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A: Flu strains constantly evolve, the reason that people need an updated vaccine every year. But the CDC says this year’s is a good match to the types that are circulating, including a new kind of the tough H3N2 strain. That family tends to be harsher than other flu types — and health officials warned last fall that it was coming and that this winter would likely be tougher than last year’s flu season, the mildest on record.

Q: But don’t some people get vaccinated and still get sick?

A: Flu vaccine never is 100 percent effective, and unfortunately it tends to protect younger people better than older ones. But the CDC released a study Friday showing that so far this year, the vaccine appears 62 percent effective, meaning it’s working about as well as it has in past flu seasons.

While that may strike some people as low, Dr. Frieden said it’s the best protection available.

“It’s a glass 62 percent full,” he said. “It’s well worth the effort.”

Q: What else can I do?

A: Wash your hands often and avoid touching your eyes, nose and mouth. Viruses can spread by hand, not just through the air. Also, cough in your elbow, not your hand. When you’re sick, protect others by staying home.

And people who are in those high-risk groups should call a doctor if they develop symptoms, added CDC spokesman Tom Skinner. They might be prescribed antiviral medication, which works best if given within the first 48 hours of symptoms.

• Lindsey Tanner and Mike Stobbe contributed to this article.