Antibodies cross-reactive to seasonal influenza A(H1N1)pdm09 viruses in human sera of elderly people immunized with trivalent influenza vaccines containing the A/California/7/09 vaccine strain

Bakground: Of primer importance in immunity to influenza is the production of antibody to the virus haemagglutinin (HA) protein because such antibodies can neutralize the infectivity of the viruses and their presence in the blood has been shown to correlate with the level of protection. For this reason, influenza vaccine virus selection is based on the antigenic characterization of virus HA using haemagglutination inhibition (HAI) assay using sera isolated from naïve ferrets recovering from primary influenza infection with vaccine viruses or viruses representative of circulating strains. Vaccine strains are updated when influenza viruses acquired mutations that prevent the binding of primary ferret anti-influenza sera. However, unlike ferrets, most humans are repetitively infected with different viruses, and prior influenza exposure influence the development of new antibodies against drifted influenza strains. This aspect may be particularly relevant in the choice of the H1N1 vaccine component since different A/H1N1 viruses have circulated over the past 100 years, and have left an important mark in human immunity. The aim of the study was: - verify the ability of human sera to react with 6 different A/H1N1 viral strains circulated in Italy between 20011 and 2015 that, using primary antisera of infected ferrets, were found to be antigenically similar to the A/California7/09 vaccine strain; - verify the ability of influenza vaccines containing A/California/7/2009 to induce antibodies cross reactive to this 6 field strains. Methods: We analyzed the sera of 156 volunteers (mean age = 80.3 years, range 65–104) by HAI assay using as antigens the A/H1N1 vaccine strain (A/California/7/2009) and 6 A/H1N1 isolated in our laboratory between 2010 and 2015. Results were reported as protection rate (percentage of volunteers showing HI titer ≥40, geometric mean titers (GMT), ratio of post-vaccination to pre-vaccination GMT values (GMTR), and seroconversion rate. Results: Before vaccination, human sera reacted strongly to vaccine strain and to viruses circulated immediately after 2009 but less to viruses circulated in 2013 and 2015. A certain percentage of human sera samples (between 24 and 48%) had reduced titers (at least fourfold change) to the most recent viruses. One month after vaccination, the HAI response against field strains was always lower than those against vaccine A/H1N1 strain. This tendency was most evident among older people (over 85 years). Conclusions: These study indicate that primary ferret antisera may not be fully representative of human influenza immunity. We propose that: - a greater emphasis should be placed on sera derived from humans when the circulating viral strains are characterized; - sera obtained from sequentially infected ferrets should be included in panels used to characterize viral isolates.