Press Release Embargoed Until: Thursday, January 15, 2014, 1:00 p.m. ET

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A report published in the January 16 Morbidity and Mortality Weekly Report (MMWR) estimates that getting a flu vaccine this season reduced a person’s risk of having to go to the doctor because of flu by 23 percent among people of all ages.

Since CDC began conducting annual flu vaccine effectiveness (VE) studies in 2004-2005, overall estimates for each season have ranged from 10 percent to 60 percent effectiveness in preventing medical visits associated with seasonal influenza illness. The MMWR report says this season’s vaccine offers reduced protection and this underscores the need for additional prevention and treatment efforts this season, including the appropriate use of influenza antiviral medications for treatment.

“Physicians should be aware that all hospitalized patients and all outpatients at high risk for serious complications should be treated as soon as possible with one of three available influenza antiviral medications if influenza is suspected, regardless of a patient’s vaccination status and without waiting for confirmatory testing,” says Joe Bresee, branch chief in CDC’s Influenza Division. “Health care providers should advise patients at high risk to call promptly if they get symptoms of influenza.”

One factor that determines how well a flu vaccine works is the similarity between the flu viruses used in vaccine production and the flu viruses actually circulating. During seasons when vaccine viruses and circulating influenza viruses are well matched, VE between 50 and 60 percent has been observed. H3N2 viruses have been predominant so far this season, but about 70 percent of them have been different or have “drifted” from the H3N2 vaccine virus. This likely accounts for the reduced VE.

Flu viruses change constantly and the drifted H3N2 viruses did not appear until after the vaccine composition for the Northern Hemisphere had been chosen.

Another factor that influences how well the flu vaccine works is the age and health of the person being vaccinated. In general, the flu vaccine works best in young, healthy people and is less effective in people 65 and older. This pattern is reflected in the current season early estimates for VE against H3N2 viruses. VE against H3N2 viruses was highest — 26 percent — for children age 6 months through 17 years. While not statistically significant, VE estimates against H3N2 viruses for other age groups were 12 percent for ages 18 to 49 years and 14 percent for people age 50 years and older.

CDC recommends that people get a flu vaccine even during season’s when drifted viruses are circulating because vaccination can still prevent some infections and can reduce severe disease that can lead to hospitalization and death. Also, the flu vaccine is designed to protect against three or four influenza viruses and some of these other viruses may circulate later in the season. Flu activity so far this season has been similar to the 2012-2013 flu season, a “moderately severe” flu season with H3N2 viruses predominating.

Antiviral Supply Update

While manufacturers of antiviral medications have stated that there is no national shortage of antiviral medications at this time, and that there is sufficient product available to meet high demand, there are anecdotal reports of spot shortages of these drugs. CDC’s advice for patients and doctors is that it may be necessary to contact more than one pharmacy to fill a prescription for an antiviral medication. Pharmacies that are having difficulty getting orders filled should contact their distributor or the manufacturer directly.

For large institutional outbreaks this season, CDC is taking new measures to help match demand with supply, working with commercial partners to facilitate filling of large orders of antivirals for long-term care facilities or institutions having difficulty accessing antiviral supplies in outbreak settings. More information is available at http://www.cdc.gov/flu/antivirals/supply.htm.

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