CDC Flu Update: Second Wave of Influenza B Virus Activity Hits


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. 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 percent 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 30, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased to 1.8%. This is below the national baseline of 2.1%. 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 18 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.

Puerto Rico and two states (Minnesota and New Jersey) experienced moderate ILI activity. Three states (Pennsylvania, Tennessee, and Virginia) experienced low ILI activity. New York City and 45 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 7 states (Connecticut, Delaware, Maine, New Hampshire, New York, Pennsylvania, and Virginia). This is a decrease from 13 states with widespread activity last week. Regional flu activity was reported by 15 states (Alabama, Arizona, California, Colorado, Idaho, Iowa, Kentucky, Massachusetts, Missouri, New Jersey, North Carolina, Ohio, Oklahoma, Vermont, and Wisconsin). Local flu activity was reported by the District of Columbia and 17 states (Alaska, Arkansas, Hawaii, Maryland, Michigan, Minnesota, Montana, Nebraska, New Mexico, North Dakota, Oregon, Rhode Island, South Carolina, South Dakota, Texas, Washington, and West Virginia). Sporadic flu activity was reported by the U.S. Virgin Islands and 11 states (Florida, Georgia, Illinois, Indiana, Kansas, Louisiana, Mississippi, Nevada, Tennessee, Utah, and Wyoming). Geographic spread data show how many areas within a state or territory are seeing flu activity.

Since October 1, 2015, 8,467 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 30.6 hospitalizations per 100,000 people in the United States. This is significantly lower than the hospitalization rate at this time last season (64.1 per 100,000), which was a H3N2 predominant season. During the last H1N1 predominant season (2013-2014), the cumulative hospitalization rate was 35.1. More data on hospitalization rates, including hospitalization rates during other influenza seasons, are available at and The highest hospitalization rates are among people 65 years and older (82.1 per 100,000), followed by adults 50-64 years (44.3 per 100,000) and children younger than 5 years (41.4 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 (H1N1)pdm09 virus and occurred during weeks 14 and 15 (the weeks ending April 9, and April 16, 2016).
One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 13 (the week ending April 2, 2016).
One death was associated with an influenza B virus and occurred during week 12 (the week ending March 26, 2016). This brings the total number of flu-associated pediatric deaths reported this season to 64 children.

Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending April 30 was 11.2%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 7.9% to 16.9%. During the week ending April 30, of the 1,442 influenza-positive tests reported to CDC by clinical laboratories, 550 (38.1%) were influenza A viruses and 892 (61.9%) were influenza B viruses.

The most frequently identified influenza virus type reported by public health laboratories during the week ending April 30 was influenza B viruses. During the week ending April 30, 82 (46.9%) of the 175 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 93 (53.1%) were influenza B viruses. Of the 78 influenza A viruses that were subtyped, 30 (38.5%) were H3 viruses and 48 (61.5%) were (H1N1)pdm09 viruses.

Cumulatively from October 4, 2015-April 30, 2016, influenza A (H1N1)pdm09 viruses were predominant in all four age groups: 0-4 years age group (71.6%), 5-24 years age group (50.3%),  25-64 years age group (70.4%), and in ages 65 years and older (52.4%).

CDC has characterized 1,903 specimens (822 influenza A (H1N1)pdm09, 493 influenza A (H3N2) and 588 influenza B viruses) collected in the U.S. since October 1, 2015. 821 of 822 (99.9%) 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 493 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 223 H3N2 viruses also were antigenically characterized; 215 of 223 (96.4%) 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 1,869 influenza A (H1N1)pdm09, 623 influenza A (H3N2), and 931 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, 14 (0.7%) influenza A (H1N1)pdm09 viruses have showed resistance to oseltamivir and peramivir.

The Food and Drug Administration’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

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