Influenza Season Still Going Strong, CDC Says

Troy Brown, RN

February 10, 2020

Influenza season is still going strong, although overall severity is not high, according to a report from the Centers for Disease Control and Prevention (CDC) for the week ending February 1 (week 5).

"Key indicators that track flu activity remain high and, after falling during the first two weeks of the year, increased over the last three weeks. Indicators that track overall severity (hospitalizations and deaths) are not high at this point in the season," the CDC writes in the report.

This season there have been at least 22 million influenza illnesses, 210,000 hospitalizations, and 12,000 deaths.

The percentage of respiratory specimens that test positive for influenza at clinical laboratories rose to 29.8%, up from 28.4% the week before.

Influenza B/Victoria viruses predominated earlier in the season; however, influenza A(H1N1)pdm09 viruses have increased recently, and the two strains are now approximately equal this season overall.

"The predominant virus also varies by age group. Nationally, for the season overall, influenza B viruses are the most commonly reported influenza viruses among children and young adults age 0-4 years (56% of reported viruses) and 5-24 years (73% of reported viruses), while A(H1N1)pdm09 viruses are the most commonly reported influenza viruses among persons 25-64 years (53% of reported viruses) and 65 years of age and older (60% of reported viruses)," the CDC explains.

Nationally, 6.7% of outpatient visits were for influenza-like illness — above the national baseline of 2.4%. Regionally, this percentage ranged from 4.0% to 10.9%.

Influenza activity was high in New York City, Puerto Rico, and 45 states; moderate in the District of Columbia, the US Virgin Islands, Nevada, and New Hampshire; low in Alaska and Delaware; and minimal in Idaho.

Geographically, activity was widespread in Puerto Rico and 48 states; regional in Hawaii and Oregon, local in the District of Columbia; and sporadic in the US Virgin Islands. Guam did not report on influenza activity.

Mortality

The percentage of deaths from pneumonia and influenza fell to 7.1% from 7.2% during week 4 — below the epidemic threshold of 7.2% for week 4.

Ten influenza-related pediatric deaths were reported during week 5, for a total of 78 this season. Of those 78 deaths, 52 were linked to influenza B viruses, nine of which had lineage determined and were B/Victoria viruses. Twenty-six deaths were linked to influenza A viruses, of which 16 were subtyped and found to be A(H1N1)pdm09 viruses.

During the 2009 influenza pandemic, 260 pediatric deaths were reported between August 30, 2009, and January 16, 2010. Since then, pediatric mortality at this point in the season has ranged from two in the 2011-2012 season to 80 in the 2014-2015 season.

Hospitalizations

Between October 1, 2019, and February 1, 2020, 10,314 laboratory confirmed influenza-associated hospitalizations were reported.

Of those, 6416 (62.2%) were linked to influenza A virus, 3835 (37.2%) to influenza B virus, 31 (0.3%) to co-infection with influenza A virus and influenza B, and 32 (0.3%) to influenza virus for which the type was undetermined. Among viruses with influenza A subtype information, 1397 (92.6%) were A(H1N1)pdm09 virus and 112 (7.4%) were A(H3N2).

The overall hospitalization rate rose to 35.5 per 100,000 population, up from 29.7 the week before. "Overall, hospitalization rates remain similar to this time during recent seasons, but rates among children and young adults are higher at this time than in recent seasons," the CDC notes in the report.

The hospitalization rate was highest among individuals aged 65 years and older (85.1 per 100,000), followed by children younger than 5 years (56.9). Among adults aged 18 to 49 years, the hospitalization rate was 20.2 per 100,000.

The CDC expects to have influenza vaccine effectiveness estimates later in February but emphasizes that the best way to protect against influenza and its potentially serious complications is to get vaccinated.

The CDC continues to recommend antiviral medications for those with influenza. "Antiviral medications are an important adjunct to flu vaccine in the control of influenza. Almost all (> 99%) of the influenza viruses tested this season are susceptible to the four FDA-approved influenza antiviral medications recommended for use in the U.S. this season."

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