According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), influenza activity is still elevated but is decreasing and some key indicators that measure severity are leveling off or decreasing also. Flu activity is likely to continue for several more weeks. The cumulative hospitalization rate for people 65 years and older is the highest of any age group but the rate of increase has slowed. Flu activity has been elevated for 12 consecutive weeks nationally. An average season lasts about 13 weeks. However, because this season started relatively early, it could last longer than average.
For the week ending Feb. 7, 2015, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased to 3.8%, but remains above the national baseline (2.0%) for the twelfth 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, with an average of 13 weeks.
Puerto Rico and 15 states experienced high ILI activity; a decrease from 26 states during the previous week. States reporting high ILI activity for the week ending February 7, 2015 include Arkansas, Colorado, Connecticut, Kansas, Louisiana, Mississippi, New Jersey, New York, North Carolina, Oklahoma, Pennsylvania, Rhode Island, Tennessee, Texas, and West Virginia.
New York City and 15 states (Alabama, Arizona, California, Hawaii, Idaho, Massachusetts, Missouri, Nebraska, Nevada, New Mexico, Utah, Vermont, Virginia, Washington, and Wyoming) experienced moderate ILI activity. Eight states (Delaware, Georgia, Maine, Minnesota, New Hampshire, North Dakota, South Carolina, and South Dakota) experienced low ILI activity. Twelve states (Alaska, Florida, Illinois, Indiana, Iowa, Kentucky, Maryland, Michigan, Montana, Ohio, Oregon, 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 Puerto Rico and 32 states. This decreased from 40 states in the previous week. Guam, the U.S. Virgin Islands and 16 states (Alabama, Colorado, Georgia, Hawaii, Illinois, Kansas, Kentucky, Louisiana, Michigan, Missouri, Nevada, South Dakota, Tennessee, Texas, Utah, and West Virginia) reported regional geographic influenza activity. Local flu activity was reported by the District of Columbia and two states (Alaska and Minnesota). Geographic spread data show how many areas within a state or territory are seeing flu activity.
A total of 12,065 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 44.1 hospitalizations per 100,000 population. Last week, the overall cumulative rate was 43.5 hospitalizations per 100,000 population. This is slightly higher than seen for the same week during the 2012-2013 season when the overall hospitalization rate was 33.1 per 100,000 people. The number of new hospitalizations reported per week is decreasing.
◦The hospitalization rate is high among people 65 and older, who account for more than 60% of all reported hospitalizations. The hospitalization rate in people 65 years and older is 217.3 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 213.8 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 2012-13 season. (The 2012-2013 season was the last H3N2-predominant season.)
◦The hospitalization rate for children 0-4 years is 40.4 per 100,000 population. During 2012-2013, the hospitalization rate for that age group for the same week was 45.7 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 slightly to 8.1% this week, but remains high and above the epidemic threshold of 7.2%. Last week, P&I was 8.5%. (During 2012-2013, P&I peaked at 9.9%. This was the highest recorded P&I in nearly a decade, but was comparable to recorded percentages for past severe seasons, including the 2003-2004 season when P&I reached 10.4%.)
Eleven influenza-associated pediatric deaths were reported to CDC during the week ending February 7. ◦Four deaths were associated with an influenza A (H3) virus and occurred during weeks 52, 3 and 4 (weeks ending December 27, 2014, January 24 and January 31, 2015). Five deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 53, 1, 2, and 3 (weeks ending January 3, January 10, January 17, and January 24, 2015). One death was associated with an influenza B virus and occurred during week 4. One death was associated with an influenza A and influenza B virus co-infection and occurred during week 5 (week ending February 7, 2015).
◦A total of 80 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 7 decreased from 17.5% to 14.9%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 10.1% to 27.4%.
Influenza A (H3N2) viruses have been most common in the United States this season. Few influenza B viruses have been detected and even fewer influenza A (H1N1)pdm09 viruses have been detected. During the week ending February 7, 2,768 (87.2%) of the 3,174 influenza-positive tests reported to CDC were influenza A viruses and 406 (12.8%) were influenza B viruses. Of the 1,064 influenza A viruses that were subtyped, 99.4 % were influenza A (H3) viruses and 0.6% were influenza A (H1N1)pdm09 viruses.
The CDC has antigenically or genetically characterized 809 influenza viruses, including 21 influenza A (H1N1)pdm09, 634 influenza A (H3N2) viruses and 154 influenza B viruses, collected in the United States since October 1, 2014. ◦All 21 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.
◦199 (31.4%) of the 634 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 435 (68.6%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 435 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 (93.4%) 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 (6.6%) 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 Oct. 1, 2014, the CDC has tested 29 influenza A (H1N1)pdm09, 1,213 influenza A (H3N2), and 163 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 3.4% 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 Dec. 19, 2014, the 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.