Differential Antibody Recognition of H3N2 Vaccine and Seasonal Influenza Virus Strains Based on Age, Vaccine Status, and Sex in the 2017-2018 Season

Abstract

Background: An antigenic mismatch between the vaccine and circulating H3N2 strains was hypothesized to contribute to the severity of the 2017-2018 season in North America.

Methods: Serum and nasal washes were collected from influenza positive and negative patients during the 2017-2018 season to determine neutralizing antibody (nAb) titers and for influenza virus sequencing, respectively.

Results: The circulating and vaccine H3N2 virus strains were different clades, with the vaccine strain being clade 3C.2a and the circulating viruses being 3C.2a2 or 3C.3a. At enrollment, both the H3N2 negative and positive patients had greater nAb titers to the egg-adapted vaccine virus compared to the cell-grown vaccine but the H3N2-negative population had significantly greater titers to the circulating 3C.2a2. Among H3N2-positive patients, vaccination, younger age, and female sex were associated with greater nAb responses to the egg-adapted vaccine H3N2 virus but not to the cell-grown vaccine or circulating viruses.

Conclusions: For the 2017-2018 circulating viruses, mutations introduced by egg adaptation decreased vaccine efficacy. No increased protection was afforded by vaccination, younger age, or female sex against 2017-2018 circulating H3N2 viruses.

Keywords: circulating influenza strain; egg adaptation; human surveillance; neutralizing antibody; seasonal influenza vaccine; sex differences.

Figure 1.

The hemagglutinin (HA) of circulating H3N2 viruses lacks a key glycosylation site and includes numerous mutations as compared with the egg-adapted H3N2 vaccine strain, which differentially impacts neutralizing antibody responses to the egg-adapted vaccine virus and 2017–2018 circulating H3N2 viruses in influenza A virus (IAV)-negative and -positive patients. A, A trimer of the HA protein structure for the HK14-like and HK14 vaccine viruses are shown on a modeled H3 crystal structure (PDB: 4WE8) using University of California, San Francisco’s Chimera software. The receptor binding site is shown in cyan and the N158, T160 glycosylation site (using H3 numbering) is shown in red with a simple carbohydrate attached using GlyProt. Following passage through hen eggs, the H3 trimer of the HK14 virus lacks the glycosylation site at the T160K position, marked in red. B, A trimeric HA protein structure of H3N2 viruses is shown using an H3 crystal structure of A/Aichi/2/1968 (PDB: 2YPG) using PyMOL. The 5 major antigenic sites A, B, C, D, and E are indicated in pastel purple, pink, blue, orange, and green, respectively [20–22]. The sialic acid (SA) receptor binding pocket is between sites A and B, which are indicated with black arrows [23]. Mutations that arose in egg adaptation are marked in red, while amino acid differences in the 3C.2a2 viruses compared to the HK14-like (3C.2a) virus are shown in green. C, Amino acid differences in 3C.3a compared to 3C.2a are marked in blue using PyMOL (PDB: 2YPG). D, Serum collected at enrollment (baseline) from H3N2-infected individuals was used to perform microneutralization assays to measure neutralizing antibody (nAb) titers in both IAV-negative (n = 49) and IAV-positive (n = 32) patients against the egg-adapted HK14 vaccine and cell-grown HK14-like viruses and the Balt17-2a2 and Balt18-3a circulating H3N2 viruses. Significant differences in nAb titers against the HK14 relative to the HK14-like or circulating H3N2 viruses as well as between IAV-negative and IAV-positive patients for an individual virus are represented with *. The limit of detection (LOD) is labeled with a stippled line at 1:10 dilution and the World Health Organization cutoff for seroprotection is indicated with a stippled line at 1:40. The percentage of patients with nAb titers > 1:40 is indicated below each virus. For both the IAV-negative and IAV-positive patients, significant differences in the proportion of patients with a ≥ 1:40 nAb titer against the HK14 virus relative to the HK14-like or circulating viruses are indicated with #. For an individual virus, significant differences in the proportion of IAV-positive and IAV-negative patients are illustrated with †, and significant differences in proportions between circulating viruses to the HK14-like cell-grown viruses are shown with ‡.

Figure 2.

Among influenza A virus (IAV)-negative patients, neither vaccination nor patient sex affected baseline neutralizing antibody (nAb) titers to the H3N2 vaccine relative to the circulating H3N2 viruses during the 2017–2018 season. A, Baseline serum samples from male and female patients were used to measure nAb titers against the HK14 and HK14-like H3N2 vaccine viruses and the Balt17-2a2 and Balt18-3a circulating viruses. The limit of detection (LOD) is labeled with a stippled line as is the cutoff for seroprotection (1:40). The percentage of patients with nAb titers > 1:40 is indicated below each virus. For both male and female IAV-positive patients, significant differences in the proportion of patients with a > 1:40 nAb titer against the HK14 virus relative to the HK14-like or circulating viruses are indicated with #. B, Baseline serum samples from vaccinated and unvaccinated patients were used to measure nAb titers against the HK14 and HK14-like H3N2 vaccine viruses and the Balt17-2a2 and Balt18-3a circulating viruses during the 2017–2018 influenza season in asymptomatic, IAV-negative patients. Significant differences in nAb titers against the HK14 relative to the HK14-like or circulating H3N2 viruses as well as between IAV-negative and IAV-positive patients for an individual virus are represented with *. For both vaccinated and unvaccinated IAV-negative patients, significant differences in the proportion of patients with a > 1:40 nAb titer against the HK14 virus relative to the HK14-like or circulating viruses are indicated with #.

Figure 3.

The association between age and baseline neutralizing antibody (nAb) titers against H3N2 vaccine and circulating viruses in influenza A virus (IAV)-positive patients. Baseline nAb titers for IAV-positive patients were graphed relative to patient age for (A) HK14, (B) HK14-like, (C) Balt17-2a2, and (D) Balt18-3a. The limit of detection (LOD) is labeled with a stippled line as is the cutoff titer for seroprotection (1:40). The r² and P values for each virus are given.

Figure 4.

Among influenza A virus (IAV)-positive patients, female patients had greater baseline neutralizing antibody (nAb) titers to the H3N2 vaccine relative to circulating H3N2 viruses resulting in reduced seroconversion during convalescence in the 2017–2018 season. A, Baseline serum samples from male and female patients were used to measure neutralizing antibody (nAb) titers against the HK14 and HK14-like H3N2 vaccine viruses and the Balt17-2a2 and Balt18-3a circulating viruses during the 2017–2018 influenza season. The limit of detection (LOD) is labeled with a stippled line as is the cutoff for seroprotection (1:40). The percentage of patients with nAb titers > 1:40 is indicated below each virus. For both male and female IAV-positive patients, significant differences in the proportion of patients with a > 1:40 nAb titer against the HK14 virus relative to the HK14-like or circulating viruses are indicated with #. B, Baseline and convalescent nAb titers for individual male and female patients against each H3N2 virus. C, The fold change in nAb titers (convalescent divided by baseline) for male and female patients against each virus. Significant differences in nAb titers between male and female patients against the HK14 relative to the HK14-like or circulating H3N2 viruses (A), between baseline and convalescent samples (B), and between male and female IAV-positive patients (C) are represented with *.

Figure 5.

Among influenza A virus (IAV)-positive patients, vaccination increased baseline neutralizing antibody (nAb) titers to the H3N2 vaccine relative to circulating H3N2 viruses resulting in reduced seroconversion during convalescence in the 2017–2018 season. A, Baseline serum samples from vaccinated and unvaccinated patients were used to measure neutralizing antibody (nAb) titers against the HK14 and HK14-like H3N2 vaccine viruses and the Balt17-2a2 and Balt18-3a circulating viruses during the 2017–2018 influenza season. The limit of detection (LOD) is labeled with a stippled line as is the cutoff for seroprotection (1:40). The percentage of patients with nAb titers > 1:40 is indicated below each virus. For both vaccinated and unvaccinated IAV-positive patients, significant differences in the proportion of patients with a > 1:40 nAb titer against the HK14 virus relative to the HK14-like or circulating viruses are indicated with #. B, Baseline and convalescent nAb titers from each IAV-positive vaccinated and unvaccinated patient. C, The fold change in nAb titers (convalescent divided by baseline) for vaccinated and unvaccinated patients for each virus. Significant differences in nAb titers against the HK14 relative to the HK14-like or circulating H3N2 viruses (A), between baseline and convalescent samples (B), and between vaccinated and unvaccinated IAV-positive patients (C) are represented with *.