H1N1 Crowds Out Other Flu Viruses

by Aaron Atkinson on April 30, 2010

More than 99 percent of specimens tested for influenza were positive for H1N1 over approximately the past 11 months, according to a report released by Quest Diagnostics. The Quest Diagnostics Health Trends™ Report suggests that H1N1 “crowded out” other influenza viruses to be the dominant flu virus of the 2009-2010 flu season.

The Quest Diagnostics Health Trends Report also shows that H1N1 positivity was highest among children during two waves of H1N1 infections, according to the company’s national testing data. Among those aged 5 to 9 years who were tested, 76 percent were H1N1 positive in late June, during the first wave of H1N1 activity, and 78 percent were positive in late October, the peak of the second wave. Among children aged 10 to 14 years, 83 percent of tests were positive for H1N1 in late June while 82 percent were positive in late October. By comparison, positivity for adults aged 25 to 49 years tested was 46 percent in late June and 50 percent in late October.

Children have continued to experience higher rates of positive H1N1 test results in 2010. Eighteen percent of children aged 5 to 9 years tested and 26 percent of children aged 10 to 14 years tested were positive for H1N1 during the four weeks ending April 15, 2010. By comparison, 13 percent of adults aged 25 to 49 years tested were positive for H1N1 during the same period.

“The most interesting finding from our Health Trends report is that seasonal flu viruses were virtually nonexistent during the past year’s flu season,” said Jay Lieberman, M.D., Medical Director, Infectious Diseases. “This finding is consistent with the behavior of prior influenza pandemics.

“Another key finding is that the higher positivity rate in children reflects the important role school-aged children play in the spread of influenza, as close physical contact in schools promotes the transmission of flu viruses. It is no coincidence that the start of the second wave of H1N1 infections coincided with children returning to school in late August and early September last year,” said Dr. Lieberman.

Infection from pandemic influenza viruses often occurs in waves of activity. Since peaking in late October 2009, H1N1 testing and positivity rates have both declined, indicating that a third wave has not occurred. The Quest Diagnostics investigators believe the declines in testing and positivity rates may be due primarily to lower rates of infection from millions of Americans having already been infected with H1N1 and millions more receiving the H1N1 vaccine, which have reduced the number of people susceptible to infection.  In addition, changes in physician test-ordering practices may influence testing patterns.

However, the Quest Diagnostics Health Trends™ Report also shows that H1N1 has not disappeared, and that the rate of positive H1N1 tests in the south was higher than the rate in the rest of the country in recent weeks.  The most recent data for the four weeks ending April 15, 2010, shows that about 26 percent of patients in all age groups tested in the southeast and 22 percent in the central south were positive for H1N1, compared to 6 percent for the remainder of the U.S. The southeast region includes Alabama, Florida, Georgia, Kentucky, Mississippi, North and South Carolina and Tennessee. The central south region includes Arkansas, Louisiana, New Mexico, Oklahoma and Texas.

The Quest Diagnostics Health Trends Report is based on an analysis of more than 195,000 H1N1 de-identified laboratory tests performed by the company nationally since it launched its first H1N1 test in May 2009. It is believed to be the largest analysis of H1N1 testing data in the U.S. by a private organization. The CDC identified H1N1 as a novel influenza strain in April 2009.

To read the full report, please visit QuestDiagnostics.com

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