Situation Update: Summary of Weekly FluView

Key Flu Indicators

According to this week’s FluView report, flu activity continues to decline and flu-like illness is below national baseline levels for the sixth week. However, one state (New York) continues to report widespread flu activity. Influenza B viruses now account for 88% of all influenza viruses reported. Influenza viruses circulate year-round, though at low levels in the summer in the United States. Below is a summary of the key flu indicators for the week ending May 16, 2015:

For the week ending May 16, the proportion of people seeing their health care provider for influenza-like illness (ILI) was 1.3%, and is below the national baseline of 2.0% for the sixth week. All 10 U.S. regions reported flu-like illness below region-specific baseline levels. ILI was above or at baseline for 20 weeks this season, making this the longest season in more than a decade. For the 13 seasons previous to this one ILI had remained at or above the national baseline for between one and 19 weeks each season.
New York City and 49 states experienced minimal ILI activity. The District of Columbia did not have sufficient data to calculate an activity level. Puerto Rico and one state (Louisiana) experienced low ILI activity. ILI activity data indicate the amount of flu-like illness that is occurring in each state.
Widespread influenza activity was reported by one state (New York); a decrease from three states during the previous week. Guam and one state (Connecticut) reported regional geographic influenza activity. Local flu activity was reported by Puerto Rico and nine states. Sporadic flu activity was reported by the District of Columbia and 31 states. The U.S. Virgin Islands and eight states (Arkansas, Kansas, Mississippi, Nebraska, North Carolina, Oregon, Rhode Island, and Virginia) reported no influenza activity; an increase from six states during the previous week. Geographic spread data show how many areas within a state or territory are seeing flu activity.
A total of 17,906 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 65.5 hospitalizations per 100,000 population. This is higher than the cumulative overall hospitalization rate during 2012-2013, which was 43.9 per 100,000 people.
- The hospitalization rate in people 65 years and older is 322.8 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 322.2 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 2012-2013 season. (The 2012-2013 season was the last H3N2-predominant season.)
- The hospitalization rate for children 0-4 years is 57.2 per 100,000 population. During the 2012-2013 season, the overall hospitalization rate for that age group was 67.0 per 100,000 cumulatively that season.
- 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 to 6.0%, and remains below the epidemic threshold of 6.6%. The percentage of P&I attributed deaths was at or above the epidemic threshold for 12 consecutive weeks this season. The highest P&I percentage this season was 9.3% and occurred during week 2. During 2012-2013, P&I peaked at 9.9%. This is comparable to recorded percentages for past severe seasons, including the 2003-2004 season when P&I reached 10.4%.
Three influenza-associated pediatric deaths were reported to CDC during the week ending May 16.
- One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 5 (the week ending February 7, 2015). Two influenza-associated pediatric deaths reported during week 19 occurred during the 2013-14 season and brings the total number of reported pediatric deaths occurring during that season to 112. One death was associated with an influenza A (H1N1)pdm09 virus and one death was associated with an influenza B virus.
- A total of 139 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 May 16 slightly decreased from 4.9% to 3.5%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 1.3% to 8.1%.
Influenza A (H3N2) viruses have predominated overall during the 2014-2015 flu season, accounting for more than 99% of all subtyped influenza A viruses. However influenza B viruses have accounted for the largest proportion of circulating viruses since early March. During week 19, 88% of all influenza positive specimens reported were influenza B viruses, and influenza B viruses predominated in all 10 regions. It is not uncommon for there to be a second wave of flu activity toward the end of the flu season with another seasonal influenza virus. Influenza A (H1N1) pdm09 viruses have been detected rarely this season.
CDC has antigenically or genetically characterized 2,083 influenza viruses, including 59 influenza A (H1N1)pdm09, 1,272 influenza A (H3N2) viruses and 752 influenza B viruses, collected in the United States since October 1, 2014.
- All 59 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.
- 244 (19.2%) of the 1,272 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 1,028 (80.8%) influenza A (H3N2) viruses tested were different from A/Texas/50/2012. The majority of these 1,028 influenza A (H3N2) viruses were antigenically similar to A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015 Southern Hemisphere influenza vaccine and 2015-2016 Northern Hemisphere influenza vaccine.
- 513 (97.9%) of the 524 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. Eleven (2.1%) of the B/Yamagata-lineage viruses tested showed reduced titers to B/Massachusetts/2/2012.
- 223 (97.8%) of the 228 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. Five (2.2%) of the B/Victoria-lineage viruses tested showed reduced titers to B/Brisbane/60/2008.
Since October 1, 2014, CDC has tested 59 influenza A (H1N1)pdm09, 3,187 influenza A (H3N2), and 809 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 1.7% 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) are found among influenza A (H1N1)pdm09 and influenza A (H3N2) viruses. Adamantanes are not effective against influenza B viruses.

FluView is available – and past issues are archived – on the CDC website.

Note: Delays in reporting may mean that data changes over time. The most up to date data for all weeks during the 2014-2015 season can be found on the current FluView.