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According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), influenza activity is still elevated but continues to decrease in the United States. The number of states with widespread or high flu activity is decreasing, however, another six flu deaths in children were reported this week, bringing the total number of flu-associated pediatric deaths reported so far this season to 92. While H3N2 viruses remain most common, an increase in influenza B viruses has been detected in parts of the country. Flu activity has been elevated for 14 consecutive weeks nationally. The average length of a flu season for the past 13 seasons has been 13 weeks. Because this season started relatively early, it is expected to last longer.
For the week ending February 21, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased from 3.2% to 3.0% but remains above the national baseline (2.0%) for the fourteenth consecutive week. All 10 U.S. regions reported ILI activity at or above region-specific baseline levels. For the past 13 seasons ILI has remained at or above the national baseline for between one and 19 weeks each season.
Puerto Rico and 11 states experienced high ILI activity; a decrease from 12 states during the previous week. States reporting high ILI activity for the week ending February 21, 2015 include Arkansas, Connecticut, Kansas, Louisiana, Mississippi, New Jersey, New York, North Carolina, Oklahoma, Texas, and West Virginia. Three states (Colorado, Idaho, and Nevada) experienced moderate ILI activity; a decrease from 5 states during the previous week. Sixteen states (Alabama, California, Georgia, Hawaii, Massachusetts, Minnesota, Missouri, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Utah, Vermont, Virginia, and Wyoming) experienced low ILI activity. New York City and 20 states (Alaska, Arizona, Delaware, Florida, Illinois, Indiana, Iowa, Kentucky, Maine, Maryland, Michigan, Montana, Nebraska, New Hampshire, New Mexico, North Dakota, Ohio, Oregon, Washington, and Wisconsin) experienced minimal ILI activity and the District of Columbia 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 influenza activity was reported by Guam and 20 states; a decrease from 30 states during the previous week. Puerto Rico, the U.S. Virgin Islands and 25 states (Arizona, Arkansas, Florida, Georgia, Hawaii, Kansas, Kentucky, Louisiana, Michigan, Missouri, Nebraska, Nevada, New Mexico, North Dakota, Ohio, Oregon, Pennsylvania, South Carolina, Tennessee, Texas, Utah, Washington, West Virginia, Wisconsin, and Wyoming) reported regional geographic influenza activity. Local flu activity was reported by the District of Columbia and five states (Alaska, Colorado, Illinois, Minnesota, and South Dakota). Geographic spread data show how many areas within a state or territory are seeing flu activity.
A total of 14,162 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 51.7 hospitalizations per 100,000 population. This is higher than seen for the same week during the 2012-2013 season when the overall hospitalization rate was 36.7 per 100,000 people. Last week, the overall cumulative rate was 48.6 hospitalizations per 100,000 population. The hospitalization rate in people 65 years and older is 258.0 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 242.2 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 2012-13 season. (The 2012-2013 season was the last H3N2-predominant season.)
The hospitalization rate for children 0-4 years is 45.7 per 100,000 population. During 2012-2013, the hospitalization rate for that age group during the same week was 51.9 hospitalizations per 100,000 population.
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 decreased to 7.4% this week, but remains high and above the epidemic threshold of 7.2%. Last week, P&I-associated deaths was 8.4%. (The highest P&I 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%.)
Six influenza-associated pediatric deaths were reported to CDC during the week ending February 21. Three deaths were associated with an influenza A (H3) virus and occurred during weeks 51, 4, and 5 (the weeks ending December 20, 2014, January 31, and February 7, 2015, respectively). Two deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 5 and 6 (the weeks ending February 7 and February 14, 2015, respectively). One death was associated with an influenza B virus and occurred during week 53 (the week ending January 3, 2015). A total of 92 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 February 21 decreased from 13.0% to 12.1%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 8.0% to 23.8%.
Influenza A (H3N2) viruses have been most common in the United States this season accounting for more than 99% of all subtyped influenza A viruses. This week, however, the proportion of influenza B viruses increased to 30.9%. Influenza A (H1N1) pdm09 viruses have been detected rarely.
CDC has antigenically or genetically characterized 933 influenza viruses, including 27 influenza A (H1N1)pdm09, 752 influenza A (H3N2) viruses and 154 influenza B viruses, collected in the United States since October 1, 2014. All 27 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 vaccines.
228 (30.3%) of the 752 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 524 (69.7%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 524 influenza A (H3N2) viruses were antigenically similar to A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015 Southern Hemisphere influenza vaccine. 100 (93.5%) of the 107 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. Seven (6.5%) of the B/Yamagata-lineage viruses tested showed reduced titers to B/Massachusetts/2/2012. Forty-three (91.5%) of the 47 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. Four (8.5%) of the B/Victoria-lineage viruses tested showed reduced titers to B/Brisbane/60/2008.
Since October 1, 2014, CDC has tested 32 influenza A (H1N1)pdm09, 1,762 influenza A (H3N2), and 217 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 3.1% 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.