2009 H1N1 influenza – the pandemic continues

Originally published in Journal Watch Infectious Diseases

by Stephen G. Baum, MD

Since the emergence of this virus in spring 2009, the number of infections has remained uncharacteristically high, presaging a bad winter for influenza.

journal watch logo The first cases of influenza A (H1N1) in the U.S. were identified in April 2009. By August 30, at least 1 million cases had occurred in this country; 9079 influenza-related hospitalizations and 593 deaths had been reported to the CDC. New cases appeared during the summer months, when influenza activity is usually rare, with incidence approaching winter levels in August in six states and Puerto Rico. Although the proportion of deaths attributed to influenza or pneumonia did not exceed epidemic thresholds for ≥2 consecutive weeks at any time during the summer, the number of pediatric deaths (47) was elevated.

The CDC and the WHO reported that since May 3, most influenza viruses identified from patients in the U.S. have been 2009 pandemic influenza A (H1N1) viruses. All the 2009 H1N1 viruses characterized have been related antigenically to the reference strain chosen for the vaccine currently in production. Of 1372 H1N1 virus isolates tested, 99.4% were susceptible to oseltamivir, all were susceptible to zanamivir, and all were resistant to amantadine and rimantadine.

Comment: The 2009 pandemic influenza A (H1N1) virus came out of nowhere and is acting in an unusual and perplexing way, confounding predictions for the upcoming flu season. Its swine origin is particularly worrisome. Fortunately, the mortality rate is not as high as initially feared, but children and pregnant women seem to be at increased risk. Vaccination against seasonal strains, self-quarantine until 24 hours after fever abates, observance of respiratory etiquette and hand hygiene, and oseltamivir treatment for persons at high risk for complications are the only measures available until an H1N1 vaccine arrives. For the first time, some states have mandated that all healthcare workers receive seasonal and H1N1 vaccines.


Centers for Disease Control and Prevention (CDC). Update: Influenza activity — United States, April–August 2009. MMWR Morb Mortal Wkly Rep 2009 Sep 18; 58:1009.

Copyright © 2009. Massachusetts Medical Society. All rights reserved.

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