Most barometers of influenza circulation in the United States ticked upward again last week, with 37 states reporting geographically widespread activity, the Centers for Disease Control and Prevention (CDC) reported today.
That number was up from 33 states a week earlier. Puerto Rico also reported widespread flu activity both weeks.
Ten states and Puerto Rico reported high influenza-like illness (ILI) activity, compared with eight states and Puerto Rico the week before, the CDC said. Thirteen states and New York City reported moderate ILI levels, versus nine states and New York the previous week.
The estimated share of outpatient medical visits prompted by ILI was 3.5%, up from 3.2% a week earlier and well above the national baseline of 2.1%.
Another indicator still rising was the proportion of respiratory specimens that tested positive for flu: 20.6% of 23,910 specimens, compared with 17.6% of 21,615 a week earlier.
Deaths and hospitalizations
Two flu-related deaths in children were reported, raising the season's total to 20, the CDC said. The previous week brought reports of four pediatric deaths.
The CDC reported conflicting signals from its two surveillance systems for the proportion of deaths due to pneumonia and flu. One system (122 Cities Mortality Reporting) put this figure at 7.0%, down from 7.4% last week, with an epidemic threshold of 7.0%. The other system (National Center for Health Statistics) estimated the number at 6.9%, up from 6.7% last week, with an epidemic threshold of 7.7%.
The estimate of cumulative flu-related hospitalizations for the whole season reached 10.4 per 100,000 people, up from 7.8 per 100,000 last week, the CDC reported. Elderly people (65 and older) have been hit hardest, with a rate of 27.6 hospitalizations per 100,000, followed by children under 5 years old (15.8 per 100,000).
In the previous week, the hospitalization rate in the elderly was 21.3 per 100,000 population, and in children younger than 5 it was 11.8 per 100,000.
Virologic testing showed that influenza A viruses continued to heavily outnumber type B viruses last week (77.6% to 22.4%), the report says. Of type A viruses that were characterized, 83.5% were 2009 H1N1 isolates.
Mar 11 CDC FluView report