The nation's flu activity continues to rise, with a jump last week in the number of states reporting widespread disease, marking the fifth week of elevated activity, the US Centers for Disease Control and Prevention (CDC) said today in its latest update.
Twenty-three states are now experiencing widespread flu, up from 16 reported the week before. Most of those states are in the south, west, and east.
However, one flu-tracking marker—percentage of outpatient visits for flulike illness—fell slightly from 3.5% to 3.2% last week, which the CDC said could reflect fewer routine healthcare visits during Thanksgiving week, a pattern seen in previous seasons. The level remains above the national baseline, though, and all parts of the country are at or above their specific regional baselines for flulike illness clinic visits.
Flu B still predominant, but H1N1 rising
Influenza B is still the most commonly detected strain, accounting for 68.5% of flu positives at clinical labs. High levels of influenza B are typically seen late in the flu season, but circulation of the strain at the end of the last US flu season was very low. High numbers at the start of a flu season are very unusual.
Though all three strains are circulating, the CDC is seeing some shift in the proportions of influenza A viruses, with the proportion of 2009 H1N1 viruses increasing in some regions.
CDC said the predominant virus still varies by region as well as age group, with influenza B infections more common in young children and people ages 5 to 24 years old and H3N2 more common in seniors.
The CDC regularly tests flu viruses to see how well they match the vaccine viruses and to look for antiviral resistance. Results so far show that the all circulating strains are similar to cell-grown vaccine strains. And of 415 viruses tested, nearly all (99%) were susceptible to the four approved antiviral medications.
Hospitalizations rise; 4 more pediatric deaths
Flu hospitalizations rose from 2.9 per 100,000 population to 3.9 per 100,000 population, which is normal for this point in the flu season, the CDC said. The highest rates are in seniors, followed by young children and adults ages 50 to 64. Influenza A is slightly more common than influenza B in hospitalized patients, and of the subtyped influenza A viruses, 70.5% were due to 2009 H1N1.
Four more pediatric flu deaths were reported, raising the season's total to 10. Of the latest deaths, two were from influenza A and two were due to influenza B.
Overall deaths from pneumonia and flu remained below the seasonal baseline.
This season the CDC has started reporting weekly flu burden estimates, based on a mathematical model that contains data on lab-confirmed flu hospitalization numbers from a surveillance network that covers 8.5% of the US population.
It estimates that, from Oct 1 through Dec 7, flu has sickened between 2.6 million and 3.7 million people, resulted in 1.2 million to 1.8 million medical visits, led to 23,000 to 41,000 hospitalizations, and caused 1,300 to 3,300 deaths.
See also:
Dec 13 CDC FluView report
Dec 13 CDC preliminary flu burden estimates
