Influenza Activity Increasing in United States, CDC Says

Lara C. Pullen, PhD
December 11, 2017

Influenza activity has increased since early November, according to the influenza activity update from the Centers for Disease Control and Prevention (CDC), published in the December 8 issue of the Morbidity and Mortality Weekly Report.

Although the United States had low levels of influenza activity in October, several influenza activity indicators are now higher than is typically seen this time of year.

Vivien G. Dugan, PhD, deputy chief of the Virology, Surveillance and Diagnosis Branch of the Influenza Division at the CDC in Atlanta, Georgia, and colleagues note that the new data should remind readers that even though influenza vaccine effectiveness can vary widely from season to season, influenza vaccination remains the most effective method of preventing influenza and its complications.

Vaccination, the authors emphasize, not only reduces the likelihood of becoming ill but also reduces the likelihood of transmitting the virus to others. Annual influenza vaccination is recommended for all persons aged 6 months or older, and there is adequate vaccine supply to meet current need.

The timing of influenza activity and the predominant circulating influenza viruses can vary by season. Between October 1 and November 25, the weekly percentage of outpatient visits to healthcare providers who were participating in the Outpatient Influenza-like Illness Surveillance Network (ILINet) for influenza-like illness have been rising steadily. During the week of November 24, the rate was 2.3%, which is above the national baseline and considerably higher than that seen at the same time in 2016 and 2015, when it was below 2%.

Public health laboratories tested 8777 specimens between October 1 and November 25, 22.4% of which were positive for influenza. Of these, 87% were positive for influenza A and 13% were positive for influenza B.

Analysis of influenza-positive patients revealed that 9.4% were aged 0 to 4 years, 27.3% were aged 5 to 24 years, 33.2% were aged 25 to 64 years, and 30.1% were aged at least 65 years.

The majority of the influenza viruses collected so far correspond to the cell-grown reference viruses representing the 2017 to 2018 Northern Hemisphere influenza vaccine viruses. The most commonly identified virus was influenza A(H3N2) virus. Unfortunately, in recent past seasons in which A(H3N2) viruses predominated, the effectiveness of the vaccine was lower, and hospitalizations and deaths were higher than average.

Influenza antiviral medications can help with the treatment and prevention of influenza, the authors write. Thus far, all the influenza viruses tested this year have been sensitive to oseltamivir, zanamivir, and peramivir.

The CDC and major medical associations recommend early treatment with influenza antiviral medications for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness. Antivirals are also recommended for individuals who require hospitalization and/or those who are at high risk for influenza-rated complications.

The CDC continues to collect, compile, and analyze data on influenza activity in the United States. Data come from the World Health Organization and National Respiratory and Enteric Virus Surveillance System laboratories throughout the United States. Weekly influenza surveillance reports can be found on the CDC's website.

One coauthor reports US patent 6,196,175 for "Preparation and use of recombinant influenza A virus M2 construct vaccine" and US patent 8,163,545 for "An effective vaccine against pandemic strains of influenza viruses." The other authors have disclosed no relevant financial relationships.

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SOURCE: December 11, 2017. Morb Mortal Wkly Rep. 2017;66:1318-1326.

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