Influenza Activity in US Remains High, and Still Rising

Influenza activity is still high and rose again during the week ending February 8 (week 6), according to a February 14 report from the Centers for Disease Control and Prevention (CDC).

Nationally, and in some specific regions of the United States, influenza A(H1N1)pdm09 viruses are increasing in proportion to influenza B viruses.

So far this season, at least 26 million influenza illnesses, 250,000 hospitalizations, and 14,000 deaths have occurred.

The percentage of outpatient visits for influenza-like illness (ILI) activity rose from 6.6% last week to 6.8% — above the national baseline of 2.4%. All regions were above their baselines. Regionally, the percentage ranged from 3.6% to 10.8%.

ILI activity was high in New York City, Puerto Rico, and 44 states. Activity was moderate in Nevada and Oregon; low in the District of Columbia, Alaska, and Florida; and minimal in Idaho. There were insufficient data to calculate ILI activity from the US Virgin Islands and Delaware.

Geographically, influenza 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.

Hospitalization Rate Still Highest Among Elderly Adults

Overall, the cumulative hospitalization rate was 41.9 per 100,000 population — similar to that seen at this time of year during recent previous influenza seasons.

The highest rate was seen among adults aged 65 years and older (101.6 per 100,000 population), followed by children younger than 5 years (65.9), adults aged 50 to 64 years (53.9), adults aged 18 to 49 years (23.5), and children aged 5 to 17 years (17.3).

"Rates in children and young adults are higher than at this time in recent seasons," the CDC notes in the report.

Between October 1, 2019 and February 8, 2020, 12,167 laboratory-confirmed influenza-associated hospitalizations were reported; 7881 (64.8%) were linked to influenza A virus, 4213 (34.6%) to influenza B virus, 39 (0.3%) to influenza A virus and influenza B virus co-infection, and 34 (0.3%) to influenza virus for which the type was undetermined.

Among influenza A viruses with subtype information, 1782 (93.2%) were A(H1N1)pdm09 virus and 129 (6.8%) were A(H3N2) virus.

Underlying Medical Conditions Common

Information on underlying medical conditions was available for 1573 hospitalized adults; of those, 91.9% had one or more underlying medical condition. The most frequently reported conditions were "cardiovascular disease, metabolic disorder, obesity, and chronic lung disease."

Information on underlying medical conditions was available for 333 hospitalized children; of those, 46.2% had one or more underlying medical condition. The most frequently reported underlying medical condition among the children was asthma.

Mortality Below Epidemic Threshold

As of February 13, 6.8% of the deaths that occurred during the week ending February 1 (week 5) were caused by pneumonia and influenza, below the epidemic threshold of 7.3% for that week.

A total of 14 influenza-related pediatric deaths that occurred between week 52 of 2019 and week 6 of 2020 were reported during week 6. Of those deaths, 10 were linked to influenza B viruses (one of which had lineage identified and was a B/Victoria virus) and 4 were linked to influenza A viruses; of two that underwent subtyping, one was an A(H1N1)pdm09 virus and the other was an A(H3) virus.

So far this season, 92 influenza-related pediatric deaths have occurred and been reported to the CDC. Of these, 62 were linked to influenza B viruses and lineage determination found them to be B/Victoria viruses. Approximately one third (30 deaths) were linked to influenza A viruses; of 18 that underwent subtyping, 17 were A(H1N1)pdm09 viruses and one was an A(H3) virus.

The CDC expects to make influenza vaccine effectiveness estimates available this week and they continue to recommend vaccination as "the best way to prevent flu and its potentially serious complications. Antiviral medications are an important adjunct to flu vaccine in the control of influenza."

The four US Food and Drug Administration-approved influenza antiviral medications recommended in the United States this season have been effective against more than 99% of the influenza viruses tested this season.

US Influenza Surveillance Report. Published February 14, 2020. Full text

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