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According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), influenza activity in the U.S. remains high overall and is likely to continue for several weeks. The severity of flu disease so far this season is similar to some previous seasons during which influenza A (H3N2) viruses have circulated predominantly. H3N2-predominant seasons often cause more severe disease among children younger than 5 years and adults 65 years and older compared to H1N1 seasons. The CDC continues to recommend vaccination as long as flu viruses are circulating. Prompt treatment with flu antiviral drugs for people who are hospitalized, very sick with flu or people who are at high risk of serious flu complications also is recommended.
For the week ending January 3, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) decreased slightly to 5.6%, but is above the national baseline (2.0%) for the seventh consecutive week. Activity is beginning to decline in parts of the country and is increasing in others. It is too soon to tell whether influenza activity has peaked yet this season. All 10 U.S. regions reported ILI activity at or above region-specific baseline levels. The length of a flu season can vary. For the past 13 seasons ILI has remained at or above the national baseline for between one and 19 weeks each season, with an average of 13 weeks.
Puerto Rico and 26 states experienced high ILI activity; a decrease from 29 states during the previous week. New York City and eight states (California, Florida, Illinois, Michigan, Nebraska, New Jersey, South Dakota, and Wyoming) experienced moderate ILI activity. Seven states experienced low ILI activity. Eight states experienced minimal ILI activity, and the District of Columbia and one state 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 Guam and 46 states, an increase from 43 states during the previous week. Puerto Rico, the U.S. Virgin Islands, and three states (Alaska, Arizona, and California) reported regional geographic influenza activity. Local flu activity was reported by the District of Columbia and one state (Hawaii). Geographic spread data show how many areas within a state or territory are seeing flu activity.
A total of 5,492 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 20.1 hospitalizations per 100,000 population. Last week, the overall cumulative rate was 12.6 hospitalizations per 100,000 population.
The hospitalization rate in people 65 years and older is 91.6 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 79.1 per 100,000. The 2012-2013 flu season was the last influenza A (H3N2)-predominant season in the United States.)
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 is now slightly above the epidemic threshold of 6.9%.
Five influenza-associated pediatric deaths were reported to the CDC during the week ending Jan. 3, 2014. All five deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 50, 51, 52, and 53 (weeks ending Dec. 13, Dec. 20, Dec. 27, 2014, and Jan. 3, 2015, respectively). A total of 26 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 January 3 was 24.7%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 18.9% to 37.2%.
Influenza A (H3N2) 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 Jan. 3, 7,218 (96.0%) of the 7,515 influenza-positive tests reported to CDC were influenza A viruses and 297 (4.0%) were influenza B viruses. Of the 2,494 influenza A viruses that were subtyped, 99.7 % were influenza A (H3) viruses and 0.3% were influenza A (H1N1) pdm09 viruses.
The CDC has antigenically or genetically characterized 355 influenza viruses, including 10 influenza A (H1N1) pdm09, 288 influenza A (H3N2) viruses and 57 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 vaccines.
Ninety-one (31.6%) of the 288 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 197 (68.4%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 197 influenza A (H3N2) viruses were antigenically similar to A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015 Southern Hemisphere influenza vaccine.
Forty (70.2%) of the 57 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 (88.2%) 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 (11.8%) of the B/Victoria-lineage viruses tested showed reduced titers to B/Brisbane/60/2008.
Since Oct. 1, 2014, the CDC has tested 11 influenza A (H1N1) pdm09, 450 influenza A (H3N2), and 85 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, 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.