December 08, 2017
3 min read

H3N2, known to evade vaccine, is predominant flu strain in US

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Influenza A(H3N2) — a strain known for evading the seasonal influenza vaccine and causing relatively severe illness — has been the predominant strain this year in the United States, according to nationwide surveillance data.

Writing in MMWR, Vivien G. Dugan, PhD, deputy chief of the Virology, Surveillance and Diagnosis Branch in the CDC’s Influenza Division, and colleagues said influenza activity has been increasing since the beginning of November, with four states reporting widespread activity.

So far, most tested viruses have been good matches for the reference viruses used in seasonal vaccines. However, Dugan and colleagues said circulating H3N2 viruses have been antigenically less similar to the egg-grown H3N2 viruses used for producing the majority of influenza vaccines in the U.S. — an occurrence that can be caused by several variables, including mutations related to vaccine viruses grown in eggs or the genetic diversity and rapid evolution of the strain.

H3N2-prevalent seasons are typically more severe, causing more deaths and hospitalizations than seasons dominated by H1N1 or B viruses. Recently, H3N2 was the most prevalent strain during an historically bad influenza season in Australia, where preliminary data showed that the H3N2 component in the Southern Hemisphere’s vaccine was just 10% effective.

According to researchers, Australia saw a record number of laboratory-confirmed influenza hospitalizations this year, raising concern — but not guaranteeing — that the U.S. will have a similarly bad season.

Anthony Fauci
Anthony S. Fauci

“The implications for the Northern Hemisphere are not clear, but it is of note that the vaccine for this upcoming season has the same composition as that used in the Southern Hemisphere,” National Institute of Allergy and Infectious Diseases Director Anthony S. Fauci, MD, and colleagues wrote recently in The New England Journal of Medicine.

Fauci and colleagues warned of a potentially severe influenza season and called for a “transformative approach” to creating a universal influenza vaccine that would be long-lasting and widely protective.

The potential implications of the Australian findings have been widely reported, but the CDC thinks recent data from the U.S. are a better indicator of how things will go, according to Dugan. She said the same H3N2 component used in last year’s vaccine was 32% effective against the strain — only slightly lower than the overall effectiveness of 39%.

“Based on these data,” Dugan told Infectious Disease News, “CDC believes U.S. estimates from last season are likely to be a better predictor of the flu vaccine benefits to expect this season against circulating H3N2 viruses in the United States … assuming minimal change to circulating H3N2 viruses.”

As of Nov. 25, of 135,202 specimens tested in clinical laboratories, 3.7% were positive for influenza, including 73.4% that tested positive for influenza A viruses, according to the MMWR. Among influenza A viruses subtyped by U.S. public health laboratories, 90% were H3N2 viruses.

According to the report, there were 566 laboratory-confirmed influenza-related hospitalizations in the U.S. as of Nov. 25, most in patients aged 65 years or older. Over 85% of the patients who were hospitalized had an influenza A virus, mostly H3N2.

Georgia, Louisiana, Massachusetts and Oklahoma all reported widespread influenza activity, and 10 states reported regional activity. Dugan and colleagues said all U.S. viruses that have been tested were sensitive to antiviral drugs.

During the week ending Nov. 25, 2.3% of outpatient visits reported through a CDC influenza surveillance program were for influenza-like illness (ILI) — above the national baseline for the first time this season. Dugan said the CDC cannot predict when influenza season will peak. Health officials indicated that influenza activity will increase in the coming weeks. – by Gerard Gallagher


CDC. Weekly U.S. influenza surveillance report. 2017. Accessed December 7, 2017.

Dugan VG, et al. MMWR Morb Mortal Wkly Rep. 2017;doi:10.15585/mmwr.mm6648a2.

Paules CI, et al. N Engl J Med. 2017;doi:10.1056/NEJMp1714916.

Sullivan SG, et al. Euro Surveill. 2017;doi:10.2807/1560-7917.ES.2017.22.43.17-00707.

Disclosures: The authors report no relevant financial disclosures.