With seven more pediatric deaths reported last week and influenza-like illness (ILI) numbers that are nearing those seen during the 2009 pandemic, the Centers for Disease Control and Prevention (CDC) said today that this year's flu season will most likely be considered severe.
"It's been a tough flu season so far," said Dan Jernigan, MD, MPH, director of the influenza division at the CDC's National Center for Immunization and Respiratory Diseases. "This season is now looking like 2014-2015 season, which was categorized as a high-severity season."
In its weekly FluView report, the CDC said the total of pediatric deaths has now reached 37.
Last week the percentage of patients visiting clinics with ILL was 6.2%; this week that number is 6.6% and doesn't show signs of slowing. At the height of the 2009 pandemic, when the novel 2009 H1N1 strain infected humans for the first time, ILI visits reached 7.7%. The year's ILI percentages surpasses the 6% number reached during the 2014-15 flu season, which was the previous post–2009 pandemic high.
Hospitalizations also continued to rise, with this season on pace to match 2014-15 in that category. Last week the overall hospitalization rate was 41.9 per 100,000 population, a jump from the previous week's 31.5 per 100,000. Adults 65 and older were most likely to be hospitalized—at a rate of 183.1 per 100,000 population—followed by adults aged 50 to 64 (44.2 per 100,000 population) and children aged 0 to 4 years (27.0 per 100,000 population).
At this point in the severe 2014-15 season, overall flu hospitalizations were at 40.5 per 100,000 populations.
The vast majority (88.7%) of hospitalizations were associated with influenza A. Of those A strain infections that required hospitalization, 86.4% were H3N2 and 13.6% were 2009 H1N1.
Jernigan said that H3N2, the influenza A strain that dominated in 2014-15 and is predominant this year, has led to high flu activity for two reasons: Seasonal flu vaccines tend to perform poorly against H3N2, and Baby Boomers and those over age 65 are more susceptible to severe infections from this strain.
H3N2 was first detected in humans 50 years ago, in 1968. Many older Americans were thus never exposed to the strain in childhood, leading to an absence of any natural immunity, Jernigan explained during a CDC press conference.
Influenza B cases are slowly rising across the country, typical at this point in the flu season. Among all laboratory samples reported to the CDC last week, 78.5% were influenza A while 21.5% were influenza B. For the season, the split is 82.1%-17.9%.
Widespread geographic activity
Jernigan said the pervasive and widespread flu activity across the country has been notable this year.
"Flu activity has stayed at the same high level for 3 weeks now," Jernigan said. Thirty-Nine states, up from last week's 32, are reporting high activity. For the third straight week, flu is widespread in 49 states. It is also geographically widespread in Puerto Rico.
"There was a rapid increase after the holidays, with kids returning to school," said Jernigan. According to a story in the Wall Street Journal, 11 states have had school district closings because of high flu activity.
The flu season typically lasts 16 to 20 weeks, and last week was the 9th of this season.
"We still have several more weeks to go, but there are some signs flu activity has peaked in some places," Jernigan said. "California is seeing some decline."
The CDC recommends that anyone who has yet to get vaccinated receive a flu vaccine, especially children.
Jan 26 CDC FluView
Jan 26 Wall Street Journal story