According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), influenza activity in the U.S. remains high but there are early signs that activity has begun to decrease in parts of the country. While influenza-like-illness (ILI) and the percentage of respiratory specimens testing positive for flu declined in the most recent report, severity indicators used to track hospitalizations and deaths rose sharply. It’s typical for increases in ILI to be followed by increases in hospitalizations and then subsequently increases in deaths. While early data indicates this season’s vaccine is not working as well as usual against circulating H3N2 viruses, the CDC continues to recommend vaccination. It’s common for other influenza viruses to circulate later in the season and flu vaccines are designed to protect against three or four influenza viruses. However because the vaccine is offering reduced protection, 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 is especially important this season.
• For the week ending Jan. 10, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) decreased to 4.4%, but remains above the national baseline (2.0%) for the eighth consecutive week. 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 24 states experienced high ILI activity; a decrease from 26 states during the previous week. New York City and seven states (California, Indiana, Maryland, Nebraska, New Mexico, Ohio, and Wyoming) experienced moderate ILI activity. Seven states (Arizona, Connecticut, Massachusetts, North Dakota, Rhode Island, South Dakota, and Wisconsin) experienced low ILI activity. Eleven states experienced minimal ILI activity, and the District of Columbia and one state (Delaware) 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. The same number of states reported widespread activity during the previous week. Puerto Rico, the U.S. Virgin Islands, and three states (Alabama, Georgia, and Hawaii) reported regional geographic influenza activity. Local flu activity was reported by the District of Columbia and one state (Alaska). Geographic spread data show how many areas within a state or territory are seeing flu activity.
• A total of 8,199 laboratory-confirmed influenza-associated hospitalizations have been reported through the Influenza Hospitalization Surveillance Network (FluSurv-NET) since Oct. 1, 2014. This translates to a cumulative overall rate of 29.9 hospitalizations per 100,000 population. Last week, the overall cumulative rate was 20.1 hospitalizations per 100,000 population.
The hospitalization rate in people 65 years and older is 143.3 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 the 2012-2013 season 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 increased sharply this week to 8.5% and is above the epidemic threshold of 7.0%. Last week, P&1 was 7.2%. (During 2012-2013, P & I peaked at 9.9%. This was the highest recorded P & I in nearly a decade, but was comparable to recorded percentages for past severe seasons, including the 2003-2004 season when P&I reached 10.4%.)
• Nineteen influenza-associated pediatric deaths were reported to CDC during the week ending Jan. 10. Eight deaths were associated with an influenza A (H3) virus and occurred during weeks 51, 52, 53, and 1 (weeks ending Dec. 20, Dec. 27, Jan. 3, and Jan. 10, 2015, respectively). Nine deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 50, 52, 53, and 1 (weeks ending Dec. 13, Dec. 27, and Jan. 3, and January 10, 2015, respectively). One death was associated with an influenza virus for which the type was not determined and occurred during week 53, and one death was associated with an influenza B virus and occurred during week 1. A total of 45 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 Jan. 10 was 20.2%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 20.2% to 34.8%.
• 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. 10, 5,051 (95.6%) of the 5,284 influenza-positive tests reported to CDC were influenza A viruses and 233 (4.4%) were influenza B viruses. Of the 1,875 influenza A viruses that were subtyped, 99.6 % were influenza A (H3) viruses and 0.4% were influenza A (H1N1) pdm09 viruses.
• CDC has antigenically or genetically characterized 462 influenza viruses, including 10 influenza A (H1N1) pdm09, 349 influenza A (H3N2) viruses and 103 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.
122 (35.0%) of the 349 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 227 (65.0%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 227 influenza A (H3N2) viruses were antigenically similar to A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015 Southern Hemisphere influenza vaccine.
Sixty-nine (67.0%) of the 103 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.
Thirty (88.2%) of the 34 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 (11.8%) 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, 521 influenza A (H3N2), and 87 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.