Flu-Like Illness is Below the National Baseline for the First Time This Season, CDC Reports

According to this week's FluView report from the Centers for Disease Control and Prevention (CDC), levels of flu-like illness are now below the national baseline for the first time this season since mid- November. However, 5 of 10 U.S. regions continue to experience flu-like illness above their regional baseline. An additional 2 pediatric deaths were reported this week. Influenza B viruses now account for 86% of all influenza viruses reported. While flu activity persists in parts of the country, the 2014-2015 flu season is winding down. Below is a summary of the key flu indicators for the week ending April 4, 2015:

For the week ending April 4, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased to 1.8%, and is now below the national baseline of 2.0% for the first time this season since mid-November. ILI was at or above baseline for 19 weeks this season.  For the past 13 seasons ILI has remained at or above the national baseline for between one and 19 weeks each season.

No jurisdictions experienced high ILI activity; this is a decrease from three jurisdictions during the previous week. Puerto Rico experienced moderate ILI activity. Seven states (Connecticut, Illinois, Louisiana, Nebraska, Oklahoma, Texas, and Vermont) experienced low ILI activity. New York City and 43 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 four states (Connecticut, Massachusetts, Maryland, and New York); the same number of states reported widespread flu activity during the previous week. Seventeen states reported regional geographic influenza activity. Local flu activity was reported by Guam, the District of Columbia, and 22 states. Sporadic flu activity was reported by Puerto Rico, the U.S. Virgin Islands, and seven states. Geographic spread data show how many areas within a state or territory are seeing flu activity.

A total of 16,720 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 61.1 hospitalizations per 100,000 population. The cumulative overall hospitalization rate during 2012-2013 was 43.9 per 100,000 people. The hospitalization rate in people 65 years and older is 301.8 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 296.2 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 2012-2013 season. (The 2012-2013 season was the last H3N2-predominant season.) The hospitalization rate for children 0-4 years is 53.2 per 100,000 population. During the 2012-2013 season, the hospitalization rate for that age group during the same week was 63.7 hospitalizations per 100,000 population and was 67.0 per 100,000 cumulatively that season. 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 again this week to 6.5%. This is below the epidemic threshold of 7.1%. P & I was above the epidemic threshold for 12 consecutive weeks this season. Last week, P&I associated deaths was 6.9%. 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%.

Two influenza-associated pediatric deaths were reported to CDC during the week ending April 4. Both deaths were associated with an influenza B virus and occurred during week 9 (the week ending March 7, 2015).
A total of 125 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 April 4 decreased slightly from 10.8% to 10.7%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 7.3% to 17.1%.

Influenza A (H3N2) viruses have predominated overall during the 2014-2015 flu season, accounting for more than 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 13, 86% of all influenza positive specimens reported were influenza B viruses, and influenza B viruses predominated in all 10 regions. 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 1,562 influenza viruses, including 39 influenza A (H1N1)pdm09 viruses, 1,102 influenza A (H3N2) viruses and 421 influenza B viruses, collected in the United States since October 1, 2014. All 39 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. 243 (22.1%) of the 1,102 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 859 (77.9%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 859 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. 283 (96.3%) of the 294 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. Eleven (3.7%) of the B/Yamagata-lineage viruses tested showed reduced titers to B/Massachusetts/2/2012. 122 (96.1%) of the 127 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. Five (3.9%) of the B/Victoria-lineage viruses tested showed reduced titers to B/Brisbane/60/2008.

Since October 1, 2014, CDC has tested 44 influenza A (H1N1)pdm09, 2,658 influenza A (H3N2), and 465 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 2.3% 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 Food and Drug Administration (FDA) 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