Flu Activity Continues to Decrease, CDC Reports

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 pediatric deaths reported so far this season to 86. While H3N2 viruses remain most common, an increase in influenza B viruses has been detected in some parts of the country. Flu activity has been elevated for 13 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 7, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) decreased from 3.7% to 3.2% but remains above the national baseline (2.0%) for the thirteenth 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 12 states experienced high ILI activity; a decrease from 15 states during the previous week. States reporting high ILI activity for the week ending February 14, 2015 include Arkansas, Connecticut, Hawaii, Idaho, Kansas, Mississippi, Nevada, New York, North Carolina, Oklahoma, Texas, and West Virginia. Five states (California, Colorado, Louisiana, New Mexico, and Rhode Island) experienced moderate ILI activity; a decrease from 15 states during the previous week. New York City and 12 states (Alabama, Arizona, Massachusetts, Minnesota, Missouri, New Jersey, Pennsylvania, South Carolina, South Dakota, Tennessee, Utah, and Virginia) experienced low ILI activity. Twenty-one states (Alaska, Delaware, Florida, Georgia, Illinois, Indiana, Iowa, Kentucky, Maine, Maryland, Michigan, Montana, Nebraska, New Hampshire, North Dakota, Ohio, Oregon, Vermont, Washington, Wisconsin, and Wyoming) 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 30 states. This decreased from 32 states during the previous week. Guam, the U.S. Virgin Islands and 15 states (Florida, Georgia, Hawaii, Illinois, Kansas, Kentucky, Louisiana, Michigan, Missouri, Nebraska, South Dakota, Tennessee, Texas, Utah, and West Virginia) reported regional geographic influenza activity. Local flu activity was reported by the District of Columbia and five states (Alabama, Alaska, Colorado, Minnesota, and Nevada). Geographic spread data show how many areas within a state or territory are seeing flu activity.

A total of 13,320 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 48.6 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 35.0 per 100,000 people. Last week, the overall cumulative rate was 44.1 hospitalizations per 100,000 population. The hospitalization rate in people 65 years and older is 242.2 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 217.3 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 2012-13 season. (The 2012-2013 season was the last H3N2-predominant season.)
The hospitalization rate for children 0-4 years is 43.4 per 100,000 population. During 2012-2013, the hospitalization rate for that age group during the same week was 48.8 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 slightly to 8.4% this week, but remains high and above the epidemic threshold of 7.2%. Last week, P&I-associated deaths was 8.1%. This increase in P&I is small and could be a result of reporting delays associated with inclement weather. (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 14. Four deaths were associated with an influenza A (H3) virus and occurred during weeks 51 [1 death], 3 [2 deaths], and 4 [1 death] (the weeks ending December 20, 2014, January 24 and January 31, 2015 respectively). Two deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 4 and 5 (weeks ending January 31 and February 7, 2015).
A total of 86 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 14 decreased from 14.9% to 13.0%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 8.8% to 24.6%.

Influenza A (H3N2) viruses have been most common in the United States this season. This week there was an increase in the proportion of influenza B viruses detected in some parts of the country. Very few influenza A (H1N1) pdm09 viruses have been detected. During the week ending February 14, 1,833 (77%) of the 2,381 influenza-positive tests reported to CDC were influenza A viruses and 548 (23%) were influenza B viruses. Of the 518 influenza A viruses that were subtyped, 98.8 % were influenza A (H3) viruses and 1.2% were influenza A (H1N1)pdm09 viruses.

The CDC has antigenically or genetically characterized 871 influenza viruses, including 21 influenza A (H1N1)pdm09, 696 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.
214 (30.7%) of the 696 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 482 (69.2%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 482 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, the CDC has tested 30 influenza A (H1N1)pdm09, 1,418 influenza A (H3N2), and 194 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 3.3% 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.

Source: CDC