CDC: Flu Activity Continues in US, Vaccine 48% Effective

Megan Brooks

February 16, 2017

"Elevated" influenza activity is expected for several more weeks in parts of the United States, federal health officials with the US Centers for Disease Control and Prevention (CDC) said today.

Influenza activity in the United States began to increase in mid-December and has remained elevated as of February 4, report Lenee Blanton, MPH, and colleagues from the CDC's Influenza Division at the National Center for Immunization and Respiratory Diseases in the Morbidity and Mortality Weekly Report (MMWR).

During the most recent weeks, decreases in activity have been observed in the Northwest, while activity has continued to increase in the rest of the country, they report.

Between October 2, 2016, and February 4, 2017, influenza A (H3N2) viruses made up the majority of circulating influenza viruses, but influenza A (H1N1)pdm09 and influenza B viruses also were identified, they note.

"Influenza activity has been moderate so far this season, and severity indicators are within the range of what has been observed during previous seasons when influenza A (H3N2) viruses predominated," the researchers report.

A companion report in MMWR provides interim estimates of vaccine effectiveness (VE) based on data collected from November 28, 2016, through February 4, 2017.

The data suggest that this year's influenza vaccine has been 48% (95% confidence interval [CI], 37% - 57%) effective in preventing influenza-related medical visits across all age groups.

VE was estimated to be 43% (95% CI, 29% - 54%) effective against illness caused by influenza A (H3N2) and 73% (95% CI, 54% - 84%) effective against influenza B viruses, report the CDC's Brendan Flannery, PhD, and colleagues.

"These interim VE estimates indicate that influenza vaccination reduced the risk for outpatient medical visits by almost half," they write. "Because influenza activity remains elevated, CDC and the Advisory Committee on Immunization Practices recommend that annual influenza vaccination efforts continue as long as influenza viruses are circulating."

Influenza vaccination "remains the most effective way to prevent influenza illness," and antiviral medications are "an important adjunct to vaccination in the treatment and prevention of influenza," Dr Blanton and colleagues note in their report.

They say no antiviral resistance to the neuraminidase inhibitors oseltamivir (Tamiflu, Roche), zanamivir (Relenza, GlaxoSmithKline), or peramivir (Rapivab, Biocryst) has been detected among influenza viruses collected since October 1, 2016.

The CDC recommends starting these medications early for patients with severe, complicated, or progressive influenza illness and for those at higher risk for influenza complications, including adults aged 65 years and older.

"Concerning" Case of Nonhuman Influenza Virus Infection

Dr Blanton and her colleagues also make note of an "unusual outbreak" of avian lineage influenza A (H7N2) virus infection among cats in an animal shelter in New York City. The outbreak was first reported to public health officials on December 14, 2016.

Roughly 350 people who were exposed to infected cats during this outbreak were screened or tested for infection, and one human infection with avian influenza A (H7N2) was identified.

"This is the first influenza A (H7N2) virus infection in humans identified in the United States since 2003 and the first known human infection with an influenza A virus likely acquired through exposure to an ill cat," they point out. "The finding of an avian lineage influenza virus in an unexpected host, such as a domestic cat, or any human infection with a nonhuman influenza virus is concerning.

"Early identification and investigation of human infections with novel influenza A viruses are critical so that the risk of infection can be more fully understood and appropriate public health measures can be taken," they write.

If clinical laboratories test a respiratory specimen that they cannot type or subtype using commercially available rapid or molecular influenza diagnostic tests, the CDC recommends they contact their state public health laboratory to arrange for additional testing.

"Public health laboratories should immediately send virus specimens that they cannot type or subtype using standard methods to CDC and submit all specimens that are otherwise unusual as soon as possible after identification," Blanton and colleagues write.

MMWR Morb Mortal Wkly Rep. 2017;66:159-171. First report, full text; Second report, full text

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