Flu Activity in the U.S. Continues to Fluctuate

According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), influenza activity decreased slightly, but remains widespread with key indicators that measure severity still high. Parts of the country that saw earlier increases in flu activity are now experiencing declines while regions where flu activity started later are still experiencing increases in activity. Pneumonia and flu-associated deaths declined slightly from last week but remain high and the hospitalization rate for people 65 years and older is the highest recorded since CDC began tracking that information. Flu activity has been elevated for 11 consecutive weeks nationally. 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. 31, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased slightly to 4.1%, but remains above the national baseline (2.0%) for the eleventh consecutive week. All 10 U.S. regions reported ILI activity at or above region-specific baseline levels. 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. States reporting high ILI activity for the week ending January 31, 2015 include Arizona, Arkansas, California, Colorado, Connecticut, Hawaii, Idaho, Kansas, Louisiana, Maine, Mississippi, Missouri, Nebraska, Nevada, New Jersey, New York, North Carolina, Oklahoma, Pennsylvania, Rhode Island, Tennessee, Texas, Vermont, Virginia, Washington, and West Virginia.

New York City and eight states (Alabama, Delaware, Massachusetts, Minnesota, New Mexico, North Dakota, Utah, and Wyoming) experienced moderate ILI activity. Six states (Georgia, Kentucky, Maryland, Montana, South Carolina, and South Dakota) experienced low ILI activity. Ten states (Alaska, Florida, Illinois, Indiana, Iowa, Michigan, New Hampshire, Ohio, Oregon, and Wisconsin) 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 Puerto Rico and 40 states. This decreased from 44 states in the previous week. Guam, the U.S. Virgin Islands and nine states (Alabama, Colorado, Georgia, Hawaii, Illinois, Minnesota, South Dakota, Tennessee, and Texas) 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 11,897 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 43.5 hospitalizations per 100,000 population. Last week, the overall cumulative rate was 40.5 hospitalizations per 100,000 population. This is slightly higher than seen for the same week during 2012-2013 when the overall hospitalization rate was 30.5 per 100,000 people. The hospitalization rate in people 65 years and older is 213.8 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 198.4 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 2012-13 season. (The 2012-2013 season was the last H3N2-predominant season.)
The hospitalization rate for children 0-4 years is 40.2 per 100,000 population. During 2012-2013, the hospitalization rate for that age group for the same week was 41.8 hospitalizations per 100,000 population.
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 slightly to 8.5% this week, but remains high and above the epidemic threshold of 7.2%. Last week, P&I was 9.1%. (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%.)

Eight influenza-associated pediatric deaths were reported to CDC during the week ending Jan. 24. Four deaths were associated with an influenza A (H3) virus and occurred during weeks 1, 2, and 4 (weeks ending Jan. 10, Jan. 17, and Jan. 31, 2015, respectively). Four deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 50, 51, 52, and 4 (weeks ending Dec. 13, Dec. 20, Dec. 27, 2014 and Jan. 31, 2015).
A total of 69 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. 31 decreased from 19.9 to 17.5%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 12.2% to 28.2%.

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. 31, 3,535 (91.4%) of the 3,869 influenza-positive tests reported to CDC were influenza A viruses and 334 (8.6%) were influenza B viruses. Of the 1,350 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 734 influenza viruses, including 21 influenza A (H1N1)pdm09, 569 influenza A (H3N2) viruses and 144 influenza B viruses, collected in the United States since October 1, 2014. All 21 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.
178 (31.3%) of the 569 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 391 (68.7%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 391 influenza A (H3N2) viruses were antigenically similar to A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015 Southern Hemisphere influenza vaccine.
Ninety-two (92.9%) of the 99 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. Seven (7.1%) of the B/Yamagata-lineage viruses tested showed reduced titers to B/Massachusetts/2/2012.
Forty-one (91.1%) of the 45 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 (8.9%) of the B/Victoria-lineage viruses tested showed reduced titers to B/Brisbane/60/2008.

Since Oct. 1, 2014, CDC has tested 25 influenza A (H1N1)pdm09, 1,035 influenza A (H3N2), and 150 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 4.0% 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 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.

Source: CDC