Flu Activity High in Much of the Country, CDC Reports

According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), flu activity continued to increase in the United States and is high in about half of the country with national influenza-like-illness (ILI) now approaching the peak level seen during the 2012-2013 season. The 2012-2013 flu season was the last influenza A (H3N2)-predominant season in the United States. The timing of flu activity can vary in different parts of the country. Flu activity is expected to continue in the coming weeks, with increases occurring especially in those states that have not yet had significant activity. The CDC continues to recommend vaccination as long as flu viruses are circulating. If you have not been vaccinated yet this season, get vaccinated now.

Below is a summary of the key flu indicators for the week ending Dec. 20, 2014:

- For the week ending Dec. 20, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) increased to 5.5% and is above the national baseline (2%) for the fifth consecutive week. All 10 U.S. regions reported ILI activity at or above region-specific baseline levels. The length of a flu season can vary. The national baseline was calculated beginning in the 2001-2002 season, and since then ILI has remained at or above the national baseline for between 1 and 19 weeks each season, with an average of 13 weeks.

- Puerto Rico and 22 states experienced high ILI activity, an increase from thirteen states during the previous week. Six states (Colorado, Michigan, New Mexico, Oklahoma, Pennsylvania, and West Virginia) experienced moderate ILI activity. New York City and eight states experienced low ILI activity. Fourteen states experienced minimal ILI activity. 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 36 states. Guam, Puerto Rico and 10 states reported regional geographic influenza activity. The U.S. Virgin Islands, The District of Columbia and two states (Alaska and Oregon) reported local activity. Two states (Hawaii and California) reported sporadic influenza activity. Geographic spread data show how many areas within a state or territory are seeing flu activity.

- 2,643 laboratory-confirmed influenza-associated hospitalizations have been reported thru the Influenza Hospitalization Surveillance Network (FluSurv-NET) since October 1, 2014. This translates to a cumulative overall rate of 9.7 hospitalizations per 100,000 population. The hospitalization rate in people 65 years and older is 38.3 per 100,000. This is the highest rate of any age group. (The hospitalization rate for people 65 and older for the same week during 2012-2013 was 28.4 per 100,000.) 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 again this week and was at the epidemic threshold of 6.8%.

- Four influenza-associated pediatric deaths were reported to CDC during the week ending Dec. 20. Three deaths were associated with an influenza A (H3) virus and occurred during weeks 49 and 50 (weeks ending Dec. 6 and Dec. 13, 2014, respectively). One death was associated with an influenza B virus and occurred during week 51 (week ending Dec. 20, 2014). A total of 15 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 Dec. 20 increased once again to 28.1%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 11.3% to 35.9%.

- 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 December 20, 5,987 (97.3%) of the 6,152 influenza-positive tests reported to CDC were influenza A viruses and 165 (2.7%) were influenza B viruses. Of the 2,023 influenza A viruses that were subtyped, 99.9 % were influenza A (H3) viruses and 0.05% were influenza A (H1N1) pdm09 viruses.

The CDC has antigenically or genetically characterized 305 influenza viruses, including 10 influenza A (H1N1) pdm09, 239 influenza A (H3N2) viruses  and 56 influenza B viruses, collected in the United States since October 1, 2014.

- All 10 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.
- Seventy-eight (32.6%) of the 239 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 161 (67.4%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 161 influenza A (H3N2) viruses were antigenically similar to A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015 Southern Hemisphere influenza vaccine.
- Thirty-nine (70%) of the 56 influenza B viruses tested belonged to the B/Yamagata/16/88 lineage and were characterized as B/Massachusetts/2/2012-like. This is an influenza B component of the 2014-2015 Northern Hemisphere trivalent and quadrivalent influenza vaccine.
- Fifteen (88%) of the 17 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. Two (12%) of the B/Victoria-lineage viruses tested showed reduced titers to B/Brisbane/60/2008.

Since Oct. 1, 2014, CDC has tested 11 influenza A (H1N1) pdm09, 248 influenza A (H3N2), and 55 influenza B viruses for resistance to neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir). All viruses showed susceptibility to oseltamivir, zanamivir and peramivir.

Previously, the neuraminidase inhibitors oseltamivir and zanamivir were the only recommended influenza antiviral drugs. On Dec. 19, 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.

Source: CDC