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According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), flu activity remains elevated in the United States. Influenza-like-illness levels remain above the national baseline, and there are now 33 states reporting widespread flu activity. 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 27, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) decreased slightly to 3.2%, which is above the national baseline (2.1%). Nine of 10 regions (Regions 2, 3, 4, 5, 6, 7, 8, 9 and 10) 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 7 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 eight states (Arizona, Arkansas, Illinois, Maryland, New Mexico, North Carolina, Tennessee, and Utah) experienced high ILI activity. New York City and nine states (Alabama, California, Florida, Hawaii, Kentucky, Mississippi, New Jersey, Oklahoma, and South Carolina) experienced moderate ILI activity. 13 states (Colorado, Georgia, Indiana, Kansas, Louisiana, Michigan, Minnesota, Missouri, New York, Oregon, Pennsylvania, Texas, and Virginia) experienced low ILI activity. 20 states experienced minimal ILI activity. The District of Columbia and Virginia 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 Puerto Rico and 33 states. Regional flu activity was reported by Guam and 14 states (Alabama, Arkansas, Colorado, Florida, Georgia, Hawaii, Illinois, Indiana, Louisiana, Maine, Missouri, Oklahoma, South Carolina, and Tennessee). The District of Columbia and one state (Mississippi) reported local influenza activity. The U.S. Virgin Islands and 2 states (Oregon and 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, 2,163 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 7.8 hospitalizations per 100,000 people in the United States. This is significantly lower than the hospitalization rate at this time last season (55.9 per 100,000). More data on hospitalization rates, including hospitalization rates during other influenza seasons, are available at http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html. The highest hospitalization rates are among people 65 years and older (21.3 per 100,000), followed by children younger than 5 years (11.8 per 100,000), and adults 50-64 years (10.6 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 NCHS Mortality Surveillance System and above the system-specific epidemic threshold in the 122 Cities Mortality Reporting System.
Four influenza-associated pediatric deaths were reported to CDC during the week ending February 27. Two deaths were associated with an influenza A (H1N1)pdm09 virus and occurred during week 7 (the week ending February 20, 2016). One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 8 (the week ending February 27, 2016) and one death was associated with an influenza B virus and occurred during week 7 (the week ending February 20, 2016). A total of 18 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 27 was 17.6%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 6.2% to 21.7%. During the week ending February 27, of the 3,803 influenza-positive tests reported to CDC by clinical laboratories, 2,978 (78.3%) were influenza A viruses and 825 (21.7%) were influenza B viruses.
The most frequently identified influenza virus type reported by public health laboratories during the week ending February 27 was influenza A viruses, with influenza A (H1N1)pdm09 viruses predominating. During the week ending February 27, 791 (73.4%) of the 1,078 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 287 (26.6%) were influenza B viruses. Of the 779 influenza A viruses that were subtyped, 72 (9.2%) were H3 viruses and 707 (90.8%) were (H1N1)pdm09 viruses. Cumulatively from October 4, 2015-February 27, 2016, influenza A (H1N1)pdm09 viruses were predominant in all four age groups (0-4 years age group (69.9%), 5-24 years age group (48.3%), 25-64 years age group (71.2%), and in ages 65 years and older (45.2%).
CDC has characterized 783 specimens (331 influenza A (H1N1)pdm09, 251 influenza A (H3N2) and 201 influenza B viruses) collected in the U.S. since October 1, 2015. All 331 (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 251 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 142 (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 554 influenza A (H1N1)pdm09, 326 influenza A (H3N2), and 321 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).