The flu season in the United States has now reached 18 straight weeks above baseline levels but shows signs of easing, the Centers for Disease Control and Prevention (CDC) said today in its weekly FluView update.
Partly because of increased activity of the H3N2 strain, however, the CDC issued a Health Alert Network (HAN) advisory yesterday to clinicians about the prolonged season and the need for antiviral drugs for some patients. The past five seasons have averaged 16 weeks, with the longest being 20 weeks.
The agency noted that, while H3N2 cases have climbed in recent weeks, the 2009 H1N1 strain is still circulating, and low levels of influenza B have been detected, as well.
"Because influenza A(H3N2) viruses may be associated with severe disease in older adults, this health advisory serves as a reminder that early empiric treatment with influenza antiviral medications is recommended for hospitalized and high-risk patients, especially those 65 years and older," the CDC said in the HAN notice. "Antiviral treatment should be started as soon as possible after illness onset and should not wait for laboratory confirmation."
The CDC said it recommends antiviral drugs for all hospitalized, severely ill, and high-risk patients with suspected or confirmed influenza. It recommends them for treating the flu regardless of the patient's vaccination status because of their proven efficacy in reducing illness and severe outcomes.
The agency said in a summary of the FluView report, "While CDC continues to recommend influenza vaccination as long as influenza viruses are circulating, influenza antiviral drugs are an important second line of defense that can be used to treat flu illness."
The CDC said in the HAN advisory that the season "is likely to last several more weeks."
Several key indicators down
In the FluView report, the CDC noted that the percentage of clinic visits for influenza-like illness (ILI) has taken a healthy downturn. At 3.8% for the week ending Mar 23, it is still well above the national baseline of 2.2% but dropped from 4.4% the week before. All 10 US regions reported ILI at or above their region-specific baselines.
Twenty states experienced high ILI activity for the week, which is down from 26 the week before. Flu is geographically widespread in Puerto Rico and 34 states, compared with 44 states the week before.
H3N2 has now become more than twice as common as H1N1 among specimens tested at public health labs. Among 841 influenza A viruses subtyped, 68.1% were H3N2 and 31.9% were H1N1. The week before H3N2 accounted for 65.4% and H1N1 34.6%.
Only 5% of samples that tested positive for influenza were influenza B strains.
One new pediatric death
The CDC confirmed 1 new flu-related death in a child, bringing the number of pediatric flu deaths to 77 for the season. For comparison, the three previous flu seasons saw 95, 110, and 185 pediatric flu deaths over the entire season, respectively, according to CDC data.
As typically happens late in the season, the overall hospitalization rate for flu continued to climb. It rose to 52.5 per 100,000 population, up from 47.1 per 100,000 the week before.
The rate in seniors was 167.0 per 100,000 population, up from 146.0 per 100,000 the week before. People aged 50 to 64 were the next hardest-hit group (67.4 per 100,000 population), then children 4 years old and younger (63.6).
The overall rate of deaths from pneumonia and flu was 7.4%, up from 7.1% and back up above the epidemic threshold of 7.2% for the week, the sixth week this season it has been above the threshold.
Mar 29 CDC FluView
Mar 29 CDC FluView summary
Mar 28 CDC HAN notice