CDC Flu Update: Activity Likely to Continue for Several More Weeks

According to the CDC's FluView report for the week ending March 25, 2017 (week 12), flu activity remains elevated in the United States. While the 2016-2017 flu season has peaked, 31 states continue to report widespread flu activity and significant flu activity is likely to continue for several more weeks. Influenza A (H3N2) viruses have been most common overall this season and they continue to predominate in public health laboratories, but there has been an increasing proportion of influenza B viruses detected in recent weeks. Based on interim estimates, flu vaccines this season have reduced a vaccinated person’s risk of getting sick and having to go to the doctor because of flu by about half (48%). CDC recommends annual flu vaccination for everyone 6 months of age and older. Vaccination efforts should continue for as long as influenza viruses are circulating.

Influenza-like Illness Surveillance: For the week ending March 25, the proportion of people seeing their health care provider for influenza-like illness (ILI) was 3.2%. This remains above the national baseline of 2.2%. Eight regions (Regions 1, 3, 4, 5, 6, 7, 8 and 10) reported ILI at or above their region-specific baseline level. This is the 15th week during the 2016-2107 flu season that influenza-like-illness has been at or above baseline. For the last 15 seasons, the average duration of a flu season by this measure has been 13 weeks, with a range from one week to 20 weeks.

Influenza-like Illness State Activity Indicator Map: 10 states (Alabama, Arkansas, Georgia, Kentucky, Minnesota, North Carolina, Oklahoma, South Carolina, Tennessee, and Virginia) experienced high ILI activity. Eight states (Illinois, Indiana, Kansas, Louisiana, Maryland, Michigan, Mississippi, and Rhode Island) experienced moderate ILI activity. Eight states (Alaska, Colorado, Massachusetts, Missouri, New Mexico, Pennsylvania, West Virginia, and Wyoming) experienced low ILI activity. New York City, Puerto Rico and 24 states (Arizona, California, Connecticut, Delaware, Florida, Hawaii, Idaho, Iowa, Maine, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New York, North Dakota, Ohio, Oregon, South Dakota, Texas, Utah, Vermont, Washington, and Wisconsin) experienced minimal ILI activity. 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.

Geographic Spread of Influenza Viruses: Widespread influenza activity was reported by 31 states (Alabama, Arkansas, Connecticut, Delaware, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Nebraska, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Vermont, Virginia, and Wisconsin). Regional influenza activity was reported by Guam, Puerto Rico and 12 states (Alaska, Arizona, California, Idaho, Louisiana, Mississippi, Nevada, New Mexico, South Dakota, Tennessee, Texas, and West Virginia). Local influenza activity was reported by the District of Columbia and five state (Colorado, Montana, Oregon, Washington, and Wyoming). Sporadic activity was reported by two states (Hawaii and Utah). No influenza activity was reported by the U.S. Virgin Islands. Geographic spread data show how many areas within a state or territory are seeing flu activity.

Flu-Associated Hospitalizations: Since October 1, 2016, a total of 15,137 laboratory-confirmed influenza-associated hospitalizations have been reported. This translates to a cumulative overall rate of 54.1 hospitalizations per 100,000 people in the United States. This is higher than the hospitalization rate for week 12 (42.1 per 100,000) during the 2012-2013 flu season, when influenza A H3N2 viruses also predominated, but lower than the cumulative hospitalization rate during 2014-2015 (60.6 per 100,000) which also was an H3N2 predominant season. Vaccine effectiveness during 2012-13 was 49%, similar to interim estimates for the current season, but was 19% during 2014-2015 as a result of a high proportion of drifted influenza viruses during that season. The hospitalization rate among people 65 years and older is 243.6 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 the 2012-2013 flu season was 178.4 per 100,000. For week 12 during 2014-2015, it was 294.7 per 100,000.
The hospitalization rate among adults 50-64 years is 52.9 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 38.9 per 100,000 and 49.5 respectively.
The hospitalization rate among children younger than 5 years is 37.5 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 62.5 per 100,000 and 53.9 per 100,000 respectively.
During most seasons, children younger than 5 years and adults 65 years and older have the highest hospitalization rates.
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. Additional data, including hospitalization rates during other influenza seasons, can be found at http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html.

Mortality Surveillance:  The proportion of deaths attributed to pneumonia and influenza (P&I) was 7.8% for the week ending March 11, 2017 (week 10). This percentage is above the epidemic threshold of 7.5% for week 10 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. The weekly percentage of deaths attributed to P&I has exceeded the epidemic threshold for eight consecutive weeks this season.

Pediatric Deaths: Six influenza-associated pediatric deaths are being reported by CDC for the week ending March 25, 2017.
Three deaths were associated with an influenza A (H3) virus and occurred during weeks 8, 10, and 11 (the weeks ending February 25, March 11, and March 18, 2017, respectively).
Two deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 8 and 11.
One death was associated with influenza B and occurred during week 11.
A total of 61 influenza-associated pediatric deaths have been reported for the 2016-2017 season.
Additional information on pediatric deaths for the 2016-2017 season is available on FluView Interactive at: https://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html.

Laboratory Data: Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending March 25 was 20.1%.
Regionally, the three week average percent of specimens testing positive for influenza in clinical laboratories ranged from 6.7% to 25.2%.
During the week ending March 25, of the 4,768 (20.1%) influenza-positive tests reported to CDC by clinical laboratories, 2,253 (47.3%) were influenza A viruses and 2,515 (52.7%) were influenza B viruses.
The most frequently identified influenza virus type reported by public health laboratories during the week ending March 25 was influenza A viruses, with influenza A (H3N2) viruses predominating.
During the week ending March 25, 320 (53.2%) of the 602 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 282 (46.8%) were influenza B viruses. Of the 301 influenza A viruses that were subtyped, 291 (96.7%) were H3N2 viruses and 10 (3.3%) were (H1N1)pdm09 viruses.
Since October 1, 2016, antigenic and/or genetic characterization shows that the majority of the tested viruses remain similar to the recommended components of the 2016-2017 Northern Hemisphere vaccines.
Since October 1, 2016, CDC tested 2,191 specimens (222 influenza A (H1N1)pdm09, 1,455 influenza A (H3N2), and 514 influenza B viruses) for resistance to the neuraminidase inhibitors antiviral drugs. None of the tested viruses were found to be resistant to oseltamivir, zanamivir, or peramivir.

Source: CDC