CDC Flu Update: All Areas of the U.S. are Experiencing Elevated Influenza-Like Illness Levels

February 28, 2016

According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), flu activity continues to increase in the United States. All 10 regions of the country are experiencing elevated influenza-like illness levels. H1N1 viruses are most common at this time. This is the same H1N1 virus that emerged in 2009 to cause a pandemic. Seasonal flu vaccines have included the H1N1 pandemic virus since 2010. CDC reports this season's vaccine is offering significant protection against circulating viruses this season. CDC recommends an annual flu vaccine for everyone 6 months of age and older. If you have not gotten vaccinated yet this season, you should get vaccinated now.

For the week ending February 20, the proportion of people seeing their health care provider for influenza-like illness (ILI) increased from 3.1% to 3.2%, which is above the national baseline (2.1%). All 10 regions reported ILI at or above their region-specific baseline levels. One way that CDC measures the length of the influenza season is the number of consecutive weeks during which ILI is at or above the national baseline. ILI has been at or above the national baseline for 6 consecutive weeks so far this season. For the last 13 seasons, the average duration of a flu season by this measure has been 13 weeks, with a range from 1 week to 20 weeks.

Puerto Rico and six states (Arizona, California, New Mexico, North Carolina, Texas, and Utah) experienced high ILI activity. New York City and six states (Arkansas, Connecticut, Florida, Illinois, New Jersey, and Oregon) experienced moderate ILI activity. 13 states (Alabama, Georgia, Hawaii, Kansas, Kentucky, Louisiana, Massachusetts, Michigan, Minnesota, Mississippi, Oklahoma, South Carolina, and Virginia) experienced low ILI activity. 24 states experienced minimal ILI activity. The District of Columbia and one state (Colorado) did not have sufficient data to calculate an activity level. ILI activity data indicate the amount of flu-like illness that is occurring in each state.

Widespread flu activity was reported by Guam, Puerto Rico and 21 states (Arizona, California, Connecticut, Iowa, Kentucky, Maryland, Massachusetts, Michigan, Minnesota, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Dakota, Pennsylvania, Texas, Utah, Vermont, Virginia, and Washington). Regional flu activity was reported by 18 states (Alabama, Florida, Georgia, Hawaii, Idaho, Indiana, Kansas, Maine, Montana, Nebraska, North Carolina, Ohio, Oklahoma, Oregon, Rhode Island, South Dakota, Tennessee, and Wyoming). The District of Columbia and 10 states (Alaska, Arkansas, Colorado, Delaware, Illinois, Louisiana, Mississippi, Missouri, South Carolina, and Wisconsin) reported local influenza activity. The U.S. Virgin Islands and 1 state (West Virginia) reported sporadic influenza activity. Geographic spread data show how many areas within a state or territory are seeing flu activity.

Since October 1, 2015, 1,594 laboratory-confirmed influenza-associated hospitalizations have been reported through FluSurv-NET, a population-based surveillance network for laboratory-confirmed influenza-associated hospitalizations. This translates to a cumulative overall rate of 5.8 hospitalizations per 100,000 people in the United States. This is significantly lower than the hospitalization rate at this time last season (54.4 per 100,000). More data on hospitalization rates, including hospitalization rates during other influenza seasons, are available at and The highest hospitalization rates are among people 65 years and older (16.7 per 100,000), followed by children younger than 5 years (8.6 per 100,000), and adults 50-64 years (7.4 per 100,000). During most seasons, children younger than 5 years and adults 65 years and older have the highest hospitalization rates. FluSurv-NET hospitalization data are collected from 13 states and represent approximately 8.5% of the total U.S. population. The number of hospitalizations reported does not reflect the actual total number of influenza-associated hospitalizations in the United States.

The proportion of deaths attributed to pneumonia and influenza (P&I) was below the system-specific epidemic threshold in both the NCHS Mortality Surveillance System and the 122 Cities Mortality Reporting System. One influenza-associated pediatric death was reported to CDC during the week ending February 20. This death was associated with an influenza B virus and occurred during week 7 (the week ending February 20, 2016). A total of 14 influenza-associated pediatric deaths have been reported during the 2015-2016 season.

Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending February 20 was 13.8%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 4.3% to 16.1%. During the week ending February 20, of the 2,599 influenza-positive tests reported to CDC by clinical laboratories, 1,978 (76.1%) were influenza A viruses and 621 (23.9%) were influenza B viruses.

The most frequently identified influenza virus type reported by public health laboratories during the week ending February 20 was influenza A viruses, with influenza A (H1N1)pdm09 viruses predominating. During the week ending February 20, 694 (77.8%) of the 892 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 198 (22.2%) were influenza B viruses. Of the 647 influenza A viruses that were subtyped, 83 (12.8%) were H3 viruses and 564 (87.2%) were (H1N1)pdm09 viruses. Cumulatively from October 4, 2015-February 20, 2016, influenza A (H1N1)pdm09 viruses were predominant in all four age groups (0-4 years age group (67.8%), 5-24 years age group (45.6%),  25-64 years age group (70.1%), and in ages 65 years and older (42.4%).

CDC has characterized 660 specimens (271 influenza A (H1N1)pdm09, 242 influenza A (H3N2) and 147 influenza B viruses) collected in the U.S. since October 1, 2015. All 271 (100%) influenza A (H1N1)pdm09 viruses were antigenically characterized as similar to A/California/7/2009, the influenza A (H1N1) component of the 2015-2016 Northern Hemisphere vaccine. All 242 H3N2 viruses were genetically sequenced and all viruses belonged to genetic groups for which a majority of viruses antigenically characterized were similar to cell-propagated A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015-2016 Northern Hemisphere vaccine. A subset of 109 H3N2 viruses also were antigenically characterized; 102 of 109 (93.6%) H3N2 viruses were similar to A/Switzerland/9715293/2013 by HI testing or neutralization testing. All 88 (100%) of the B/Yamagata-lineage viruses were antigenically characterized as similar to B/Phuket/3073/2013, which is included in both the 2015–16 Northern Hemisphere trivalent and quadrivalent vaccines. Fifty-eight of 59 (98.3%) of the B/Victoria-lineage viruses were antigenically characterized as similar to B/Brisbane/60/2008, which is included in the 2015-16 Northern Hemisphere quadrivalent vaccine.

Since October 1, 2015, CDC has tested 452 influenza A (H1N1)pdm09, 307 influenza A (H3N2), and 269 influenza B viruses for resistance to the neuraminidase inhibitors antiviral drugs. While the vast majority of the viruses that have been tested are sensitive to oseltamivir, zanamivir, and peramivir, so far this season, 2 (0.4%) influenza A (H1N1)pdm09 viruses have showed resistance to oseltamivir and peramivir (but both were sensitive to zanamivir).

Source: CDC