Since the end of August -- the official start of the 2009-2010 flu season -- influenza activity has increased rapidly across the U.S., nearly all of it attributable to the pandemic 2009 H1N1 influenza virus.
And through the end of October, all but two states -- Mississippi and Hawaii -- and the District of Columbia were reporting widespread disease, CDC researchers reported in the Nov. 13 issue of Morbidity and Mortality Weekly Report.
The activity is "substantially elevated compared with data recorded in previous years over the same period," the researchers said.
For the 2008-2009 flu season, widespread activity was not reported before the week ending Jan. 10.
Over the first two months of the current flu season U.S. laboratories tested 163,123 respiratory specimens for influenza viruses and 30% were positive. Nearly all of those that tested positive were the pandemic strain.
Since Sept. 1, 239 pandemic H1N1 viruses have been antigenically characterized, and all but one were related to the reference virus used to create the pandemic vaccine that is being distributed.
Another 256 pandemic viruses were tested for antiviral resistance in September and October. All were susceptible to zanamivir (Relenza) and all but four were susceptible to oseltamivir (Tamiflu).
Since the beginning of the outbreak in April, researchers have identified 14 isolates resistant to oseltamivir.
All of the pandemic isolates have been resistant to the adamantanes, a fact known since the early stages of the epidemic.
Some of the other findings from the most recent surveillance data:
- The weekly percentage of outpatient visits for influenza-like illness increased from 3.5% in the week ending Sept. 5 to 7.7% in the week ending Oct. 31.
- Cumulative influenza hospitalization rates for people younger than 65 were elevated in September and October.
- In the week ending Oct. 31, pneumonia or influenza was reported as an underlying or contributing cause in 7.4% of all deaths, which was above the week-specific epidemic threshold of 6.7%. This statistic exceeded the epidemic threshold for the fifth straight week.
- During September and October, there were 85 flu-related pediatric deaths reported; 73 were confirmed as related to the pandemic strain and the other 12 were related to an influenza A virus of undetermined subtype.
- Of the children who died, 72% had an underlying medical condition that increased the risk of flu-related complications.
- There have been 145 flu-related pediatric deaths since April, with the pandemic strain responsible for 89%. The average number of pediatric deaths in the previous five flu seasons has been 82.
In an accompanying unsigned comment, the MMWR editors cautioned that even though most of the influenza viruses identified so far this year have been the pandemic strain, seasonal viruses continue to circulate at low levels.
"Influenza circulation patterns remain unpredictable, and seasonal influenza viruses might circulate more widely later in the influenza season," they said.
Morbidity and Mortality Weekly ReportSource Reference: WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza "Update: Influenza activity -- United States, August 30-October 31, 2009" MMWR 2009; 58: 1236-41.