Following multiple reassortment events, the A(H1N1)pdm09 virus emerged from North American swine to infect humans and possessed an HA gene belonging to the classical swine lineage (12
). The ancestry of the classical swine influenza HA can be traced back to H1N1 viruses that circulated in pigs since 1918. The presence of a swine influenza-like HA in the genome of the A(H1N1)pdm09 virus which has largely remained antigenically intact suggested that seasonal influenza vaccination would provide some level of protection against the 1918 pandemic virus. Using the ferret model, regarded as the most relevant model for influenza vaccine efficacy assessments, we showed that the 2010-2011 seasonal TIV was able to induce neutralizing antibodies that reacted with 1918 virus and provided a significant reduction in morbidity and virus shedding compared to these parameters in 1918-challenged control ferrets. Seasonal TIV was also effective at reducing virus replication and pathology in the lung associated with 1918 virus infection.
Although the prechallenge cross-neutralizing titers against the 1918 virus were generally low (GMT of 23 and 28) relative to homologous titers to seasonal B/Brisbane/60 and Mex/4482 H1N1 viruses (GMT of 121 to 485), the antibody titers to one homologous virus, Perth/16 (H3N2) were also low (GMT of 35 and 47) and in the same titer range as the neutralizing titers generated to the 1918 virus. In general, inactivated influenza vaccines are poorly immunogenic in immunologically naïve animals and appreciable antibody responses are induced only at high vaccine doses or when vaccines are adjuvanted (6
). In particular, purified surface antigen preparations of TIV (also referred to as “split” vaccines) are weakly immunogenic in naïve ferrets, often necessitating multiple rounds of vaccine boosting when adjuvants are not used (2
). For the current study, ferrets were vaccinated with TIV manufacturer-filled syringes designed for single-dose administration. We chose not to use an adjuvant to vaccinate animals since the standard TIVs against influenza are mostly nonadjuvanted vaccines (21
). It is worth noting that adjuvanted vaccines have demonstrated the ability, particularly in immunologically naïve animals, to significantly augment the cross-reactivity of antibodies induced by influenza virus vaccines (23
). Thus, it would be of interest to evaluate the potential for adjuvants in TIV formulations to induce greater cross-neutralizing antibody titers against the 1918 virus than were shown in this study for the group that received TIV alone. Although the precise immunological mechanism conferring cross-protection was not addressed in the current study, the generation of TIV-induced neutralizing antibodies against the 1918 virus most certainly played a prominent role in decreasing viral shedding and replication in trachea and lung tissues. TIV generally does not effectively induce mucosal immune responses or influenza-specific CD8+
T cells, two immune effectors capable of protecting the host (10
The antigenic similarities between the 1918 and A(H1N1)pdm09 viruses are supported by the results of passive transfer experiments in which 1918 HA-specific monoclonal antibodies administered to mice protected them against the A(H1N1)pdm09 virus (25
). Taking an opposite experimental approach, Medina and colleagues showed that human A(H1N1)pdm09-positive sera was sufficient to protect mice from 1918 lethal challenge (26
). The same study demonstrated that a partially purified whole-virus vaccine preparation from a recombinant influenza virus carrying the HA gene from A/California/04/2009 virus provided protection against 1918 virus challenge in mice. Conversely, immunization with an inactivated 1918 virus vaccine but not a former seasonal H1N1 influenza vaccine conferred protection against the A(H1N1)pdm09 virus in mice, further demonstrating the antigenic similarity between these viruses (34
The A(H1N1)pdm09 virus is now part of seasonal influenza virus circulation and is a component of the seasonal trivalent vaccine. The 2009 H1N1 antigen became one of three components in the 2010-2011 seasonal TIV and is the same vaccine virus as was used in the 2009 H1N1 monovalent vaccine. Moreover, there was no change in vaccine components for the 2011-2012 Northern Hemisphere TIV. The current worldwide production capacity for trivalent influenza vaccine is approximately 300 million doses per year (20
). Among persons 6 months of age or older, an estimated 130.9 million people, or 43% of the U.S. population, received a seasonal influenza vaccine during August 2010 through May 2011 (http://www.cdc.gov/flu/professionals/vaccination/vaccinecoverage.htm
). The results of our study suggest that individuals who receive the contemporary seasonal influenza vaccine, which contains the A(H1N1)pdm09 virus component, may develop neutralizing antibodies that cross-react against the 1918 virus. Indeed, it was demonstrated that the 2009 H1N1 monovalent vaccine induced cross-reactive antibodies to the 1918 virus among adults ≥18 years enrolled in an A(H1N1)pdm09 influenza vaccine trial (26
). Furthermore, human serology studies showed a high prevalence of A(H1N1)pdm09 cross-reactive antibodies in people born before 1930 due to probable exposure to 1918 or 1918-like H1N1 virus (15
). This correlated with a reduced frequency of hospitalization during the 2009 pandemic among individuals aged ≥65 years when compared with the risks for complications and hospitalizations from seasonal influenza virus infection (4
). A recent study by Reed et al. (C. Reed, J. M. Katz, K. Hancock, A. Balish, J. Singleton, and A. M. Fry for the H1N1 Serosurvey Working Group, unpublished data) estimated that by December 2009, approximately 20% of the U.S. population, including over half of school-aged children, were infected with A(H1N1)pdm09 virus. The extent to which human infection with A(H1N1)pdm09 virus elicits neutralizing antibody against the 1918 virus warrants further study.
Improving biosafety and assessing the dual-use potential and risks of working with highly pathogenic influenza viruses are integral components of public health research. This is the first report to demonstrate that the seasonal trivalent influenza vaccine for 2010-2011 offers protection against the reconstructed 1918 virus. Although more human serology data are needed to determine the frequency of cross-reactive antibodies to the 1918 H1N1 virus, these data (along with other data cited in the text) suggest that there is significant preexisting immunity to potential 1918 virus infection in the human population, which is likely to increase as more individuals acquire immunity through influenza vaccination or infection.