H3N2 Viruses Remain the Most Common Cause of Flu in the U.S.

According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), flu activity is widespread in most of the country and key indicators used to measure severity are climbing sharply. H3N2 viruses remain the most common. Flu activity has been elevated for nine consecutive weeks. An average season lasts about 13 weeks. However, because this season started relatively early, it could last longer than average.

For the week ending Jan. 17, the proportion of people seeing their health care provider for influenza-like illness (ILI) increased slightly to 4.5%, and remains above the national baseline (2.0%) for the ninth 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 23 states experienced high ILI activity; a decrease from 24 states during the previous week. New York City and ten states (Delaware, Hawaii, Indiana, Maryland, Massachusetts, New Jersey, New Mexico, Pennsylvania, Rhode Island, and Tennessee) experienced moderate ILI activity. Ten states (Arizona, Florida, Iowa, Kentucky, Maine, Michigan, Montana, Nebraska, Ohio, and South Carolina) experienced low ILI activity. Seven states (Alaska, Georgia, Illinois, New Hampshire, Oregon, Vermont, and Wisconsin) experienced minimal ILI activity, and the District of Columbia did not have sufficient data to calculate an activity level. ILI activity data indicates the amount of flu-like illness that is occurring in each state.

Widespread influenza activity was reported by 44 states; a decrease from the 46 states that reported widespread activity during the previous week. Guam, Puerto Rico, the U.S. Virgin Islands, and five states (Alabama, Georgia, Hawaii, Minnesota, and Tennessee) 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 9,926 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 36.3 hospitalizations per 100,000 population. Last week, the overall cumulative rate was 29.9 hospitalizations per 100,000 population. The hospitalization rate in people 65 years and older is 176.1 per 100,000. This is the highest rate of any age group. (During the 2012-13 season, the hospitalization rate for people 65 and older for the same week was 103.5 per 100,000, and for the following week was 120.1 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 continues to climb sharply, reaching 9.3% this week, which is above the epidemic threshold of 7.1%. Last week, P&I was 8.5%.  (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%.)

Eleven influenza-associated pediatric deaths were reported to CDC during the week ending January 17. Three deaths were associated with an influenza A (H3) virus and occurred during weeks 51, 53, and 1 (weeks ending Dec. 20, 2014, Jan. 3, and Jan. 10, 2015, respectively).  Eight deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 51, 52, 53, 1, and 2 (weeks ending Dec. 20, Dec. 27, 2014, and Jan. 3, Jan. 10, and Jan. 17, 2015, respectively).
A total of 56 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. 17 was 19.5%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 16.6% to 29.1%.

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 Jan. 17, 4,800 (94%) of the 5,104 influenza-positive tests reported to CDC were influenza A viruses and 304 (6.0%) were influenza B viruses. Of the 1,824 influenza A viruses that were subtyped, 99.8 % were influenza A (H3) viruses and 0.1% were influenza A (H1N1) pdm09 viruses.

CDC has antigenically or genetically characterized 508 influenza viruses, including 10 influenza A (H1N1) pdm09, 395 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.
141 (35.7%) of the 395 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 254 (64.3%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 254 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 14 influenza A (H1N1) pdm09, 724 influenza A (H3N2), and 127 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, one influenza A (H1N1) pdm09 virus reported this week showed resistance to oseltamivir and peramivir. (Because H1N1 viruses have been so rare this season, one virus accounts for 7.1 percent 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 Dec. 19, 2014, 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