Influenza B continues to circulate as some US flu metrics drop

Some key indicators that track US flu activity declined a bit last week but remain high, and influenza B continues to slightly predominate the 2019-20 flu season, the Centers for Disease Control and Prevention (CDC) said today in its weekly FluView update.

Hospitalization and death rates are not high at this point in the season, the CDC said as it reported data through Jan 11, but it confirmed seven new flu-related deaths in children.

In addition, the CDC estimates that flu has sickened at least 13 million people so far this season, hospitalized at least 120,000, and killed at least 6,600. A week ago, the estimated death toll was 4,800.

ILI and flu-positive test rates down

Outpatient visits for influenza-like illness (ILI) declined from 5.7% the previous week to 4.7% last week. Two weeks earlier, the rate reached 6.9%, the highest so far this season. But the ILI rate in clinics can peak multiple times in one season, as it did last year.

"It is too early to know whether the season has peaked or if flu activity will increase again," the CDC said.

The percentage of respiratory specimens testing positive for influenza also dropped, from 23.6% to 22.9%. In clinical labs, 58.0% of positive specimens were influenza B and 42.0% influenza A. In public health labs, 50.4% were influenza A and 49.6% influenza B.

Of subtyped A strains, 93.3% were 2009 H1N1 and 6.7% were H3N2, the strain known for more severe disease overall. Of subtyped B strains, 98.8% were from the Victoria lineage, which has been by far the most common B lineage all season.

Pediatric deaths rise to 39

All seven new flu-related deaths in children were caused by influenza B, and they occurred from the last week in December through last week. So far 28 of the 39 total pediatric deaths have been attributed to influenza B.

Over the entire previous three flu seasons, pediatric flu deaths totaled 110, 187, and 143, respectively.

The rate of influenza hospitalizations for the week was 19.9 per 100,000 population, up from 14.6 per 100,000 and similar to what has been seen during this time in previous seasons, the CDC said. The highest rate of hospitalization is among adults aged 65 and older (47.6 per 100,000), followed by children aged 0 to 4 years old (34.4) and adults aged 50 to 64 (23.2).

The rate of pneumonia and influenza deaths was 6.9% last week, up from 5.8% the week before but still below the epidemic threshold of 7.0% for the week.

ILI activity is high in 32 states and New York City, down from 33 states the previous week. Flu is widespread in 48 states and Puerto Rico, up from 46 states the week before.

"Flu vaccine effectiveness estimates are not available yet this season," the CDC said, "but vaccination is always the best way to prevent flu and its potentially serious complications."

See also:

Jan 17 CDC FluView report

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