The U.S. Food and Drug Administration authorized the Pfizer COVID-19 vaccine in youngsters 12 to 15 years old on Monday, expanding access to all teenagers in time for swim camp, soccer practice and school.

With the U.S. Centers for Disease Control and Prevention also expected to sign off as early as Wednesday, the next step is up to parents: Is it worth the time and trouble to immunize your healthy adolescent?

“Yes — don’t hesitate for a second,” said Dr. George Rutherford, pediatrician and epidemiologist at UC San Francisco. “They should be vaccinated. Absolutely.”

Clinical trial data shows that the two-dose shot was safe, triggered a strong immune response and was 100% effective in fending off the disease among this age group. The vaccine is already available for those ages 16 and up.

Vaccinating young teens will move us closer to the 80% threshold needed to obtain “herd immunity” and slow viral spread, a wide-range of public health experts say. And it offers peace of mind to families as teens return to typical activities.

The Santa Clara County Department of Public Health says it will administer the vaccine to young teens as soon as the CDC approves an anticipated recommendation by a vaccine advisory panel.

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Other vaccination clinics are likely to take the same step. The Pfizer vaccine is already widely available in storage, so there is no need to wait for shipments of additional supplies.

Its authorization comes as youth make up a growing share of California’s pandemic, according to data compiled by infectious disease epidemiologist George Lemp, former director of the University of California’s HIV/AIDS Research Program.

A year ago, children and adolescents younger than 18 represented 5% of the state’s reported COVID-19 cases. Now they account for about 18.7% of the cases.

“We need to move as quickly as we can to vaccinate Californians, particularly younger populations, to drive these numbers down,” Lemp said.

The pandemic’s changing demographics — infections shifting from adults to youth — are not entirely unexpected. Because older Americans were offered the vaccine first, young people now represent a growing proportion of cases — although the number of COVID-19 infections is declining across all age groups.

Among adults, the nation’s vaccination campaign has passed a halfway point: According to the CDC, 58% have now received at least one shot of a COVID-19 vaccine. About 44% are fully vaccinated.

But other dynamics are also in play, such as the loosening of restrictions on youth activities — especially sports.

“Now we’re letting the kids get COVID, whereas they were protected before,” according to Dr. Daniel Griffin of Columbia University.

Last winter, an outbreak of 77 cases was traced to an unauthorized youth basketball tournament in the Placer County town of Rocklin. Officials said the outbreak — which included 39 middle- and high-school players, three coaches and 35 “additional contacts” — stemmed from a “Fall Fest” tournament in the town outside Sacramento.

Between January and March, Michigan saw 291 outbreaks linked to youth sports teams that involved at least 1,091 people, according to state health officials.

Transmission may not occur during the practices and games, when youth are tested, masked and supervised. Instead, the virus may spread through informal team carpools, celebrations and sleepovers, according to the CDC.

Meanwhile, our understanding of COVID-19 in youth has changed, say experts

Initially, it was presumed that children rarely got infected with the virus. And if they did get sick, the thinking was, they escaped any long-term problems.

“Now we know that there is transmission in this group. There is disease in this group. And there are long-term consequences of disease in this group,” said Rutherford.

At least 14,849 children have been hospitalized with COVID-19. Of these, at least 3,000 suffered from a rare but very serious condition associated with COVID-19, called multisystem inflammatory syndrome in children.

At least 297 children have died of COVID-19. At particular risk are teenagers with underlying health conditions such as weakened immune systems, obesity or chronic lung conditions.

In addition to protecting children, immunization may protect the adults around them, Rutherford said.

The 12-to-15-year-old age group is already eligible for the vaccine in Canada. In Europe and Israel, regulators are now conducting assessments of its use.

There is no category of teenagers who shouldn’t get the vaccine, unless they have a known allergy to one of the vaccine’s components, according to Dr. Angela Dangvu, a pediatrician in the Children’s Health Orange County Primary Care Network.

Because the Pfizer product isn’t a live vaccine — one that uses a weakened form of a germ to prompt an immune response — the possibility for reaction is very small. There have been reports of allergic reactions to the vaccine, but these occurrences are rare.

Like adults, teens may feel feverish, achy or tired, she said. Because teens have more robust immune systems, it’s possible that they may feel these side effects more strongly.

Parents should accompany their child to the vaccination site. Anyone under 18 must have a parent or legal guardian present to receive the vaccine.

Bring a photo ID and a document verifying your teen’s date of birth. There’s no need to bring pediatric vaccine records, as the teen will be given a card specific to the COVID-19 vaccine.

“This is a disease to be avoided,” said Rutherford. “You are going to protect your child and the rest of the family.”