Situation Update: Summary of Weekly FluView

Key Flu Indicators

According to this week’s FluView, influenza activity continues to decrease, but remains elevated in the United States. While H3N2 viruses have been most common this season, influenza B viruses have accounted for the largest proportion of circulating viruses in recent weeks. This week, influenza B viruses accounted for 75% of all influenza viruses reported and were predominant in 9 of 10 U.S. regions. Flu activity has been elevated for 18 consecutive weeks nationally. The average length of a flu season for the past 13 seasons has been 13 weeks. This flu season started relatively early. Below is a summary of the key flu indicators for the week ending March 21, 2015:

For the week ending March 21, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased from 2.3% to 2.2% but remains above the national baseline (2.0%) for the eighteenth consecutive week. Six 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 one state (Oklahoma) experienced high ILI activity; a decrease from four states during the previous week. Four states (Georgia, Kansas, Minnesota, and Texas) experienced moderate ILI activity; a decrease from six states during the previous week. Twelve states (Arkansas, Colorado, Connecticut, Hawaii, Idaho, Illinois, Louisiana, Mississippi, Missouri, Nebraska, New York, and Wyoming) experienced low ILI activity. New York City and 33 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 seven states (Connecticut, Massachusetts, Mississippi, New Hampshire, New Jersey, New York, and Oklahoma); the same number of states reported widespread activity during the previous week. Guam, the U.S. Virgin Islands, and 27 states (Arizona, Arkansas, Florida, Hawaii, Idaho, Illinois, Indiana, Iowa, Kentucky, Louisiana, Maine, Missouri, Montana, Nebraska, New Mexico, North Carolina, North Dakota, Ohio, Oregon, Pennsylvania, Texas, Utah, Vermont, Virginia, Washington, Wisconsin, and Wyoming) reported regional geographic influenza activity. Local flu activity was reported by Puerto Rico, the District of Columbia, and 15 states. Sporadic flu activity was reported by one state (Alaska). Geographic spread data show how many areas within a state or territory are seeing flu activity.
A total of 15,964 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 58.4 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 41.2 per 100,000 people. Last week, the overall cumulative rate was 57.1 hospitalizations per 100,000 population.
- The hospitalization rate in people 65 years and older is 289.7 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 284.3 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 52.0 per 100,000 population. During 2012-2013, the hospitalization rate for that age group during the same week was 60.7 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 to 7.4% this week and was above the epidemic threshold of 7.2%. Last week, P&I associated deaths was 7.6%. 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%.
Nine influenza-associated pediatric deaths were reported to CDC during the week ending March 21.
- Three deaths were associated with an influenza A (H3) virus and occurred during weeks 3, 4, and 9 (the weeks ending January 24, January 31, and March 7, 2015, respectively). Two deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 6 and 10 (the weeks ending February 14 and March 14, 2015, respectively). Four deaths were associated with an influenza B virus and occurred during weeks 9 [2 deaths], 10 [1 death], and 11 [1 death] (weeks ending March 7, March 14, and March 21, 2015, respectively).
- A total of 116 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 21 decreased from 11.2% to 10.6%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 7.2% to 15.0%.
Influenza A (H3N2) viruses have predominated overall during the 2014-15 flu season, accounting for over 99% of all subtyped influenza A viruses. However influenza B viruses have accounted for the largest proportion of circulating viruses in recent weeks. During week 11, 75% of all influenza positive specimens reported were influenza B viruses, and influenza B viruses predominated in nine of ten regions (Regions 2, 3, 4, 5, 6, 7, 8, 9, and 10). 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 1,346 influenza viruses, including 27 influenza A (H1N1)pdm09, 1,026 influenza A (H3N2) viruses and 293 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.
- 242 (23.6%) of the 1,026 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 784 (76.4%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 784 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..
- 198 (95.7%) of the 207 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. Nine (4.3%) of the B/Yamagata-lineage viruses tested showed reduced titers to B/Massachusetts/2/2012.
- Eighty-two (95.3%) of the 86 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 (4.7%) of the B/Victoria-lineage viruses tested showed reduced titers to B/Brisbane/60/2008.
Since October 1, 2014, CDC has tested 38 influenza A (H1N1)pdm09, 2,443 influenza A (H3N2), and 338 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 2.6% 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.