CDC Flu Update: H3N2 Viruses Continue to Predominate

According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), seasonal influenza activity increased sharply in the United States. The number of states reporting widespread flu activity jumped from 12 to 23, the proportion of samples testing positive for influenza at clinical laboratories went from 8.4% to 14.0%, and the percentage of people seeking outpatient care for influenza-like illness (ILI) increased from 2.7% to 3.5% over last week’s report. ILI has been at or above the national baseline for four weeks so far this season. During recent seasons, ILI has remained at or above baseline for 13 weeks on average. H3N2 viruses continue to predominate.

While flu vaccination is still recommended for people who have not yet gotten vaccinated, antiviral drugs are an important second line of defense that can be used to treat flu illness. CDC recommends the use of antiviral drugs as early as possible to treat flu illness in people who are very sick with flu and those at high risk of serious flu complications.

Influenza-like Illness Surveillance: For the week ending December 16, the proportion of people seeing their health care provider for influenza-like illness (ILI) was 3.5%, which is above the national baseline of 2.2%. Nine of 10 regions (regions 1, 2, 4, 5, 6, 7, 8, 9 and 10) reported a proportion of outpatient visits for ILI at or above their region-specific baseline levels. Additional ILINet data, including national, regional, and select state-level data for the current and previous seasons, can be found at

Influenza-like Illness State Activity Indicator Map: Ten states experienced high ILI activity (Alabama, Arizona, Arkansas, Kentucky, Louisiana, Mississippi, Nevada, Oklahoma, South Carolina, and Texas). Puerto Rico and eight states (California, Georgia, Illinois, Kansas, Missouri, Nebraska, Oregon, and West Virginia) experienced moderate ILI activity. New York City, the District of Columbia and 11 states (Alaska, Colorado, Connecticut, Massachusetts, Michigan, Minnesota, New Jersey, New Mexico, South Dakota, Virginia, and Wyoming) experienced low ILI activity. 21 states experienced minimal ILI activity (Delaware, Florida, Hawaii, Idaho, Indiana, Iowa, Maine, Maryland, Montana, New Hampshire, New York, North Carolina, North Dakota, Ohio, Pennsylvania, Rhode Island, Tennessee, Utah, Vermont, Washington, and Wisconsin). Additional data, including data for previous seasons, can be found at

Geographic Spread of Influenza Viruses: Widespread influenza activity was reported by 23 states (Alabama, Arizona, Arkansas, California, Connecticut, Idaho, Illinois, Indiana, Kentucky, Louisiana, Maryland, Massachusetts, Mississippi, Missouri, New York, North Dakota, Ohio, Oklahoma, Oregon, South Carolina, Texas, Virginia, and Wisconsin). Regional influenza activity was reported by Puerto Rico and 23 states (Alaska, Colorado, Florida, Georgia, Iowa, Kansas, Maine, Michigan, Minnesota, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, North Carolina, Pennsylvania, Rhode Island, South Dakota, Tennessee, Vermont, Washington, and Wyoming). Local influenza activity was reported by the District of Columbia and four states (Delaware, Hawaii, Utah, and West Virginia). Sporadic activity was reported by the U.S. Virgin Islands. Guam did not report. Geographic spread data show how many areas within a state or territory are seeing flu activity. Additional data are available at:

Flu-Associated Hospitalizations: Since October 1, 2017, 1,772 laboratory-confirmed influenza-associated hospitalizations have been reported through the Influenza Hospitalization Network (FluSurv-NET), a population-based surveillance network for laboratory-confirmed influenza-associated hospitalizations. This translates to a cumulative overall rate of 6.2 hospitalizations per 100,000 people in the United States.

The highest hospitalization rates are among people 65 years and older (25.3 per 100,000), followed by adults aged 50-64 years (6.5 per 100,000), and children younger than 5 years (4.7 per 100,000). During most seasons, children younger than 5 years and adults 65 years and older have the highest hospitalization rates. Additional data, including hospitalization rates during other influenza seasons, can be found at and

Mortality Surveillance:
The proportion of deaths attributed to pneumonia and influenza (P&I) was 6.2% for the week ending December 2, 2017 (week 48). This percentage is below the epidemic threshold of 7.4% for week 48 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. Region and state-specific data are available at

Pediatric Deaths:
One influenza-associated pediatric death was reported to CDC during week 50.
This death was associated with an influenza A(H1N1)pdm09 virus and occurred during week 50 (the week ending December 16, 2017).
A total of 9 influenza-associated pediatric deaths for the 2017-2018 season have been reported to CDC.
Additional information on pediatric deaths is available on FluView Interactive at:

Laboratory Data:
Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending December 16 was 14.0%.
Regionally, the three week average percent of specimens testing positive for influenza in clinical laboratories ranged from 4.1% to 17.6%.
During the week ending December 16, of the 3,308 (14.0%) influenza-positive tests reported to CDC by clinical laboratories, 2,812 (85.0%) were influenza A viruses and 496 (15.0%) were influenza B viruses.

The most frequently identified influenza virus subtype reported by public health laboratories was influenza A(H3N2) virus.
During the week ending December 16, 689 (86.7%) of the 795 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 106 (13.3%) were influenza B viruses. Of the 669 influenza A viruses that were subtyped, 611 (91.3%) were H3N2 viruses and 58 (8.7%) were (H1N1)pdm09 viruses.

The majority of the influenza viruses collected from the United States during October 1 through December 9, 2017 were characterized antigenically and genetically as being similar to the cell-grown reference viruses representing the 2017–18 Northern Hemisphere influenza vaccine viruses.
Of the influenza viruses tested and collected during October 1-December 16, 2017, none were found to be resistant to antiviral medications (i.e. oseltamivir, zanamivir, or peramivir).

Source: CDC