Dr. Joe Bresee, chief of the epidemiology and prevention branch at the Center's for Disease Control and Prevention's Influenza Division, said anyone with severe influenza illness, or at high risk of serious influenza-related complications, should get treated with influenza antiviral medications if they get flu symptoms regardless of whether or not they got vaccinated.
"Also, you don't need to wait for a positive laboratory test to start taking antivirals," Bresee said in a statement.
"Reports of influenza-like illness are nearing what have been peak levels during moderately severe seasons and while we can't say for certain how severe this season will be, we can say that a lot of people are getting sick with influenza and we are getting reports of severe illness and hospitalizations."
Anyone who has not already been vaccinated should do so now, Bresee said.
The CDC tracks influenza activity year-round and the proportion of people seeing their healthcare provider for influenza-like illness was elevated for four consecutive weeks, but climbed sharply from 2.8 percent to 5.6 percent during that time. Some cities such as Chicago and Boston have had very high rates of patients being treated in hospitals.
However, last season, which was relatively mild, influenza-like illness peaked at 2.2 percent; during 1998-99 and 200304, which were moderately severe flu seasons, influenza-like-illness peaked at 7.6 percent. During the 2007-08 flu season another moderately severe season, influenza-like-illness peaked at 6 percent, but during the 2009 H1N1 pandemic, influenza-like-illness peaked at 7.7 percent, the CDC said.
So far this season, 91 percent of the influenza viruses analyzed by the CDC match the viruses included in this year's influenza vaccine. The match between the vaccine virus and circulating viruses is one factor that impacts how well the vaccine works. But Bresee cautioned other factors are involved such as the health of the patient.
"While influenza vaccination offers the best protection we have against influenza, it's still possible that some people may become ill despite being vaccinated," Bresee said.
CDC recommends antiviral medications -- sold commercially as Tamiflu and Relenza -- to treat influenza as early as possible after becoming ill. The antivirals are recommended for any patient with confirmed or suspected influenza who are hospitalized, seriously ill, or ill and at high risk of serious influenza-related complications, including young children, people age 65 and older, people with certain underlying medical conditions and pregnant women.
Treatment should begin as soon as influenza is suspected, regardless of vaccination status or rapid test results and should not be delayed for confirmatory testing, the CDC said.