Background: The 2011-12 seasonal trivalent inactivated influenza vaccine was overall poorly protective against influenza A (H3N2) infection. We evaluated whether the low vaccine effectiveness (VE) in this season was associated with mutation in hemagglutinin (HA) of the egg-derived vaccine strain. Additionally, it was investigated whether the egg-derived vaccine seed strain obtains further mutations in the receptor binding site while being propagated in the eggs through the manufacturing process.
Materials and methods: We performed hemagglutination inhibition (HI) assays and compared the reactivities of human sera from the recipients of 2011-12 influenza vaccination against the X-187 vaccine seed virus (with Ser-to-Thr mutation at residue 228 [Ser228Thr]), vaccine prototype virus and the reversed X-187 vaccine virus (with Thr228Ser).And the reactivities of the anti-sera from the 2011-12 influenza vaccinees against circulated influenza viruses were assessed. We analysed the sequences of the HAs of repetitively propagated influenza vaccine viruses and compared it to the sequence of the original vaccine virus.
Results: The reactivities of human sera from the 2011-12 influenza-vaccinated subjects against the X-187 vaccine virus, vaccine prototype virus and the reversed X-187 vaccine virus were not statistically different, as measured by geometric mean titers (GMT) [177 (95% CI: 151-208) vs. 150 (95% CI: 128-177) vs. 142 (95% CI: 120-167); P = 0.139]. (Fig. 3) The seroprotection rates of 2011-12 influenza vaccinees against three kinds of viruses were 99.2% (X-187 vaccine virus group), 98.5% (vaccine prototype virus group) and 98.5% (reversed X-187 virus group) (P = 0.582). (Fig. 4) The GMTs and the seroprotection rates of 80 influenza vaccinees during the 2011-12 season against the circulated A/H3N2 influenza viruses were much lower compared with those of the X-187 vaccine virus, vaccine prototype virus and reversed X-187 vaccine virus. (Table 1) Any further mutation did not occur in the receptor binding site of the X-187 and reversed X-187 virus while being propagated in the eggs.
Conclusion: The low influenza VE during the 2011-12 season was may be the result from mismatch of the vaccine strains with circulating viruses rather than from the development of mutation in the egg-derived vaccine virus. In addition, it is thought that eggs are still able to function as the economical and practical vaccine production substrate.