CDC Flu Update: Activity Decreases for Fifth Consecutive Week


According to the CDC's FluView report for the week ending March 18, 2017 (week 11), flu activity remains elevated, but decreased for the fifth consecutive week. While the 2016-2017 flu season has peaked, 36 states continue to report widespread flu activity and significant flu activity is expected to continue for several more weeks. Influenza A (H3N2) viruses have been most common overall this season and they continue to predominate, 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 18, 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%. Seven regions (Regions 1, 3, 4, 5, 6, 7 and 10) reported ILI at or above their region-specific baseline level. This is the 14th 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: 12 states (Alabama, Georgia, Indiana, Kansas, Kentucky, Louisiana, Maryland, Minnesota, Mississippi, Oklahoma, South Carolina, and Virginia) experienced high ILI activity. Six states (Arkansas, Illinois, Michigan, New Mexico, North Carolina, and Texas) experienced moderate ILI activity. Nine states (Alaska, Arizona, California, Missouri, Pennsylvania, Rhode Island, South Dakota, Tennessee, and Wisconsin) experienced low ILI activity. New York City, Puerto Rico and 23 states (Colorado, Connecticut, Delaware, Florida, Hawaii, Idaho, Iowa, Maine, Massachusetts, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New York, North Dakota, Ohio, Oregon, Utah, Vermont, Washington, West Virginia, and Wyoming) 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 36 states (Alabama, Alaska, Arkansas, California, Connecticut, Delaware, Florida, Georgia, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, 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 10 states (Arizona, Hawaii, Idaho, Illinois, Oregon, South Dakota, Tennessee, Texas, West Virginia, and Wyoming). Local influenza activity was reported by the District of Columbia and two states (Colorado and Montana). Sporadic activity was reported by two states (Utah and Washington).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 14,103 laboratory-confirmed influenza-associated hospitalizations have been reported. This translates to a cumulative overall rate of 50.4 hospitalizations per 100,000 people in the United States. This is higher than the hospitalization rate for week 11 (41.3 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 (59.5 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 228.6 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 176.2 per 100,000. For week 11 during 2014-2015, it was 290.7 per 100,000.
The hospitalization rate among adults 50-64 years is 49.2 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 38.2 per 100,000 and 48.2 respectively.
The hospitalization rate among children younger than 5 years is 33.6 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 60.7 per 100,000 and 53.0 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 and

Mortality Surveillance:  The proportion of deaths attributed to pneumonia and influenza (P&I) was 7.8% for the week ending March 4, 2017 (week 9). This percentage is above the epidemic threshold of 7.5% for week 9 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: Two influenza-associated pediatric deaths are being reported by CDC for the week ending March 18, 2017.
Both deaths were associated with an influenza A (H3) virus and occurred during weeks 7 and 8 (the weeks ending February 18, and February 25, 2017, respectively).
A total of 55 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:

Laboratory Data: Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending March 18 was 17.9%.
Regionally, the three week average percent of specimens testing positive for influenza in clinical laboratories ranged from 7.5% to 24.3%.
During the week ending March 18, of the 4,416 (17.9%) influenza-positive tests reported to CDC by clinical laboratories, 2,343 (53.1%) were influenza A viruses and 2,073 (46.9%) were influenza B viruses.
The most frequently identified influenza virus type reported by public health laboratories during the week ending March 18 was influenza A viruses, with influenza A (H3N2) viruses predominating.
During the week ending March 18, 428 (60.5%) of the 707 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 279 (39.5%) were influenza B viruses. Of the 392 influenza A viruses that were subtyped, 375 (95.7%) were H3N2 viruses and 17 (4.3%) 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 2,088 specimens (208 influenza A (H1N1)pdm09, 1,404 influenza A (H3N2), and 476 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

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