While flu vaccines offer protection, they are not failsafe. This year’s flu vaccine is 62 percent effective, scientists reported on Friday in the CDC’s weekly publication, meaning that almost four in 10 people who receive the vaccine and are exposed to the virus will nevertheless become infected.

This is considered “moderate” effectiveness and is in line with previous years’ flu vaccines, which range from 50 percent to 70 percent effective, Dr. Joseph Bresee, chief of the CDC’s influenza division, told reporters.

Experts recommend the vaccine for everyone over 6 months of age. Even if it does not prevent flu, immunization can reduce the severity of the illness, preventing pneumonia and other life-threatening results of flu.

Public health authorities were correct in their forecast of which flu strains would emerge this season and therefore what vaccine to make: one that contains two strains of influenza A and one strain of influenza B. An A strain, called H3N2, predominates this season, though the B strain has caused about 20 percent of cases.

About 10 percent of cases have been caused by a B strain that is not in the vaccine, which “has space for only three strains,” CDC Director Dr. Tom Frieden said.

Dr. Arnold Monto of the University of Michigan, a co-author of the vaccine-effectiveness study told Reuters this year’s vaccine was “a good vaccine, but not a great vaccine.”

It is less effective for the frail elderly, for people receiving chemotherapy for cancer, and for people taking oral steroids, as their immune systems have been weakened and are often unable to produce an effective number of antibodies in response to the vaccine.

One reason flu vaccines are far from perfect, said Monto, is where in the body the viruses find a home – congregating on the surface of small airways in the respiratory tract, while virus-fighting antibodies that are stimulated by vaccines mostly stay in the bloodstream.