Influenza activity continues to accelerate in the United States, with influenza-like illness (ILI) levels elevated for the sixth straight week, 30 states reporting widespread flu, and nine newly confirmed flu-related deaths in children, the Centers for Disease Control and Prevention (CDC) said today in its weekly update.

The season had started earlier than recent post–2009-pandemic years and has seen the influenza B strain predominating, which is unusual early in a flu season.

At least 3.7 million ill

The CDC said outpatient visits for ILI rose from 3.2% last week to 3.9% this week, and the number has been above the baseline of 2.4% for 6 straight weeks now. Nineteen states and Puerto Rico experienced high ILI activity, compared with 11 states and Puerto Rico the week before.

The agency estimates that at least 3.7 million people have fallen ill from the flu so far this season, with at least 32,000 hospitalizations and 1,800 deaths. For comparison, in 2017-18—the last full season for which the CDC produced estimates—for the entire season flu sickened an estimated 45 million, hospitalized 810,000, and killed 61,000.

The 9 pediatric flu-related deaths occurred within the past 6 weeks. Of them, 7 were caused by influenza B and 2 by 2009 H1N1, which is one of the two circulating "A" strains. The CDC has now reported 19 flu-associated pediatric deaths for the season, 13 caused by influenza B and 6 by influenza A, 4 of which were H1N1 viruses. The agency confirmed 143 kids' deaths for all of last season.

Influenza B continues to predominate among circulating strains, with 68.5% of specimens tested at clinical labs positive for that strain, the same percentage as the previous week. Of the influenza A strains subtyped at public health labs, 87% were H1N1 and 13% H3N2.

Some 98% of influenza B strains subtyped were of the Victoria lineage, which is the "B" strain in trivalent (three-strain) flu vaccines. Quadrivalent (four-strain) vaccines contain both "B" strains.

Hospitalizations rise

The overall hospitalization rate was 5.5 per 100,000 population, up from 3.9 per 100,000 the week before. The highest rates are in those 65 and older, followed by young children and adults ages 50 to 64.

Hospitalization rates were 12.7 per 100,000 in seniors (up from 9.4 the week before), followed by 10.9 and 5.5 per 100,000 in children 4 years and younger and in adults 50 to 64, respectively. Both figures were up substantially from the previous week.

Flu is now widespread in 30 states and regional in 17, up from being widespread in 23 states and regional in 14 states the week before.

The CDC continues to encourage immunization for those who have not received a flu vaccine. The length and severity of flu seasons remains unpredictable.

'A' strains dominating in Europe

In a Dec 18 update from the European Centre for Disease Prevention and Control (ECDC), the agency noted that influenza A strains make up 71% of circulating viruses, compared with 29% for influenza B. Of the type "A" strains, 68% have been H3N2 and 32% H1N1.

The ECDC says the mix could result in high mortality in the elderly and a heavy burden on healthcare centers.

Of influenza B strains that were subtyped, 95% have been from the Victoria lineage. The ECDC also noted that six nations in the World Health Organization European Region—which includes parts of Asia—are seeing influenza B strains predominating, and France and Spain are experiencing about equal levels of influenza A and B.

See also:

Dec 20 CDC FluView report

Dec 18 ECDC report