H1N1 in Transition: La persistance de dénouements

As the traditional winter flu season peaks, pandemic H1N1 (pH1N1) activity in the USA remains at low ebb. Overall flu activity has remained below baseline for the last six weeks. Currently, pH1N1 is circulating at elevated (above baseline) levels in pockets of the southeastern and western regions of the country. Recent CDC estimates indicate that 11,690 Americans died from influenza and around 57 million have been infected with pH1N1.

H1N1 vaccine demand is perhaps at low ebb as well. As of 13 Feb, pH1N1 vaccine uptake was estimated around 86 million persons (97 million doses). Despite the safety record of pH1N1 vaccine, which parallels seasonal flu vaccine, surveys continue to indicate persistent public doubts concerning vaccine safety in general.

According to the World Health Organization, pH1N1 transmission persists at elevated levels in areas of Europe and Asia, but is otherwise at declining or low levels in the northern hemisphere. The most active areas of pH1N1 transmission are parts of Southeastern Asia (Thailand and Myanmar) and Eastern Europe (Russia, Bulgaria, Armenia and Moldova). In China, Influenza B is the predominant flu strain in circulation, accounting for 88% of influenza detected in surveillance.

Overall, pH1N1 remains treatable with antiviral medications, oseltamivir and zanamivir. Disagreement remains over the pathogenicity of oseltamivir-resistant mutations of pH1N1 (referred to as D222G/D225G). The Norwegian Institute of Public Health reported that the mutation was associated with 11 of 61 cases of severe illness and was not found in any mild cases. The Influenza Division at the US Centers for Disease Control and Prevention (CDC) countered that global H1N1 data do not show a clear association between the mutation and severe illness, and that the mutation proved fatal in three of eight cases in the USA.

Comment: A comparison of apples-to-apples appears needed. CDC contends the Norwegian data needs to control for covarying factors, such as underlying patient conditions. The three fatalities in the USA that had the mutation were patients treated at Duke University, all of whom had compromised immune systems; this association had not been highlighted in reporting. At most, the Norwegian data suggests an upper-bound on the incidence of this mutation in severe disease. Seasonal H1N1 influenza viruses are now completely resistant to oseltamivir and this is a likely evolutionary path for future pH1N1 mutations.

CDC Estimates of 2009 H1N1 Influenza Cases, Hospitalizations and Deaths in the United States, April 2009 – January 16, 2010
CIDRAP: CDC – Pandemic vaccine safety record still matches seasonal vaccine
CIDRAP: H1N1 mutation’s proposed link to severe illness debated
LATimes Health: Parents’ vaccination fears remain high
Reuters: Biggest swine flu regret for U.S.: vaccine chaos
WHO: Pandemic (H1N1) 2009 – update 90
WHO: Pandemic (H1N1) 2009 – update 90: Weekly Virological Surveillance Update
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