"People cannot become complacent this year," said Howard Koh, assistant secretary of the Department of Health and Human Services, who received his own flu shot Thursday.
A yearly vaccination now is recommended for nearly everybody, but new figures released Thursday show that last year 52 percent of children and just 39 percent of adults were immunized.
Best protected: Three-quarters of tots ages 6 months to 23 months were vaccinated. That's a significant jump from the previous year, when 68 percent of those youngsters were immunized.
But even though seniors are at especially high risk of severe illness or death if they catch the flu, just 66 percent of them were immunized, a number that has been slowly dropping for several years.
Older adults got a little lost in the recent public health push to explain that flu vaccine benefits all ages and it's time to target them again, said Daniel Jernigan, a flu specialist with the Centers for Disease Control and Prevention.
In contrast, child deaths from flu have made headlines in recent years the U.S. counted 34 pediatric deaths last year raising parents' awareness of the risk, he said.
The only ones who shouldn't get vaccinated: babies younger than 6 months and people with severe allergies to the eggs used to make the vaccine.
Flu specialists can't say how bad this winter's flu season might be. Influenza strains constantly evolve, and some cause more illness than others.
But strains from the H3N2 family tend to be harsher than some other flu types, and a new H3N2 strain is included in this year's vaccine because it is circulating in parts of the world.
Because of that strain, "I am pretty confident that this year will be a more traditional flu season" than last year, CDC's Jernigan told The Associated Press. "People won't have had any real exposure to that."
Only one ingredient in this year's flu vaccine was retained from last year's, protection against the H1N1 strain that caused the 2009 swine flu pandemic and has been the main kind of influenza circulating since. Also new in this year's shot is protection against a different Type B strain.
Vaccination rates vary widely among states, too. Nevada vaccinates a third of people who are eligible, while South Dakota reaches 51 percent. In Iowa, 76 percent of seniors get their flu shot, compared to half in Alaska. In Rhode Island, 74 percent of children are vaccinated, compared to 39 percent in Alaska.
Manufacturers are expected to make about 135 million doses of flu vaccine this year.
The vaccine is covered by insurance, and Medicare and some plans don't require a copay; drugstore vaccination programs tend to charge about $30.
People can be vaccinated anytime, but Jernigan cautioned that it takes about two weeks for protection to kick in. Flu typically starts to appear in October or November, and peaks in January or February.
Also this year, public health groups are urging workers in doctors' offices, hospitals and particularly nursing homes, where patients are especially vulnerable, to do a better job getting vaccinated.
About 67 percent of health care workers were vaccinated last year, a number that's slowly rising. Doctors are the best role models, with about 86 percent immunized. But anyone from the receptionist to the person delivering meals can spread influenza to patients, and just half of those workers in nursing homes got vaccinated last year, Koh said.
In addition to patient safety, hospitals and clinics need to have enough healthy staff to care for the sick once the flu strikes, said William Schaffner of Vanderbilt University.
When that happens, "we need to be vertical, not horizontal," he said.
Trends the CDC spotted in 2011:
• Roughly a third of teenagers got a flu vaccine.
• So did 45 percent of high-risk young and middle-aged adults, those who are particularly vulnerable to flu because they also have asthma, diabetes or any of a list of other health conditions.
• About 47 percent of pregnant women were vaccinated. Women have five times the risk of severe illness if they catch the flu when they're pregnant, and they can require hospitalization and suffer preterm labor as a result. Vaccination not only protects them, but research shows it also provides some protection to their newborns as well.
Different formsto choose from:
Traditional flu shot • For all ages and people with high-risk health conditions.
FluMist • The squirt-up-the-nose version is for healthy people ages 2 to 49 who aren't pregnant.
A high-dose shot • It is available for those over 65.
The intradermal shot • A skin-deep prick instead of the usual inch-long needle is available for 18- to 64-year-olds.