Assessment of population susceptibility to upcoming seasonal influenza epidemic strain using interepidemic emerging influenza virus strains

Abstract

Seasonal influenza virus epidemics have a major impact on healthcare systems. Data on population susceptibility to emerging influenza virus strains during the interepidemic period can guide planning for resource allocation of an upcoming influenza season. This study sought to assess the population susceptibility to representative emerging influenza virus strains collected during the interepidemic period. The microneutralisation antibody titers (MN titers) of a human serum panel against representative emerging influenza strains collected during the interepidemic period before the 2018/2019 winter influenza season (H1N1-inter and H3N2-inter) were compared with those against influenza strains representative of previous epidemics (H1N1-pre and H3N2-pre). A multifaceted approach, incorporating both genetic and antigenic data, was used in selecting these representative influenza virus strains for the MN assay. A significantly higher proportion of individuals had a ⩾four-fold reduction in MN titers between H1N1-inter and H1N1-pre than that between H3N2-inter and H3N2-pre (28.5% (127/445) vs. 4.9% (22/445), P < 0.001). The geometric mean titer (GMT) of H1N1-inter was significantly lower than that of H1N1-pre (381 (95% CI 339-428) vs. 713 (95% CI 641-792), P < 0.001), while there was no significant difference in the GMT between H3N2-inter and H3N2-pre. Since A(H1N1) predominated the 2018-2019 winter influenza epidemic, our results corroborated the epidemic subtype.

Keywords: Influenza; population susceptibility; respiratory tract infections; serology.

Fig. 1.

Influenza A strains emerging in Hong Kong. (a) Emergence of influenza A(H1N1) strains with HA S183P substitution. (b) Emergence of influenza A(H3N2) strains belonging to lineage 3C.2a1b. Amino acid sequences were downloaded from GISAID (Supplementary Table S1). Serum samples in this study were collected from April to June 2018 and are indicated by the black arrows.

Fig. 2.

Phylogenetic tree of HA showing the genetic relationship of influenza A(H3N2) in Hong Kong. Nucleotide sequences were downloaded from GISAID (Supplementary Table S2). All influenza A(H3N2) strains from Hong Kong available at GISAID as of 7 January 2019 are included. Vaccine strains recommended by the World Health Organisation are highlighted in blue. H3N2-pre and H3N2-inter used in the MN assay are highlighted in green and red, respectively. The phylogenetic trees were constructed using the maximum-likelihood method with the best-fit substitution model HKY + G. Bootstrap values were calculated from 1000 trees.