Flu-Like Illness Now Below National Baseline for the First Time Since Late December

According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), flu activity continues to decrease in the United States. Levels of flu-like illness are now below the national baseline for the first time this season since late December. While flu activity has peaked nationally for this season, some parts of the country are still experiencing widespread flu activity and an increasing proportion of influenza B viruses have been detected. Influenza B viruses now account for more than half of all influenza viruses reported. Second waves of influenza B activity occur during many flu seasons. Ongoing activity is expected to continue for a number of weeks. CDC continues to recommend influenza vaccination as long as influenza viruses are circulating. In late February, CDC reported flu vaccine effectiveness of nearly 60% this season. CDC also recommends that patients suspected of having influenza who are at high-risk of flu complications or who are very sick with flu-like symptoms should receive prompt treatment with influenza antiviral drugs without waiting for confirmatory testing.

For the week ending April 23, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased to 2.0% and is now below the national baseline of 2.1% for the first time this season since late December. Three of 10 regions (Regions 2, 3, and 4) reported ILI at or above their region-specific baseline levels. One way that CDC measures the duration of the influenza season is the number of consecutive weeks during which ILI is at or above the national baseline. ILI was at or above the national baseline for 17 consecutive weeks this season. For the last 13 seasons, the average duration of a flu season by this measure has been 13 weeks, with a range from 1 week to 20 weeks.

One  state (New Jersey) experienced high ILI activity. The same state reported high ILI activity during the previous week. Puerto Rico and two states (Arizona and Arkansas) experienced moderate ILI activity. New York City and six states (Alaska, Georgia, Kentucky, Massachusetts, North Carolina, and Virginia) experienced low ILI activity. 41 states 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.

Widespread flu activity was reported by Guam, Puerto Rico and 13 states (Connecticut, Delaware, Kentucky, Maine, Massachusetts, New Hampshire, New York, North Carolina, Ohio, Pennsylvania, Vermont, Virginia, and Wisconsin). This is a decrease from 14 states with widespread activity last week. Regional flu activity was reported by 16 states (Alabama, Alaska, Arizona, California, Colorado, Idaho, Iowa, Michigan, Minnesota, Missouri, Nebraska, New Jersey, New Mexico, North Dakota, Oklahoma, and Oregon). Local flu activity was reported by the District of Columbia and 13 states (Arkansas, Hawaii, Kansas, Maryland, Montana, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Washington, West Virginia, and Wyoming). Sporadic flu activity was reported by the U.S. Virgin Islands and eight states (Florida, Georgia, Illinois, Indiana, Louisiana, Mississippi, Nevada, and Utah). Geographic spread data show how many areas within a state or territory are seeing flu activity.

Since October 1, 2015, 8,239 laboratory-confirmed influenza-associated hospitalizations have been reported through FluSurv-NET, a population-based surveillance network for laboratory-confirmed influenza-associated hospitalizations. This translates to a cumulative overall rate of 29.8 hospitalizations per 100,000 people in the United States. This is significantly lower than the hospitalization rate at this time last season (63.8 per 100,000). More data on hospitalization rates, including hospitalization rates during other influenza seasons, are available at http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html. The highest hospitalization rates are among people 65 years and older (79.6 per 100,000), followed by adults 50-64 years (43.1 per 100,000) and children younger than 5 years (40.5 per 100,000). During most seasons, adults 65 years and older and children younger than 5 years have the highest hospitalization rates.
FluSurv-NET hospitalization data are collected from 13 states and represent approximately 8.5% 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) was below the system-specific epidemic threshold in both the NCHS Mortality Surveillance System and the 122 Cities Mortality Reporting System.

Four additional influenza-associated pediatric deaths were reported to CDC this week: Two deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 15 and 16 (the weeks ending April 16, and April 23, 2016). One death was associated with an influenza B virus and occurred during week 15 (the week ending April 16, 2016). One death was associated with an influenza virus for which the type was not determined and occurred during week 13 (the week ending April 2, 2016). This brings the total number of flu-associated pediatric deaths reported this season to 60 children.

Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending April 23 was 12.5%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 7.0% to 16.7%. During the week ending April 23, of the 1,857 influenza-positive tests reported to CDC by clinical laboratories, 881 (47.4%) were influenza A viruses and 976 (52.6%) were influenza B viruses.

The most frequently identified influenza virus type reported by public health laboratories during the week ending April 23 was influenza B viruses. During the week ending April 23, 105 (46.9%) of the 224 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 119 (53.1%) were influenza B viruses. Of the 101 influenza A viruses that were subtyped, 25 (24.8%) were H3 viruses and 76 (75.2%) were (H1N1)pdm09 viruses.

Cumulatively from October 4, 2015-April 23, 2016, influenza A (H1N1)pdm09 viruses were predominant in all four age groups (0-4 years age group (71.8%), 5-24 years age group (51.2%), 25-64 years age group (70.9%), and in ages 65 years and older (53.3%).

CDC has characterized 1,777 specimens (744 influenza A (H1N1)pdm09, 445 influenza A (H3N2) and 588 influenza B viruses) collected in the U.S. since October 1, 2015. All 744 (100%) influenza A (H1N1)pdm09 viruses were antigenically characterized as similar to A/California/7/2009, the influenza A (H1N1) component of the 2015-2016 Northern Hemisphere vaccine. All 445 H3N2 viruses were genetically sequenced and all viruses belonged to genetic groups for which a majority of viruses antigenically characterized were similar to cell-propagated A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015-2016 Northern Hemisphere vaccine. A subset of 193 H3N2 viruses also were antigenically characterized; 185 of 193 (95.9%) H3N2 viruses were similar to A/Switzerland/9715293/2013 by HI testing or neutralization testing. All 359 (100%) of the B/Yamagata-lineage viruses were antigenically characterized as similar to B/Phuket/3073/2013, which is included in both the 2015–16 Northern Hemisphere trivalent and quadrivalent vaccines. 223 of 229 (97.4%) of the B/Victoria-lineage viruses were antigenically characterized as similar to B/Brisbane/60/2008, which is included in the 2015-16 Northern Hemisphere quadrivalent vaccine.

Since October 1, 2015, CDC has tested 1,699 influenza A (H1N1)pdm09, 577 influenza A (H3N2), and 850 influenza B viruses for resistance to the neuraminidase inhibitors antiviral drugs. While the vast majority of the viruses that have been tested are sensitive to oseltamivir, zanamivir, and peramivir, so far this season, 13 (0.8%) influenza A (H1N1)pdm09 viruses have showed resistance to oseltamivir and peramivir.

The Food and Drug Administration (FDA)’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) endorsed the WHO-recommended vaccine viruses for use in all U.S. seasonal flu vaccines for the 2016-2017 flu season. These recommendations were as follows: It was recommended that trivalent vaccines for use in the 2016-2017 influenza season (Northern Hemisphere winter) contain the following: an A/California/7/2009 (H1N1)pdm09-like virus; an A/Hong Kong/4801/2014 (H3N2)-like virus; a B/Brisbane/60/2008-like virus (B/Victoria lineage).

It was recommended that quadrivalent vaccines containing two influenza B viruses contain the above three viruses and a B/Phuket/3073/2013-like virus (B/Yamagata lineage). This represents a change in the influenza A (H3) component and a change in the influenza B lineage included in the trivalent vaccine compared with the composition of the 2015-2016 influenza vaccine.

Source: CDC