Virus-specific factors associated with zoonotic and pandemic potential

Influenza A is a highly contagious respiratory virus in constant evolution and represents a threat to both veterinary and human public health. IA viruses (IAVs) originate in avian reservoirs but may adapt to humans, either directly or through the spillover to another mammalian species, to the point of becoming pandemic. IAVs must successfully be able to (i) transmit from animal to human, (ii) interact with host cells, and (iii) transmit from human to human. The mechanisms by which viruses evolve, cause zoonotic infections, and adapt to a new host species are indeed complex and appear to be a heterogeneous collection of viral evolutionary events rather than a single phenomenon. Progress has been made in identifying some of the genetic markers mainly associated with virulence and transmission; this achievement has improved our knowledge of how to manage a pandemic event and of how to identify IAVs with pandemic potential. Early evidence of emerging viruses and surveillance of animal IAVs is made possible only by strengthening the collaboration between the public and veterinary health sectors.

Addition of glycosylation to influenza A virus hemagglutinin modulates antibody-mediated recognition of H1N1 2009 pandemic viruses

Seasonal influenza A viruses (IAV) originate from pandemic IAV and have undergone changes in antigenic structure, including addition of glycans to the viral hemagglutinin (HA). Glycans on the head of HA promote virus survival by shielding antigenic sites, but highly glycosylated seasonal IAV are inactivated by soluble lectins of the innate immune system. In 2009, human strains of pandemic H1N1 [A(H1N1)pdm] expressed a single glycosylation site (Asn(104)) on the head of HA. Since then, variants with additional glycosylation sites have been detected, and the location of these sites has been distinct to those of recent seasonal H1N1 strains. We have compared wild-type and reverse-engineered A(H1N1)pdm IAV with differing potential glycosylation sites on HA for sensitivity to collectins and to neutralizing Abs. Addition of a glycan (Asn(136)) to A(H1N1)pdm HA was associated with resistance to neutralizing Abs but did not increase sensitivity to collectins. Moreover, variants expressing Asn(136) showed enhanced growth in A(H1N1)pdm-vaccinated mice, consistent with evasion of Ab-mediated immunity in vivo. Thus, a fine balance exists regarding the optimal pattern of HA glycosylation to facilitate evasion of Ab-mediated immunity while maintaining resistance to lectin-mediated defenses of the innate immune system.

Effectiveness of monovalent 2009 pandemic influenza A virus subtype H1N1 and 2010-2011 trivalent inactivated influenza vaccines in Wisconsin during the 2010-2011 influenza season

BACKGROUND

The 2009 influenza A virus subtype H1N1 (A[H1N1]pdm09) did not exhibit antigenic drift during the 2010-2011 influenza season, providing an opportunity to investigate the duration of protection after vaccination. We estimated the independent effects of 2010-2011 seasonal trivalent inactivated influenza vaccine (TIV) and A(H1N1)pdm09 vaccine for preventing medically attended influenza A virus infection during the 2010-2011 season.

METHODS

Individuals were tested for influenza A virus by real-time reverse transcription polymerase chain reaction (rRT-PCR) after a clinical encounter for acute respiratory illness. Case-control analyses compared participants with rRT-PCR-confirmed influenza A virus infection and test-negative controls. Vaccine effectiveness was estimated separately for monovalent pandemic vaccine and TIV and was calculated as 100 × [1 - adjusted odds ratio], where the odds ratio was adjusted for potential confounders.

RESULTS

The effectiveness of TIV against influenza A virus infection was 63% (95% confidence interval [CI], 37%-78%). The effectiveness of TIV against A(H1N1)pdm09 infection was 77% (95% CI, 44%-90%). Monovalent vaccine administered between October 2009 and April 2010 was not protective during the 2010-2011 season, with an effectiveness of -1% (95% CI, -146% to 59%) against A(H1N1)pdm09 infection.

CONCLUSIONS

 Monovalent vaccine provided no sustained protection against A(H1N1)pdm09 infection during the 2010-2011 season. This waning effectiveness supports the need for annual revaccination, even in the absence of antigenic drift in A(H1N1)pdm09.

Characterization of oseltamivir-resistant influenza A(H1N1)pdm09 viruses in Taiwan in 2009-2011

The early isolated swine-origin influenza A(H1N1)pdm09 viruses were susceptible to oseltamivir; however, there is a concern about whether oseltamivir-resistant influenza A(H1N1)pdm09 viruses will spread worldwide as did the oseltamivir-resistant seasonal influenza A(H1N1) viruses in 2007-2008. In this study, the frequency of oseltamivir resistance in influenza A(H1N1)pdm09 viruses was determined in Taiwan. From May 2009 to April 2011, 1,335 A(H1N1)pdm09-positive cases in Taiwan were tested for the H275Y mutation in the neuraminidase (NA) gene that confers resistance to oseltamivir. Among these, 15 patients (1.1%) were found to be infected with H275Y virus. All the resistant viruses were detected after the patients have received the oseltamivir. The overall monthly ratio of H275Y-harboring viruses ranged between 0% and 2.88%, and the peak was correlated with influenza epidemics. The genetic analysis revealed that the oseltamivir-resistant A(H1N1)pdm09 viruses can emerged from different variants with a great diversity under drug pressure. The ratio of NA/HA activities in different clades of oseltamivir-resistant viruses was reduced compared to those in the wild-type viruses, indicating that the balance of NA/HA in the current oseltamivir-resistant influenza A(H1N1)pdm09 viruses was interfered. It is possible that H275Y-bearing A(H1N1)pdm09 virus has not yet spread globally because it lacks the essential permissive mutations that can compensate for the negative impact on fitness by the H275Y amino acid substitution in NA. Continuous monitoring the evolution patterns of sensitive and resistant viruses is required to respond to possible emergence of resistant viruses with permissive genetic background which enable the wide spread of resistance.

Genetic diversity of influenza A(H1N1)2009 virus circulating during the season 2010-2011 in Spain

BACKGROUND

Genetic diversity of influenza A(H1N1)2009 viruses has been reported since the pandemic virus emerged in April 2009. Different genetic clades have been identified and defined based on amino acid substitutions found in the haemagglutinin (HA) protein sequences. In Spain, circulating influenza viruses are monitored each season by the regional laboratories enrolled in the Spanish Influenza Surveillance System (SISS). The analysis of the HA gene sequence helps to detect the genetic diversity and viral evolution.

OBJECTIVES

To perform an analysis of the genetic diversity of influenza A(H1N1)2009 viruses circulating in Spain during the season 2010-2011 based on analysis of the HA sequence gene.

STUDY DESIGN

Phylogenetic analysis based on the HA1 subunit of the haemagglutinin gene was carried out on 220 influenza A(H1N1)2009 viruses circulating during the season 2010-2011.

RESULTS

Six different genetic groups were identified among circulating A(H1N1)2009 viruses, five of them were previously reported during season 2010-2011. A new group, characterized by E172K and K308E changes and a proline at position 83, was observed in 12.27% of the Spanish viruses.

CONCLUSION

Co-circulation of six different genetic groups of influenza A(H1N1)2009 viruses was identified in Spain during the season 2010-2011. Nevertheless, at this stage, none of the groups identified to date have resulted in significant antigenic changes according to data collected by World Health Organization Collaborating Centres for influenza surveillance.