Identification of seasonal vaccine hemagglutinin subtype 1 (H1) epitopes in Mexican isolates of the new influenza A (H1N1) 2009 virus

Epidemiological characteristics and underlying risk factors for mortality during the autumn 2009 pandemic wave in Mexico

Background

Elucidating the role of the underlying risk factors for severe outcomes of the 2009 A/H1N1 influenza pandemic could be crucial to define priority risk groups in resource-limited settings in future pandemics.

Methods

We use individual-level clinical data on a large series of ARI (acute respiratory infection) hospitalizations from a prospective surveillance system of the Mexican Social Security medical system to analyze clinical features at presentation, admission delays, selected comorbidities and receipt of seasonal vaccine on the risk of A/H1N1-related death. We considered ARI hospitalizations and inpatient-deaths, and recorded demographic, geographic, and medical information on individual patients during August-December, 2009.

Results

Seasonal influenza vaccination was associated with a reduced risk of death among A/H1N1 inpatients (OR = 0.43 (95% CI: 0.25, 0.74)) after adjustment for age, gender, geography, antiviral treatment, admission delays, comorbidities and medical conditions. However, this result should be interpreted with caution as it could have been affected by factors not directly measured in our study. Moreover, the effect of antiviral treatment against A/H1N1 inpatient death did not reach statistical significance (OR = 0.56 (95% CI: 0.29, 1.10)) probably because only 8.9% of A/H1N1 inpatients received antiviral treatment. Moreover, diabetes (OR = 1.6) and immune suppression (OR = 2.3) were statistically significant risk factors for death whereas asthmatic persons (OR = 0.3) or pregnant women (OR = 0.4) experienced a reduced fatality rate among A/H1N1 inpatients. We also observed an increased risk of death among A/H1N1 inpatients with admission delays >2 days after symptom onset (OR = 2.7). Similar associations were also observed for A/H1N1-negative inpatients.

Conclusions

Geographical variation in identified medical risk factors including prevalence of diabetes and immune suppression may in part explain between-country differences in pandemic mortality burden. Furthermore, access to care including hospitalization without delay and antiviral treatment and are also important factors, as well as vaccination coverage with the 2008–09 trivalent inactivated influenza vaccine.

Characterization of antibodies specific for hemagglutinin and neuraminidase proteins of the 1918 and 2009 pandemic H1N1 viruses

Serologic studies have detected protective immunity against 2009 pandemic H1N1 influenza virus (H1N1-2009) in some people. However, further study of preexisting immunity has been complicated by the complexity of the human immunological background. Here, we immunized mice with HA- and NA-encoding plasmids. The cross-neutralizing activity of the anti-HA antisera and the effect of the anti-NA antisera on viral infectivity were evaluated using H1N1-1918- and 2009-pseudotyped particles (pps) and an H1N1-2009 isolate. Antibodies to H1N1-2009 HA (09HA) neutralized pps harboring 09HA or H1N1-1918 HA (18HA); similarly, antibodies to 18HA neutralized pps harboring 18HA or 09HA. Antibodies to 09HA and 18HA also neutralized the H1N1-2009 virus with high efficiency. Antibodies to H1N1-1918 NA (18NA) or H1N1-2009 NA (09NA) both enhanced the infectivity of pps harboring 09NA and 18NA. Although anti-09NA and -18NA antibodies significantly reduced cytopathic effects in multiple-cycle infection assays, conversely, these antibodies enhanced the infectivity of H1N1-2009 in single-cycle infection assays. Our study demonstrates the existence of cross-protection between antibodies against these two antigenically related virus strains and shows that anti-NA antibodies have a dual effect that requires reexamination of their role in human immunity.