CDC: 2017-18 Flu Season in U.S. Dominated by H3N2 Viruses

Vaccine Effectiveness Uncertain Based on Australian Data

December 13, 2017 12:32 pm News Staff

New data from the CDC indicate that so far this flu season, influenza A (H3N2) viruses have been predominant in the United States -- a fact that is troubling because in previous flu seasons, H3N2-predominant seasons have been more severe, especially among young children and older adults.

[sick woman and man with cold and flu symptoms]

Vaccine effectiveness (VE) against H3N2 viruses also is typically lower than VE levels against influenza H1N1 and influenza B viruses. It's not uncommon for a flu shot to have VE of about 30 percent against H3N2 viruses.

All of this is according to a Morbidity and Mortality Weekly Report (MMWR)( the CDC published on Dec. 8 that recapped U.S. influenza activity from Oct. 1-Nov. 25.

The CDC also released three reports on 2017-18 early-season flu vaccine coverage estimates( among pregnant women, health care professionals and the general population.

Among other things, the reports found that to date, only about 39 percent of the U.S. population reported getting a flu vaccine; this figure is similar to estimates from this time last season and means about three out of every five people remain unprotected.

Story Highlights
  • So far this flu season, influenza A (H3N2) viruses have been predominant in the United States, according to a newly released CDC Morbidity and Mortality Weekly Report.
  • The 2017-18 influenza vaccine being used in the Northern Hemisphere contains the same viral components as the 2017 vaccine used in the Southern Hemisphere.
  • Although estimated influenza vaccine effectiveness (VE) against influenza A (H3N2) viruses has been low in the Southern Hemisphere, the VE measured there may not be predictive of what will happen in the United States this season, said CDC officials.

Furthermore, the CDC said it was concerned by early-season coverage data that showed vaccination among pregnant women to be low, leaving them unprotected from flu this season.

Flu Vaccine Effectiveness

The 2017-18 influenza vaccine being used in the Northern Hemisphere contains the same viral components as the vaccine used during the 2017 season in the Southern Hemisphere, the CDC noted in a previous MMWR.( In addition, the H3N2 vaccine component Australia used during its 2017 season is the same as that contained in the U.S. influenza vaccine used during the 2016-17 flu season.

Data recorded by influenza surveillance systems in Australia between May and September 2017 were used to estimate VE of the 2017 Southern Hemisphere influenza vaccine at preventing medically attended illness in that country, said the CDC. According to those estimates, influenza VE was 33 percent against all influenza viruses circulating in the Southern Hemisphere last season. VE against influenza A (H3N2) viruses was 10 percent, while VE against influenza B viruses was estimated to be 57 percent.

Even so, said the CDC, the VE measured in Australia may not be predictive of what will happen in the United States this season.

During the U.S. 2016-17 season, overall vaccine effectiveness was 39 percent. VE against influenza A (H3N2) viruses was 32 percent, and VE against influenza B viruses was 52 percent.

"Looking at VE against H3N2 in the United States last season is likely a more appropriate comparison/predictor of what may happen here in the United States," the CDC said.

Additional Discussion

The CDC said in its most recent MMWR that health care professionals should continue to recommend influenza vaccine now and throughout the flu season to all unvaccinated patients ages 6 months and older who don't have contraindications.

Children ages 6 months to 8 years who had not previously received a total of two or more doses of any trivalent or quadrivalent influenza vaccine before July 1 of this year (doses don't have to be received during same flu season) require two doses for the 2017-18 flu season. The doses should be given at least four weeks apart.

For the 2017-18 flu season, manufacturers projected they would supply the United States with 151 million to 166 million doses of injectable influenza vaccine. As of Nov. 24, about 148.2 million doses of vaccine had been distributed.

The CDC also sought to remind health care professionals of the importance of influenza antiviral medications in treating and preventing the spread of influenza. At particular risk are patients with confirmed or suspected influenza who have severe, complicated or progressive illness; those who require hospitalization; and those who are at high risk for influenza complications. In such cases, these medications should be given as close to the onset of illness as possible.

"Antiviral treatment should be initiated as soon as possible for patients who are at high risk for complications or who are severely ill with suspected influenza infection, even if rapid antigen-detection influenza diagnostic test results are negative," the CDC concluded.

Related AAFP News Coverage
AAFP Calls on Members to Promote Influenza Vaccination This Season

CDC Breaks Down Recent Study Linking Flu Vaccine, Miscarriage
No Cause-and-Effect Relationship Demonstrated, Says FP Expert


More From AAFP
Prevention and Control of Seasonal Influenza With Vaccines