As monitored by all CDC influenza surveillance systems, influenza activity in the United States for the 2014–15 season is low but increasing. Although the timing of influenza activity varies from year to year, peak activity in the United States most commonly occurs during January–March, but there can be substantial influenza activity as early as November and December.
From September 28 to December 6, 2014, influenza A (H3N2) viruses were identified most frequently in the United States, but pH1N1 and influenza B viruses also were reported. Antigenic or genetic characterization of influenza-positive respiratory specimens submitted to CDC indicate that over half of the recently examined influenza A (H3N2) viruses show evidence of antigenic drift from the A/Texas/50/2012 (H3N2) virus (the H3N2 component on the 2014–15 Northern Hemisphere influenza vaccine).
Even during seasons when the match between the vaccine viruses and circulating viruses is less than optimal and protection against illness might be reduced, vaccination remains the most effective method to prevent influenza and its complications.
Health care providers should recommend vaccination to all unvaccinated persons aged ≥6 months now and throughout the influenza season. In 2014, the Advisory Committee on Immunization Practices recommended the preferential use of live attenuated influenza vaccine (LAIV) for healthy children aged 2 through 8 years (2). However, if LAIV is not available, inactivated influenza vaccine should be used, and vaccination should not be delayed to procure LAIV (2). Children aged 6 months through 8 years who are being vaccinated for the first time require 2 doses of influenza vaccine, administered ≥4 weeks apart (3). For children aged 6 months through 8 years who have received influenza vaccination during a previous season, health care providers should consult Advisory Committee on Immunization Practices guidelines to assess whether 1 or 2 doses are required (2).
Antiviral medications continue to be an important adjunct to vaccination for reducing the health impact of influenza. On January 21, 2011, Advisory Committee on Immunization Practices recommendations on the use of antiviral agents for treatment and chemoprophylaxis of influenza were released (4).
This guidance remains in effect for the 2014–15 season, and recommended antiviral medications include oseltamivir (Tamiflu) and zanamivir (Relenza). All influenza viruses tested for the 2014–15 season since October 1 have been susceptible to oseltamivir and zanamivir. Amantadine and rimantadine are not recommended because of high levels of resistance to these drugs among circulating influenza A viruses (4). In addition, influenza B viruses are not susceptible to amantadine or rimantadine.
Treatment with antivirals is recommended as soon as possible without waiting for confirmatory testing for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness; who require hospitalization; or who are at higher risk for influenza complications§§§ (4).
Clinical benefit is greatest when antiviral treatment is administered early. When indicated, antiviral treatment should be started as soon as possible after illness onset, ideally within 48 hours of symptom onset.
However, antiviral treatment might still have some benefits in patients with severe, complicated, or progressive illness and in hospitalized patients when started after 48 hours of illness onset. Antiviral treatment also may be considered for previously healthy, symptomatic outpatients who are not considered to be at high risk and have confirmed or suspected influenza, if treatment can be initiated within 48 hours of illness onset. Residents of long-term care facilities can experience severe and fatal illness during influenza outbreaks; residents with confirmed or suspected influenza should be treated with antivirals immediately, without waiting for laboratory confirmation of influenza (4).
During periods where two or more residents of long-term care facilities are ill within 72 hours with confirmed or suspected influenza, antivirals should be given prophylactically to residents and should be considered for any unvaccinated staff (4). Additionally, antiviral chemoprophylaxis can be considered for all staff, regardless of vaccination status, if the outbreak is caused by a strain of influenza virus that is not well matched to the vaccine (4).
Influenza surveillance reports for the United States are posted online weekly and are available at http://www.cdc.gov/flu/weekly. Additional information regarding influenza viruses, influenza surveillance, influenza vaccine, influenza antiviral medications, and novel influenza A virus infections in humans is available at http://www.cdc.gov/flu.
