SAN FRANCISCO, Jan. 25 (UPI) -- Despite its circulation since 2009, immunity to the flu strain A(H1N1)2009 is not sufficiently high and many people remain susceptible, U.S. researchers say.
U.S. influenza activity is surging and there are reports of critical illness and death in young and middle-aged adults, Dr. Timothy M. Uyeki of San Francisco General Hospital wrote in the New England Journal of Medicine.
The predominant virus so far this season is influenza A(H1N1) 2009, the cause of the 2009 H1N1 pandemic.
The spread of influenza A(H1N1) 2009 virus suggests that to date, surveillance data provide no evidence of significant antigenic drift in the circulating virus strains, so susceptibility could be due to the presence of a substantial number of previously uninfected and unvaccinated people or waning immunity from prior infection.
Although previously healthy people can have severe illness, certain groups -- including children age 2 and younger; the elderly; pregnant women; people with certain chronic conditions such as pulmonary, cardiac, renal, hepatic, metabolic, hematologic, neurologic, or neuromuscular conditions; immunosuppression; or morbid obesity; nursing home residents American Indians and Alaska Natives -- are at increased risk for complications.
The effectiveness of influenza vaccine varies depending on several factors, including the recipient's age and immune response and the match between circulating virus strains and vaccine strains, Uyeki said.
Influenza vaccine has been moderately effective in recent years, though less so among the elderly, and some vaccinated people may still develop influenza, he said.
Circulating influenza A(H1N1) 2009 virus strains are well matched to date by H1N1 strains in all available vaccines this season, and unvaccinated people should be vaccinated as soon as possible, Uyeki advised.
Annual influenza vaccination is recommended for everyone 6 months of age or older in the United States.
Resistance of influenza A(H1N1) 2009 viruses to the antiviral treatment oseltamivir is documented, but surveillance indicates the prevalence of oseltamivir-resistant influenza viruses is low to date.
For severely ill patients with influenza who have strongly suspected or documented oseltamivir resistance or malabsorption, or gastrointestinal bleeding, intravenous zanamivir -- an investigational drug -- can be considered. It is available through enrollment in a clinical trial, Uyeki said.