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Please cite this paper as: Kash et al. (2010) Prior infection with classical swine H1N1 influenza viruses is associated with protective immunity to the 2009 pandemic H1N1 virus. Influenza and Other Respiratory Viruses 4(3), 121–127.
Background The 2009 H1N1 pandemic emerged even though seasonal H1N1 viruses have circulated for decades. Epidemio-logical evidence suggested that the current seasonal vaccine did not offer significant protection from the novel pandemic, and that people over the age of 50 might were less susceptible to infection.
Objectives In a mouse challenge study with the 2009 pandemic H1N1 virus, we evaluated protective immune responses elicited by prior infection with human and swine influenza A viruses.
Results Mice infected with A/Mexico/4108/2009 (Mex09) showed significant weight loss and 40% mortality. Prior infection with a 1976 classical swine H1N1 virus resulted in complete protection from Mex09 challenge. Prior infection with either a 2009 or a 1940 seasonal H1N1 influenza virus provided partial protection and a >100-fold reduction in viral lung titers at day 4 post-infection.
Conclusions These findings indicate that in experimental animals recently induced immunity to 1918-derived H1N1 seasonal influenza viruses, and to a 1976 swine influenza virus, afford a degree of protection against the 2009 pandemic virus. Implications of these findings are discussed in the context of accumulating data suggesting partial protection of older persons during the 2009 pandemic.
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- Materials and methods
Influenza A viruses are significant causes of pandemic respiratory disease and annually recurrent seasonal influenza.1 In April 2009, a novel H1N1 influenza A virus was identified from patients in Mexico and the United States and soon spread globally.2 In June 2009, the World Health Organization (WHO) declared the first influenza pandemic since 1968.3 As of January 29, 2010 there have been millions of H1N1 infections and at least 14711 deaths worldwide, although the actual number of cases is likely much higher.4 The pandemic virus is a previously unrecognized reassortant derived from two pre-existing swine influenza A virus lineages.5
In the present mouse challenge study, we evaluated infection-induced protection against the 2009 H1N1 pandemic influenza virus (A/Mexico/4108/09; Mex09) afforded by a pre-1957-era H1N1 virus (A/Hickox/40; H40), a contemporary H1N1 virus (A/Bethesda/NIH50/09; NIH50), a contemporary H3N2 seasonal influenza A virus (A/Bethesda/NIH20/08; NIH20), and by a 1976 classical swine H1N1 virus (A/Swine/Iowa/1/76; Sw76). Classical swine lineage viruses are derived from the 1918 pandemic virus and have circulated enzootically since 1918.6
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Experimental protection against the pandemic H1N1 virus is of obvious interest because most humans have been exposed to circulating seasonal H1N1 and H3N2 viruses throughout their lifetimes, and because preliminary epidemiologic data from older persons in 2009 have suggested lower than expected influenza attack rates, severe disease, and death.13 Recent studies have suggested that the current pandemic H1N1 virus predominantly infects children and young adults, with a median age of between 12 and 22.14–16 The median age of fatal cases was recently reported as 37, with approximately 50% of fatalities in 20–49 year olds.17 This is markedly different from seasonal influenza, in which 95% of mortality usually occurs in persons >65 years old.18 A recent report showed that 2009 pandemic H1N1 cross reactive antibodies were detected in 6–7% of 18 to 64 year olds and in 8–33% of people ≥64 years.19 This suggests that individuals exposed to H1N1 influenza viruses in the first few decades after the 1918 pandemic may have some degree of cross-protective immunity against the 2009 H1N1 pandemic virus. It is thus important for pandemic and vaccination planning to evaluate whether and under what circumstances past H1N1 influenza virus exposures might afford a degree of protection against the 2009 pandemic virus.
The surface glycoprotein HA contains the four major antigenic domains of influenza A viruses.10,20 Accumulation of HA mutations by antigenic drift arising from population immune pressure is a significant cause of the emergence of new seasonal human influenza viruses.21 In contrast, antigenic drift of H1N1 viruses in pigs occurs more slowly,11 perhaps in part because of the short life span of pigs in domestic agriculture. Conservation of the Sa and Sb antigenic sites in classical swine lineage HAs might also reflect these regions being less antigenically important in antibody-mediated immune responses in swine as compared to the corresponding sites in human influenza viruses.
The 1918 pandemic H1N1 influenza virus is the likely common ancestor of both the human H1N1 and the classical swine H1N1 lineages,11,12 both of which have evolved independently since 1918. Archaeserologic data identified high levels of cross-reactive antibody titers to the 2009 pandemic virus in persons born before 1930,19,22 possibly indicating major H1N1 antigenic changes around the time of the severe 1928–1929 epidemic,23 and declining titers in ever smaller percentages of seropositive persons born in the 1930s, 1940s, and later.19 Increasing human age in 2009 should thus be highly correlated with past exposures to ever earlier drift descendants of the human 1918 virus, and thus to HAs ever more closely related antigenically to the 1918-derived classical swine lineage HAs24 from which the HA of the 2009 pandemic virus is derived.5 In contrast, HAs on more recent human seasonal H1N1 viruses are far more distantly related to the 1918 virus and to classical swine lineage viruses. However, it is not yet clear that these human serologic data can fully explain 2009 epidemiologic patterns, or whether there may be one or more additional age thresholds for immunologic protection corresponding to other past influenza events.
Taking data from the 1957 H2N2 pandemic as a benchmark for age-specific morbidity and mortality trends,25 preliminary data from the 2009 pandemic26 seem to be consistent with a protective affect against illness in persons no older than 37·5 years old, and a protective effect against pneumonia and influenza (P&I) mortality in persons older than 57·5 years old in 2009. However, such data are difficult to interpret, not only because of their preliminary nature but also because they do not distinguish between lower attack rates, “delayed” infections, lower rates of clinically apparent illness, or lower rates of complications, and because a possible contribution to protection by neuraminidase or other viral antigens has not been evaluated.
It has also been speculated that decreased morbidity/mortality in older persons in the 2009 pandemic might result in part from vaccination with the 1976 “swine flu” vaccine,19 an inactivated vaccine made from an influenza A H1N1 virus designated A/New Jersey/8/1976 that is antigenically very similar to the Sw76 virus used in the protection studies reported here, and whose HA is closely related to the HA on the current pandemic virus (Figure 1). The 1976 vaccine was administered to about 45 million persons,27 most of whom were 18 years old or older at that time. Roughly 25 million of these vaccinees (57%) are believed to be alive today, approximately half being 52–65 years old and half over 65 years old (Table 2). A possible protective effect of the 1976 vaccine can hopefully be examined by “historical” or “retrospective-prospective” cohort or other epidemiologic studies. If a protective effect is found, it will be important to try to determine its mechanisms, which might be complex and include “boosting” of vaccine responses by infection with circulating H1N1 viruses.
Table 2. Estimated number of persons alive in 2010 who had been vaccinated against influenza in the United States in 1976*
|Age in 1976||Age in 2010||Alive in 2010||Percent vaccinated in 1976||Number vaccinated in 1976|
|≤17||34–51||77 192 149||0||0|
|18–20||52–54||12 923 191||0·19||2 455 406|
|21–25||55–59||19 517 000||0·28||5 464 760|
|26–29||60–63||13 816 273||0·28||3 868 556|
|30||64||2 943 615||0·32||941 957|
|31–35||65–69||12 261 000||0·32||3 923 520|
|36–40||70–74||9 202 000||0·32||2 944 640|
|41–45||75–79||7 282 000||0·32||2 330 240|
|46–49||80–83||4 781 546||0·32||1 530 095|
|50||84||950 116||0·33||313 538|
|51–55||85–89||3 650 000||0·33||1 120 500|
|56–60||90–94||1 570 000||0·33||518 100|
|61–65||95–99||452 000||0·33||149 160|
|≥66||≥100||79 000||0·33||26 070|
|Total|| ||89 427 741||0·2871||25 670 542|
The data presented in this study may provide some support for this hypothesis, as Sw76 virus infection conferred complete protection against Mex09 challenge, whereas vaccination with human H1N1 virus H40 or NIH50 provided only partial protection. Our study also provides data consistent with the archaeserologic studies described above, in which cross-reacting antibodies to the 2009 pandemic virus were detected in the sera of persons born before 1930 and likely exposed to 1918-descended H1N1 viruses in the decade after the pandemic,19,22 since infection of mice with Sw76 resulted in cross-reactive antibodies against the reconstructed 1918 influenza virus. However, the mouse challenge studies do not fully correspond to the natural situation in humans, e.g., the 33-year gap between swine influenza vaccination in 1976 and current pandemic virus exposure, which might be associated with loss of immune memory or intermittent boosting by exposure to naturally circulating H1N1 viruses after 1977.
The current study also revealed that mice infected with a 1940s-era H1N1 virus (H40) before Mex09 challenge had somewhat less severe pathology and lower lung titers than mice infected with a 2009 seasonal H1N1 virus (NIH50). Alignment of the antigenic sites of 1918-derived human and swine lineage H1N1 HAs demonstrates that rapid antigenic drift likely occurred in the decade after the 1918 pandemic, since the earliest isolated human H1N1 virus (A/WS/1933) had accumulated a number of antigenic differences from the 1918 virus (Figure 1). Continual antigenic drift during seasonal influenza virus circulation may partially explain why an earlier human H1N1 virus offered modestly more protection against Mex09 challenge than a contemporary seasonal H1N1 virus, and supports epidemiological findings in the 2009 pandemic that people over about age 60 may have a degree of immunologic protection. Mice inoculated with NIH20 (a 2009 seasonal H3N2) had modest decreases in viral replication after challenge with Mex09 as compared to controls, suggesting that uncharacterized immune responses may be playing a partially protective role. The observation of neutralizing antibody-independent heterosubtypic immunity to influenza viruses has been described in previous studies and are associated with cytotoxic T-lymphocyte (CTL) responses.28–32 It must be stressed that experimental infection with live viruses, followed closely by viral challenge, is not an ideal model for studying human protection induced by live attenuated or inactivated vaccines. Further work is necessary to determine if inactivated vaccines would also elicit a similar effect and provide heterosubtypic immunity. Nonetheless, these studies are important in linking viral evolution at key antigenic sites to a role in protective heterologous immunity. Clearly, key antigenic sites on the HA protein of swine lineage influenza viruses have been preserved from 1918 to the present, and these epitopes may be susceptible to neutralization by antibodies induced by early descendents of the 1918 human influenza virus and conceivably also by an H1N1 swine influenza vaccine administered to millions of Americans 33 years ago.