H3N8 influenza viruses infected three people, marking thefirsthuman infections with viruses of this subtype. Methods Here, we generated one of these viruses (A/Henan/4-10CNIC/2022; hereafter called A/Henan/2022 virus)by using reverse genetics and characterized it. Findings In intranasally infected mice, reverse genetics-generated A/Henan/2022 virus caused weight loss in allfiveanimals (one of which had to be euthanized) and replicated efficiently in the respiratory tract. Intranasal infection offerrets resulted in minor weight loss and moderate fever but no mortality. Reverse genetics-generated A/Henan/2022virus replicated efficiently in the upper respiratory tract of ferrets but was not detected in the lungs. Virustransmission via respiratory droplets occurred in one of four pairs of ferrets. Deep-sequencing of nasal swabsamples from inoculated and exposed ferrets revealed sequence polymorphisms in the haemagglutinin proteinthat may affect receptor-binding specificity. We also tested 90 human sera for neutralizing antibodies againstreverse genetics-generated A/Henan/2022 virus and found that some of them possessed neutralizing antibodytitres, especially sera from older donors with likely exposure to earlier human H3N2 viruses. Interpretation Our data demonstrate that reverse genetics-generated A/Henan/2022 virus is a low pathogenicinfluenza virus (of avian influenza virus descent) with some antigenic resemblance to older human H3N2 virusesand limited respiratory droplet transmissibility in ferrets. Funding This work was supported by the Japan Program for Infectious Diseases Research and Infrastructure(JP23wm0125002), and the Japan Initiative for World-leading Vaccine Research and Development Centers(JP233fa627001) from the Japan Agency for Medical Research and Development (AMED). Copyright (c) 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/)