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Still, with the shots in short supply in some places, “common sense” preventive measures are being trotted out across the country.

Inova, which operates five hospitals in Northern Virginia, has announced that it is restricting visitation policies because of flu concerns and that visitors might be screened for flulike symptoms. Some Catholic churches across the country have omitted parts of the Mass where parishioners shake hands or sip from the same cup during Communion. News outlets in New York reported Tuesday that the Manhattan Soccer Club, which has about 750 children playing soccer throughout the city, advised parents that children should avoid high-fiving and bump elbows instead to help prevent hand-to-hand contact.

Annual flu upticks aren’t uncommon, but what makes the 2012-13 season so worrisome among health specialists is that reports started earlier than normal and that the strain making people ill is the same kind that made the 2003-04 season so severe.

The strain H3N2 is considered common but can be fatal for young children and senior citizens. The CDC’s most recent report through Jan. 5 showed that 20 people younger than 18 had died this season. About 24,000 people die each year from influenza-type illnesses.

Doctors have warned that even the flu vaccine is not 100 percent effective. That said, this year’s vaccine does offer a 60 percent effective rate, meaning that those who have been vaccinated are about 60 percent less likely to get the flu that requires a doctor’s visit, Dr. Frieden said.

The vaccine takes several weeks to take effect, CDC officials said, so a vaccinated person can catch the flu by coming into contact with the virus, but their symptoms often are not as severe.

On Tuesday, Laurel Regional Hospital in Prince George’s County had a line outside its door for a free flu-shot clinic. The clinic started at 2 p.m., and in an hour 100 people had come in, said Shervon Yancey, the community health manager for the hospital.

“It all kind of happened rapidly,” Ms. Yancey said of the rush.

As she straightened her shirt sleeve after receiving an injection, Maria Bitenas, 65, said she came to get her vaccine because “my family was bugging me this year.”

“More children are sick, my neighbor’s children,” the Laurel resident said. “I called CVS, but nobody had any [vaccine] and Target had it for $45.”

Patrick Boateng, 23, said he heard about the clinic from his mother.

“I went to the doctor and they ran out,” the Beltsville resident said. “I’m traveling in the next couple days, going to different places,” Mr. Boateng said of his reason for getting a shot. “Anyone passing through airports, being around people, it’s good to have that precaution.”

Adjusting her jacket after receiving her first-ever flu shot, Avril Abrahams, 30, said her brother persuaded her to come to the clinic.

“He wanted to come, but I was still a little skeptical,” the Hyattsville woman said. “I decided on the spot. It’s my first time.”

The shot hurt “a little.” As for the vaccine itself, “I’m still skeptical,” she said.