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Rates of 2009 H1N1 influenza virus testing in the U.S. have declined more than 75 percent since their peak in late October, suggesting that the "second wave" of virus infection that sickened tens of millions of Americans since it began four months ago may be coming to an end, according to a new report by Quest Diagnostics Incorporated, a provider of diagnostic testing, information and services.
The Quest Diagnostics Health Trends Report, "H1N1 Testing in America: End of the Second Wave?" also indicates that the virus is still the dominant influenza virus in the U.S. and that close to four in ten school-age children tested as recently as last week were infected with the pandemic virus.
"Our data are a welcome indication that H1N1 may not be the health threat during the holidays that it has been throughout the fall," said Jay M. Lieberman, MD, medical director for infectious diseases at Quest Diagnostics. "Nonetheless, this is no time for people to be complacent about protecting themselves and their families from H1N1 infection. The 2009 H1N1 influenza virus continues to be far and away the most significant source of influenza in every region of the U.S., according to our data. Many millions of people remain susceptible to the virus, which continues to cause serious disease in some individuals. In addition, the prospect of a third wave of virus activity next year means it is far too soon to declare victory over H1N1.”
Quest Diagnostics analyzed results of nearly 170,000 de-identified patient specimens it tested for the 2009 H1N1 influenza virus in the U.S. between May 11, 2009, when the company introduced its first test for detecting the pandemic virus, and Dec. 9, 2009. Quest Diagnostics is the only company in the U.S. that both performs testing for H1N1 and also provides 2009 H1N1 laboratory testing kits, developed by its Focus Diagnostics business, that are FDA authorized for emergency use by qualified molecular laboratories. The tests employ real-time reverse transcription polymerase chain reaction to qualitatively detect the virus' RNA in a patient's nasal or nasopharyngeal specimens.
Influenza viruses often circulate in waves of activity. The report indicates that two major waves of H1N1 influenza virus activity have affected the U.S., based on the company's test data. The first wave started in early May, following the discovery of the virus in the U.S. in late April. A second wave began in late August and peaked the week ending October 28. Between the weeks ending October 28 and December 9, testing rates fell by 75 percent, and now are roughly equivalent with testing rates experienced when the second wave began.
Other key findings:
-- The number of specimens that tested positive for 2009 H1N1 influenza has declined in all age groups since late October. Children ages five to 14 continue to experience the highest percentage of H1N1 positive test results compared to negative results, with a positivity rate of close to 40 percent. "Our findings suggest that children continue to be most vulnerable to the H1N1 virus," said Dr. Lieberman.
-- Nearly every region experienced declines of 60 percent or more in positive test results during the two weeks ending December 9 compared to the two weeks ending November 25. The most striking decline in positive test results occurred in the region comprised by Pennsylvania, Maryland, Delaware, Virginia and West Virginia, and the District of Columbia (87 percent decline).
-- The 2009 H1N1 influenza virus remains the predominant cause of influenza in the U.S., as more than 98 percent of positive influenza A specimens are positive for 2009 H1N1 influenza. "Our data provide no evidence that seasonal influenza viruses have begun to circulate," added Lieberman.
The Quest Diagnostics investigators believe the decline in testing demand is due to several factors, primarily lower rates of infection due to millions of Americans having already been infected with this influenza virus and the impact of H1N1 vaccines, both of which have reduced the number of people susceptible to infection, and changes in physician test-ordering practices.