Influenza Activity Dominated by H3N2 Viruses, Vaccine Only 25% Effective

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The percentage of outpatient visits that were for influenza-like illness rose sharply in late 2017 to 7.7% in early February.
The percentage of outpatient visits that were for influenza-like illness rose sharply in late 2017 to 7.7% in early February.

The Centers for Disease Control and Prevention (CDC) issued a surveillance report on influenza activity from October 2017 to February 2018. In the United States, cases of influenza started to rise in early November 2017 and increased sharply from December 2017 through February 3, 2018. CDC officials expect heightened influenza activity to persist for several more weeks.1

This season, influenza A(H3N2) viruses have predominated with reports of influenza A(H1N1)pdm09 and influenza B viruses. During the surveillance period, clinical laboratories tested 666,493 specimens for influenza virus, of which 18.7% (n=124,316) tested positive. Specifically, the percentage of samples testing positive for any influenza virus rose to 26.4% during the week ending January 13 and stayed between 26.3% to 26.7% through the week ending February 3.

During the same period, public health laboratories tested 51,014 specimens for influenza virus, of which 84.1% (n=23,257) tested positive for influenza A and 15.9% (n=4412) tested positive for influenza B viruses. Of the influenza A viruses, 89.9% were influenza A(H3N2) and 10.1% were influenza A(H1N1)pdm09 viruses. 

Most of the influenza-positive patients whose specimens were tested were aged ≥65 years (37.9%). Overall, influenza A(H3N2) viruses were most prevalent across all age groups with 68% to 72% of cases in patients aged 0-64 years, and 84% of cases among patients aged ≥65 years.

This influenza season "has been substantial, with some of the highest levels of influenza-like illness and hospitalization rates in recent years, and elevated activity occurring in most of the country simultaneously," the authors write. 

The CDC also released interim estimates of the 2017/2018 season influenza vaccine efficacy using data from 4562 children and adults enrolled in the United States Influenza Vaccine Effectiveness Network during November 2, 2017 to February 3, 2018.2 The overall adjusted vaccine effectiveness (VE) against influenza A and B virus infection associated with acute respiratory illness was 36% (95% CI: 27%-44%). VE was estimated to be 67% against influenza A(H1N1)pdm09 viruses, 42% against influenza B viruses, and 25% against influenza A(H3N2) virus.2 

Based on these findings, the CDC continues to recommend influenza vaccination at this time.  Given the current VE estimates, vaccination will still help prevent influenza illness, hospitalizations, and deaths. In particular, patients aged ≥6 months who have not been vaccinated this season should receive the vaccine. 

Early treatment with neuraminidase inhibitor antivirals (eg, oseltamivir [Tamiflu®], zanamivir [Relenza®], peramivir [Rapivab™]) is recommended for patients with severe, complicated or progressive influenza, and for patients at higher risk for complications (eg, adults aged ≥65 years). The authors emphasize that treatment should not be delayed while waiting for test results or if rapid antigen-detection influenza diagnostic test results are negative. "Treatment with influenza antiviral medications, where appropriate, is especially important this season," they conclude.

References

  1. Budd AP, Wentworth DE, Blanton L, et al. Update: Influenza Activity - United States, October 1, 2017-February 3, 2018. MMWR Morb Mortal Wkly Rep. 2018;67:169-179.
  2. Flannery B, Chung JR, Belongia EA, et al. Interim estimates of 2017-18 seasonal influenza vaccine effectiveness - United States, February 2018. MMWR Morb Mortal Wkly Rep. 2018;67:180-185.
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