"We're concerned that vaccination rates could be lower this year because the mist isn't available," said CDC Director Tom Frieden. "A lot of kids prefer the mist to the shot."

Vaccination coverage estimates from the 2015-2016 influenza season, presented Thursday by Frieden, show steady vaccination rates among children, but a concerning drop in influenza vaccination among adults 50 and older. He noted that hospitalization rates are highest for people older than 64 and second highest for baby boomers, those between 54 to 64.

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If vaccination rates could be boosted by just 5 percent across the board, Frieden said, "that would prevent 800,000 illnesses and nearly 10,000 hospitalizations."

Every year, Frieden and infectious disease experts travel to Washington to urge people to get vaccinated. To emphasize the point, Frieden rolled up his sleeves and got a shot in his right arm during a press conference Thursday at the National Press Club in Washington.

Flu is most dangerous for people older than 65, young children, pregnant women and people with certain health conditions, such as diabetes, lung disease and heart disease.

Still, many Americans don't take influenza seriously enough, Frieden said.

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"It's the Rodney Dangerfield of diseases," he said in an interview. "It gets no respect."

Every year, influenza sickens hundreds of thousands of people, sending them to the hospital. It kills, on average, about 24,000 people in the U.S. each year, including about 100 children, according to the CDC.

Influenza activity often begins to increase in October and November. Most of the time flu activity peaks between December and March and can last as late as May. For the best protection, scientists say it's optimal to get your flu shot before there's a lot of flu in your community because it takes about two weeks for full protection to kick in.

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Some recent studies have raised the possibility that getting vaccinated too early in the season, especially for the elderly, might result in less protection later in the season. Data from Canada also suggest that getting flu shots every year can gradually reduce the effectiveness of the vaccines under some circumstances.

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But the data are far from clear, experts said.

Frieden said timing of flu shots is difficult because it's hard to predict whether the season will be mild or severe, when it will peak, and whether the vaccine is a good match. Last year was a moderate flu season with a late peak, and the match wasn't ideal, he said. Based on what is known so far, people should not put off getting their shots.

"Any time you defer, the tendency is to forget about it," he said.

Wilbur Chen, an infectious disease expert at the University of Maryland's School of Medicine, said the science questioning the effectiveness of getting repeated seasonal vaccines is far from resolved.

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"This has not been replicated by the CDC in the U.S.," Chen said of the Canadian data during Thursday's press conference. "It's an interesting science question and people are exploring it, but it's not yet solidly explained...It is a little bit controversial. It is something that people are trying to address."

The vaccine is far from perfect, but health officials say it lowers a person's risk by an average of 50 to 60 percent. After the CDC recommended against using the nasal spray for this season, the agency worked with manufacturers and providers to increase production of the shots, and up to 168 million doses are expected to be available this season. More than 93 million doses have already been delivered.

"There's nothing else you can do to cut your risk in half," Frieden said.

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Experts also stress the importance for adults 65 and older to get pneumococcal vaccine at the same time they get their flu shots.

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Influenza vaccination coverage across the entire U.S. population during the 2015 to 2016 season was about 45.6 percent, down by 1.5 percentage points from the previous season, according to data released Thursday. That means about 144 million people received a vaccine last year, but it's not enough, experts said.

The largest coverage decreases were seen among older people, with a drop of 3.4 percentage points to 43.6 percent among people 50 to 64 years old, and a drop of 3.3 percentage points to 63.4 percent among people 65 years and older.

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Two new flu vaccine options are available this season. One is a four-component flu shot made with virus grown in cell culture, a different process than the more traditional egg-based method. That vaccine is for use among people 4 years and older.

Another new option this season is for adults over 65 and includes an ingredient that helps create a stronger immune response.

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Influenza viruses are constantly evolving, sometimes from one season to the next and sometimes over the course of a single influenza season. Unlike other vaccines, the makeup of flu vaccines has to be updated periodically so they are effective against what researchers anticipate to be the predominant viruses that will circulate in the upcoming season.