Why the Flu Vaccine Can't Protect Against Every Flu Strain

A mutated strain of influenza is giving public health officials a headache as they warn this year's flu vaccine will be less effective against the virulent strain.

In September, health officials detected the changes in the most prevalent flu strain so far in the U.S., the virulent H3N2, after the vaccine for this year already went into production.

But why can't health officials and vaccine producers just make a new batch of vaccine to address the mutation?

Experts say it comes down to how the vaccines are produced and how fast the influenza virus can mutate.

Officials at the World Health Organization and the U.S. Food and Drug Administration look at flu strains in the southern hemisphere every year for clues to what will hit the U.S. come fall and winter in the northern hemisphere.

Experts pick three to four types of flu strains to produce a vaccine against by February so that production can start as early as March, explained Dr. William Schaffner, an infectious disease expert at Vanderbilt University School of Medicine.

"Making flu vaccine is not something that can be done overnight," said Schaffner. "We get vaccinated in September, October and November, but they have to manufacture and ship [the flu vaccine] in August."

Flu vaccines are made by injecting fertilized chicken eggs with the flu virus. There are two main types of vaccine that can be administered -- one that protects against three strains of the flu virus and one that protects against four strains. However if the virus mutates slightly -- a process called "antigenic drift" -- it means that protection against one or a few of these strains is lessened.

"Given the fact that the flu virus has the capacity to mutate," Schaffner said, "it has plenty of time to change its coat of many colors [after] we decide what's going to be in the vaccine."

If the receptors on certain flu strains mutate slightly, "the protective antibodies we make don't fit on it quite as well," Schaffner said.

Compounding the problem this year is that the prevalent strain, H3N2, has been associated with higher mortality levels during past flu seasons. But health official note that it's still key for eligible people to get vaccinated because the flu shot can still provide important protection against other strains or lessen symptoms during infection.

"We continue to recommend flu vaccine as the single best way to protect yourself against the flu," Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, said during a news conference Thursday. "The vaccine will protect against strains covered in the vaccine, and it may have some effectiveness in the drifted strains."