December 17, 2014
CDC Warns of More Severe Influenza Season; Vaccine
Match Not Ideal 

Atlanta—About half of the H3N2 flu viruses analyzed so far this influenza season are drift variants—viruses with antigenic or genetic changes that make them different from the viruses for which the season’s vaccine offers maximum protection.

That and some other factors increase the risk that the current influenza season will be more severe than usual, according to the CDC.

The CDC said recent data suggests the current vaccine’s ability to protect against those drifting viruses could be reduced, although people who have been immunized likely will have a milder illness if they do become infected.

Public health officials emphasized that even if the vaccine isn’t spot-on, however, it still is better than going without one, urging immediate vaccination for anyone who hasn’t been immunized yet. During the 2007-2008 flu season, for example, the predominant H3N2 virus was a drift variant yet the vaccine had an overall efficacy of 37% and 42% against H3N2 viruses.

Depending on the formulation, flu vaccines protect against three or four different flu viruses. Even during a season when the vaccine is only partially protective against one flu virus, it can protect against the others.

In addition, according to Joseph Bresee, MD, chief of the CDC’s Influenza Epidemiology and Prevention Branch,

“Vaccination has been found to provide some protection against drifted viruses in past seasons. Also, vaccination will offer protection against other flu viruses that may become more common later in the season.”

A new study bolsters that argument, finding that seasonal flu vaccines protect individuals not only against the strains of flu they contain but also against many additional types. The study, published this month in mBio, the online open-access journal of the American Society for Microbiology, found that some study participants who reported receiving flu vaccines had a strong immune response not only against the seasonal H3N2 flu strain from 2010, when blood samples were collected for analysis, but also against flu subtypes never included in any vaccine formulation.

So far this year, the most common seasonal influenza viruses detected have been H3N2 viruses, which generally cause more severe flu illnesses, hospitalizations, and deaths than some other strains.

H3N2 viruses were predominant during the 2012-2013, 2007-2008, and 2003-2004 seasons, the three seasons with the highest mortality levels in the past decade and characterized as “moderately severe,” according to the CDC.

“It’s too early to say for sure that this will be a severe flu season, but Americans should be prepared,” said CDC Director Tom Frieden, MD, MPH. “We can save lives with a three-pronged effort to fight the flu: vaccination, prompt treatment for people at high risk of complications, and preventive health measures, such as staying home when you’re sick, to reduce flu spread.”

The drifted H3N2 viruses were first detected in late March 2014, after World Health Organization (WHO) recommendations for the 2014-2015 Northern Hemisphere vaccine had been made in mid-February.

Frieden noted that influenza activity currently is low in the United States as a whole but is increasing in some areas. He strongly urged the use of antiviral drugs for patients at high risk of complications who develop the flu.

Treatment of high risk patients with influenza antiviral drugs—Tamiflu (oseltamivir) and Relenza (zanamivir)—should begin as soon after symptoms develop as possible, without waiting for lab tests to confirm flu infection.

U.S. Pharmacist Social Connect