Flu activity is still elevated in the United States but is declining. It is possible, however, that flu activity will continue for several weeks in parts of the country.
While H3N2 viruses remain most common, an increase in influenza B viruses has been detected in some parts of the country. This season has been severe for people 65 years and older, with very high hospitalization rates being recorded.
Influenza antiviral drugs can treat flu illness. CDC recommends these drugs be used to treat people who are very sick or who are at high risk of serious flu-related complications who have flu symptoms. Early antiviral treatment works best.
U.S. Situation Update
Key Flu Indicators
According to this week's FluView, 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 seven flu deaths in children were reported this week, bringing the total number of flu-associated pediatric deaths reported so far this season to 104. While H3N2 viruses have been most common this season, an increase in influenza B viruses has been detected in recent weeks. This week, influenza B viruses accounted for 52% of all influenza viruses reported and were predominant in 5 of 10 U.S. regions. Flu activity has been elevated for 16 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. Below is a summary of the key flu indicators for the week ending March 7, 2015:
- For the week ending March 7, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased from 2.5% to 2.4% but remains above the national baseline (2.0%) for the sixteenth consecutive week. Eight of 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 six states experienced high ILI activity. The same number of states reported high activity during the previous week. States reporting high ILI activity for the week ending March 7, 2015 include Arkansas, Connecticut, Kansas, Mississippi, Oklahoma, and Texas). One state (Alabama) experienced moderate ILI activity; a decrease from four states during the previous week. Thirteen states (Colorado, Georgia, Hawaii, Idaho, Louisiana, Maine, Missouri, New Jersey, New York, Utah, Vermont, West Virginia, and Wyoming) experienced low ILI activity. New York City and 30 states 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 nine states (Connecticut, Indiana, Maine, Massachusetts, New Hampshire, New Jersey, New York, Oklahoma, and Vermont); a decrease from 12 states and one jurisdiction (Guam) during the previous week. Guam, Puerto Rico, the U.S. Virgin Islands and 29 states (Alabama, Arizona, Arkansas, California, Florida, Georgia, Hawaii, Idaho, Illinois, Iowa, Kansas, Louisiana, Michigan, Mississippi, Missouri, Montana, Nebraska, New Mexico, North Carolina, North Dakota, Ohio, Pennsylvania, South Carolina, Tennessee, Texas, Utah, Virginia, Washington, and Wyoming) reported regional geographic influenza activity. Local flu activity was reported by 11 states (Colorado, Delaware, Kentucky, Maryland, Minnesota, Nevada, Oregon, Rhode Island, South Dakota, West Virginia, and Wisconsin). Sporadic flu activity was reported by the District of Columbia and one state (Alaska). Geographic spread data show how many areas within a state or territory are seeing flu activity.
- A total of 15,249 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 55.7 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 39.3 per 100,000 people. Last week, the overall cumulative rate was 53.5 hospitalizations per 100,000 population.
- The hospitalization rate in people 65 years and older is 277.9 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 266.1 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 49.5 per 100,000 population. During 2012-2013, the hospitalization rate for that age group during the same week was 56.8 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 increased to 7.6% this week and was above the epidemic threshold of 7.2%. Last week, P&I-associated deaths was 7.2% and was at the epidemic threshold after eight consecutive weeks of being at or above the epidemic threshold. (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%.)
- Seven influenza-associated pediatric deaths were reported to CDC during the week ending March 7.
- One death was associated with an influenza A (H3) virus and occurred during week 8 (the week ending February 28, 2015). Two deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 5 and 8 (the weeks ending February 7 and February 28, 2015, respectively). Four deaths were associated with an influenza B virus and occurred during week 8.
- A total of 104 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 March 7 increased slightly from 10.9% to 11.4%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 7.5% to 17.3%.
- Influenza A (H3N2) viruses have been the dominant circulating viruses in the United States this season accounting for over 99% of all subtyped influenza A viruses. However in recent weeks the proportion of influenza B viruses has been increasing. During week 9, 52% of all influenza positive specimens reported were influenza B viruses, and influenza B viruses predominated in five of ten regions (Regions 4, 5, 6, 7, and 8). Influenza A (H1N1) pdm09 viruses have been detected rarely this season.
- CDC has antigenically or genetically characterized 1,150 influenza viruses, including 27 influenza A (H1N1)pdm09, 902 influenza A (H3N2) viruses and 221 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 vaccine.
- 238 (26.4%) of the 902 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 664 (73.6%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 664 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.
- 150 (95.5%) of the 157 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 (4.4%) of the B/Yamagata-lineage viruses tested showed reduced titers to B/Massachusetts/2/2012.
- Sixty (93.8%) of the 64 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 (6.2%) of the B/Victoria-lineage viruses tested showed reduced titers to B/Brisbane/60/2008.
- Since October 1, 2014, CDC has tested 34 influenza A (H1N1)pdm09, 2,053 influenza A (H3N2), and 269 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 2.9% 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.
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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