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CDC Reports This Season’s Flu Vaccine Reducing Risk by Nearly Half

Agency urges ongoing flu vaccination and appropriate antiviral treatment

February 16, 2017 – Based on early data, CDC reports that flu vaccines this season are reducing a vaccinated person’s risk of getting sick and having to go to the doctor because of flu by about half (48%). The agency also reports that there are still weeks to go during the current flu season. CDC recommends that flu vaccination efforts continue as long as influenza viruses are circulating.

Flu activity – dominated by influenza A H3N2 viruses -- has been elevated for 8 weeks this season and continues to increase in some areas. While the Northwest of the country experienced flu activity earlier and now seems to be on the downswing, the Midwest and eastern coast of the country continue to experience increases in activity. Activity this season has been dominated by influenza A H3N2 viruses. Over the past 15 years, flu seasons have averaged 13 weeks in length; with a range of 1 week to 20 weeks.

According to data from the U.S. Flu Vaccine Effectiveness Network, interim estimates show flu vaccine has been 48% effective in preventing medically-attended influenza A and B illness: Interim effectiveness estimates against the predominant influenza A (H3N2) viruses are 43% while the interim effectiveness estimate against influenza B viruses is 73%.

These interim estimates are consistent with vaccine effectiveness (VE) estimates from previous seasons during which vaccine viruses have been characterized as being “like” (well-matched to) circulating viruses based on standard characterization methods. During seasons with a less than optimal match, reduced VE has been observed.

Since 2010, CDC has published annual estimates of the number of illnesses, medical visits and hospitalizations prevented by vaccination each year. While those estimates are not yet available for the current season, CDC estimated that during 2012-2013; an H3N2-predominant season with overall VE of 49%, flu vaccine prevented 5.6 million flu illnesses, 2.7 million flu-related medical visits and about 61,500 flu hospitalizations.

With weeks to go during the current flu season, vaccination at this time could still offer protective benefit. In the past couple of weeks, a small increase in the proportion of influenza B viruses has been detected. During many seasons, there are later waves of influenza B virus activity. In the past, significant influenza activity has been observed as late as May. Vaccination efforts should continue as long as influenza viruses are circulating.

The early VE estimates also underscore that some vaccinated people may get influenza. Influenza antiviral drugs can be used to treat influenza. CDC recommends early antiviral treatment for persons with suspected influenza with severe or progressive illness (e.g., hospitalized persons) and persons at high risk for complications from influenza such as children younger than 5 years, people 65 years and older and people with certain underlying health conditions, even if illness is less severe. Early antiviral treatment can reduce severity and complications of influenza-associated illness.

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