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Situation Update: Summary of Weekly FluView

Key Flu Indicators

According to this week’s FluView report, flu activity continues to decline and flu-like illness is below national baseline levels for the third week. However, three states (Connecticut, Massachusetts, and New York) continue to report widespread flu activity. Influenza B viruses now account for 89% of all influenza viruses reported. Influenza viruses circulate year-round, though at low levels in the summer in the United States. Below is a summary of the key flu indicators for the week ending April 25, 2015:

  • For the week ending April 25, the proportion of people seeing their health care provider for influenza-like illness (ILI) remained at 1.4%, and is below the national baseline of 2.0% for the third week. However, two of 10 regions (Regions 1 and 8) reported ILI at or above region-specific baseline levels. ILI was above or at baseline for 20 weeks this season, making this the longest season in more than a decade. For the past 13 seasons ILI has remained at or above the national baseline for between one and 19 weeks each season.
  • New York City and and 49 states experienced minimal ILI activity. The District of Columbia and one state (Colorado) did not have sufficient data to calculate an activity level. Puerto Rico experienced low ILI activity. ILI activity data indicate the amount of flu-like illness that is occurring in each state.
  • Widespread influenza activity was reported by three states (Connecticut, Massachusetts, and New York); the same states reported widespread flu activity during the previous week. Guam and nine states reported regional geographic influenza activity. Local flu activity was reported by Puerto Rico and 14 states. Sporadic flu activity was reported by the District of Columbia, the U.S. Virgin Islands, and 21 states. Three states reported no influenza activity; an increase from two states during the previous week. Geographic spread data show how many areas within a state or territory are seeing flu activity.
  • A total of 17,584 laboratory-confirmed influenza-associated hospitalizations have been reported through the Influenza Hospitalization Surveillance Network (FluSurv-NET) since October 1, 2014. This translates to a cumulative overall rate of 64.3 hospitalizations per 100,000 population. This is higher than the cumulative overall hospitalization rate during 2012-2013, which was 43.9 per 100,000 people.
    • The hospitalization rate in people 65 years and older is 316.8 per 100,000, which is the highest hospitalization rate recorded since data collection on laboratory-confirmed influenza-associated hospitalization in adults began during the 2005-2006 season. This is the highest rate of any age group. Last week, the hospitalization rate in people 65 years and older was 313.8 per 100,000. Previously, the highest recorded hospitalization rate was 183.2 per 100,000, which was the cumulative hospitalization rate for people 65 years and older for the 2012-2013 season. (The 2012-2013 season was the last H3N2-predominant season.)
    • The hospitalization rate for children 0-4 years is 56.4 per 100,000 population. During the 2012-2013 season, the overall hospitalization rate for that age group was 67.0 per 100,000 cumulatively that season.
    • Hospitalization data are collected from 13 states and represent approximately 9% 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) based on the 122 Cities Mortality Reporting System was 6.7%, and remains below the epidemic threshold of 6.9%. The percentage of P&I attributed deaths was at or above the epidemic threshold for 12 consecutive weeks this season. The highest P&I percentage this season was 9.3% and occurred during week 2. During 2012-2013, P&I peaked at 9.9%. This is comparable to recorded percentages for past severe seasons, including the 2003-2004 season when P&I reached 10.4%.
  • Five influenza-associated pediatric deaths were reported to CDC during the week ending April 25.
    • Two deaths were associated with an influenza A (H3) virus and occurred during weeks 3 and 8 (the weeks ending January 24 and February 28, 2015, respectively). Three deaths were associated with an influenza B virus and occurred during weeks 10, 12, and 15 (the weeks ending March 14, March 28, and April 18, 2015, respectively).
    • A total of 133 influenza-associated pediatric deaths have been reported for the 2014-2015 season at this time.
  • Nationally, the percentage of respiratory specimens testing positive for influenza viruses in the United States during the week ending April 25 slightly decreased from 7.6% to 6.5%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 4.2% to 14.6%.
  • Influenza A (H3N2) viruses have predominated overall during the 2014-2015 flu season, accounting for more than 99% of all subtyped influenza A viruses. However influenza B viruses have accounted for the largest proportion of circulating viruses since early March. During week 16, 89% of all influenza positive specimens reported were influenza B viruses, and influenza B viruses predominated in all 10 regions. It is not uncommon for there to be a second wave of flu activity toward the end of the flu season with another seasonal influenza virus. Influenza A (H1N1) pdm09 viruses have been detected rarely this season.
  • CDC has antigenically or genetically characterized 1,865 influenza viruses, including 49 influenza A (H1N1)pdm09, 1,220 influenza A (H3N2) viruses and 596 influenza B viruses, collected in the United States since October 1, 2014.
    • All 49 influenza A (H1N1)pdm09 viruses tested were characterized as A/California/7/2009-like. This is the influenza A (H1N1) component of the 2014-2015 Northern Hemisphere quadrivalent and trivalent influenza vaccine.
    • 243 (19.9%) of the 1,220 influenza A (H3N2) viruses tested have been characterized as A/Texas/50/2012-like. This is the influenza A (H3N2) component of the 2014-2015 Northern Hemisphere quadrivalent and trivalent influenza vaccine.
    • The remaining 977 (80.1%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 977 influenza A (H3N2) viruses were antigenically similar to A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015 Southern Hemisphere influenza vaccine and 2015-2016 Northern Hemisphere influenza vaccine.
    • 396 (97.3%) of the 407 B/Yamagata-lineage viruses were characterized as B/Massachusetts/2/2012-like, which is included as an influenza B component of the 2014-2015 Northern Hemisphere trivalent and quadrivalent influenza vaccines. Eleven (2.7%) of the B/Yamagata-lineage viruses tested showed reduced titers to B/Massachusetts/2/2012.
    • 184 (97.4%) of the 189 other influenza B viruses belonged to the B/Victoria lineage of viruses, and were characterized as B/Brisbane/60/2008-like. This is the recommended influenza B component of the 2014-2015 Northern Hemisphere quadrivalent influenza vaccine. Five (2.6%) of the B/Victoria-lineage viruses tested showed reduced titers to B/Brisbane/60/2008.
  • Since October 1, 2014, CDC has tested 47 influenza A (H1N1)pdm09, 3,032 influenza A (H3N2), and 621 influenza B viruses for resistance to neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir). While the vast majority of the viruses that have been tested are sensitive to oseltamivir, zanamivir, and peramivir, so far this season, one influenza A (H1N1)pdm09 virus showed resistance to oseltamivir and peramivir. (Because H1N1 viruses have been so rare this season, one virus accounts for 2.1% of the H1N1 viruses analyzed for antiviral resistance this season.)
    • Previously, the neuraminidase inhibitors oseltamivir and zanamivir were the only recommended influenza antiviral drugs. On December 19, 2014, the U.S. Food and Drug Administration approved Rapivab (peramivir) to treat influenza infection in adults.
    • As in recent past seasons, high levels of resistance to the adamantanes (amantadine and rimantadine) continue to persist among influenza A (H1N1)pdm09 and influenza A (H3N2) viruses. Adamantanes are not effective against influenza B viruses.
  • FluView is available – and past issues are archived – on the CDC website.

Note: Delays in reporting may mean that data changes over time. The most up to date data for all weeks during the 2014-2015 season can be found on the current FluView.

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