Evolution and risk assessment of the H2 subtype of influenza viruses

Abstract

 The H2 subtype of influenza viruses caused the Asian pandemic in 1957, and disappeared from human populations since 1968. As a result, most people under the age of 50 are immunologically naïve to this subtype. Avian H2 viruses have contributed genes to the genesis of the 1957 H2 pandemic virus and a recently emerged swine H2 virus, and are still circulating in avian species. Therefore, if H2 viruses could return from the avian reservoir to infect humans, they might pose a pandemic risk. This thesis evaluated the risk of H2 viruses in multiple aspects. The surveillance and the evolutionary study were performed to understand the prevalence and ecology of H2 viruses and to identify genomic features thereof. Infection experiments with representative strains were performed in the pig and ferret models to assess whether the circulating H2 viruses could infect humans and how might this occur. Finally, to evaluate whether H2 viruses could emerge by reassorting with enzootic poultry viruses, I tested the infectivity and transmissibility in chickens of the reassortant H2 viruses that incorporated the same H9N2 internal genes contributing to the zoonotic H7N9 and H10N8 viruses. A total of 56 H2 isolates were obtained from our surveillance in southern China conducted from 1976-1980 and from 2000-2013. Although the H2 viruses were isolated at a lower rate compared with many other subtypes and were only from the aquatic birds, they were continuously being isolated throughout the study period. All avian H2 viruses were typical gene pool-like viruses with no lineage established. All the avian and the human 1957 pandemic H2 viruses tested failed to establish a productive infection in pigs, suggesting it is unlikely that pigs could act as an intermediate host for the emergence of the 1957 H2 pandemic virus nor a contemporary avian H2 virus. The human seasonal and the swine H2 viruses could replicate in pigs and the latter virus also demonstrated efficient transmission among pigs and from pigs to ferrets. Most H2 viruses could readily infect ferrets and cause significant seroconversion, indicating these viruses might cause direct infection in humans or other mammals that have similar sensitivity to ferrets. Alternatively, ferrets might be too sensitive to be a good infectivity-assessing model. The ferret antisera generated against the H2 viruses revealed the diversity of antigenic spectrum in these avian and human strains. Sixteen amino acid positions on the hemagglutinin head might be associated with these antigenic variations. Recruitment of the enzootic H9N2 internal genes didn’t allow the H2 viruses to become infective to chickens. However, the HA gene was critical for the virus infectivity in chickens while the M and NS genes could affect the transmission efficiency. This thesis provided a comprehensive risk assessment of H2 viruses with respect to their genetic and antigenic evolution, infectivity in mammals and explored the possibility for H2 viruses to emerge from poultry. These findings were critical for H2 pandemic preparedness.