Influenza H1 2009 Strain: Current Circulation and Importance
Yes, the Influenza H1 2009 strain (pandemic H1N1 2009) continues to circulate and remains an important strain in the influenza landscape, having been incorporated into seasonal influenza vaccines as the predominant H1N1 component.
Current Status of H1N1 2009 Strain
The 2009 pandemic influenza A (H1N1) virus that emerged in March 2009 has evolved from being a novel pandemic strain to becoming the predominant seasonal influenza A (H1N1) virus strain. According to the CDC, in the post-pandemic period, the 2009 H1N1 virus strains are now considered to be the predominant seasonal influenza A (H1N1) virus strains worldwide 1.
Evidence of Continued Circulation
- The 2009 H1N1 strain replaced the previous seasonal H1N1 strains that had circulated since 1977, which have been detected very rarely since 2009 1
- The strain has been incorporated into seasonal influenza vaccines since 2010, indicating its continued relevance and circulation 1
- The 2011-2012 trivalent influenza vaccine continued to include the A/California/7/2009 (H1N1)-like antigen derived from the 2009 pandemic influenza A (H1N1) virus 1
Clinical Significance and Impact
The H1N1 2009 strain remains important for several key reasons:
Age Distribution and Mortality Patterns
Unlike typical seasonal influenza strains that primarily affect the elderly, the 2009 H1N1 strain has shown a different pattern:
- Highest attack rates among children and young adults 2
- Lower incidence among adults over 60 years old, likely due to preexisting antibodies from exposure to other H1N1 viruses 2
- During the pandemic, the median age of hospitalized persons with laboratory-confirmed novel influenza A (H1N1) virus infection was 20 years 1
- The pediatric population was disproportionately affected, with nearly four times the average number of laboratory-confirmed, influenza-associated pediatric deaths compared to previous five influenza seasons 1
Continued Vigilance Required
Even after the pandemic period, the 2009 H1N1 strain continued to cause significant disease:
- In the first influenza season after the pandemic (2010-2011), the H1N1 2009 strain was a major influenza A strain in Europe, with reports of increased mortality and morbidity compared to pre-pandemic seasonal influenza 3
- The strain has shown potential for nosocomial transmission, particularly in immunocompromised patients 4
Vaccine Considerations
The continued importance of the H1N1 2009 strain is reflected in vaccine composition decisions:
- The A/California/7/2009 (H1N1)-like antigen has been included in seasonal influenza vaccines since the 2010-2011 season 1
- The World Health Organization has maintained this strain in the vaccine composition recommendations, indicating its continued circulation and relevance 1
- As of 2011, it was only the fourth time in 25 years that there was no need to change any of the influenza vaccine strains, suggesting the stability and continued importance of the included strains, including the 2009 H1N1 component 1
Treatment Considerations
For infections caused by the 2009 H1N1 strain:
- Neuraminidase inhibitors (oseltamivir or zanamivir) remain recommended for treatment 1
- Resistance to oseltamivir has been documented but remains relatively low, though vigilance is required, particularly in immunocompromised patients 4
Conclusion
The H1N1 2009 strain has transitioned from a pandemic virus to an established seasonal influenza strain that continues to circulate globally. Its inclusion in seasonal vaccines and its distinct clinical pattern compared to previous seasonal H1N1 strains underscores its continued importance in influenza surveillance, prevention, and treatment strategies.