Since adults are at high risk of Covid-19 being immunized against the coronavirus, many parents want to know: When will my child be vaccinated?

The short answer: not until late summer.

Pfizer and Moderna have enrolled children ages 12 and older in clinical trials with their vaccines and hope to see results by the summer. Depending on the performance of the vaccines in that age group, companies may then test them on younger children. It usually takes the Food and Drug Administration a few weeks to review data from a clinical trial and approve a vaccine.

Three other companies – Johnson & Johnson, Novavax and AstraZeneca – are also planning to test their vaccines in children, but are further behind.

When researchers first test drugs or vaccines in adults, they usually move down in age bracket, looking for changes in the effective dose and unexpected side effects.

“It would be quite unusual to start early with children,” said Dr. Emily Erbelding, an infectious disease doctor at the National Institutes of Health who oversees the testing of Covid-19 vaccines in specific populations.

Some vaccines – such as those that protect against pneumococcal or meningococcal bacteria, or rotavirus – were first tested in children because they could help prevent pediatric diseases. However, it made sense to test coronavirus vaccines in adults first and approve them for adults because the risk of serious illness and death from Covid-19 increases sharply with age, said Paul Offit, professor at the University of Pennsylvania and a member of the FDA vaccine advisory body.

“We’re trying to save lives, keep people out of intensive care and keep them from dying,” said Dr. Offit. That means prioritizing vaccines for the oldest people and for those with underlying diseases.

People under the age of 21 make up about a quarter of the population in the United States, but they account for less than 1 percent of deaths from Covid-19. Still, about 2 percent of children who get Covid-19 require hospital care, and at least 227 children in the United States have died from the disease.

“It’s a significant disease in children, just not necessarily when compared to adults,” said Dr. Kristin Oliver, pediatrician and vaccine expert at Mount Sinai Hospital in New York.

Children also need to be vaccinated so the United States can move closer to herd immunity – the long-promised target where the pandemic will stall because people run out of virus to infect.

Scientists have estimated that 70 to 90 percent of the population may need to be immunized against the coronavirus in order to achieve herd immunity, especially with contagious variants that are expected to be widespread in the country.

“Not all adults can get the vaccine because there is some reluctance, or there may even be a vulnerable immune system that just doesn’t respond,” said Dr. Erbelding. “I think we need to involve children if we are to achieve herd immunity.”

Immunizing children in racial and ethnic groups most affected by the pandemic will also be important, she added.

Pfizer and Moderna’s adult clinical trials each enrolled approximately 50,000 participants. They had to be large enough to show significant differences between the volunteers who received a vaccine and those who received a placebo. However, since it is less common for children to become seriously ill with Covid-19, such planning of experiments in children would not be feasible as many more participants would be required to show an effect.

Updated

Apr. 12, 2021, 7:01 p.m. ET

Instead, the companies will screen vaccinated children for signs of a strong immune response that would protect them from the coronavirus.

The Pfizer BioNTech vaccine was approved in December for people aged 16 and over. The company has continued its study with younger volunteers, recruiting 2,259 teenagers between the ages of 12 and 15. According to the Centers for Disease Control and Prevention, teenagers are roughly twice as likely to be infected with the coronavirus as younger children.

The results of this study should be in by the summer, said Keanna Ghazvini, a Pfizer spokeswoman.

“Getting under 12 will require a new study and possibly a modified formulation or dosage schedule,” said Ms. Ghazvini. These studies will most likely begin later in the year, but the plans will be final after the company has data from older children, she added.

Moderna’s vaccine, also approved in December, is on a similar path for pediatric testing. In December, the company began testing teenagers ages 12-17 and plans to add 3,000 volunteers to that age group. The company expects results “around mid-2021,” said Colleen Hussey, a spokeswoman for Moderna.

Based on the results, Moderna plans to study the vaccine in children between the ages of 6 months and 11 years of age later this year.

Infants may have some antibodies from vaccinated or infected mothers at birth, but the mother’s protection is unlikely to last until the age of one. And with their relatively weak immune systems, babies may be particularly susceptible to infection when community transmission is high.

The studies will also evaluate the safety of the vaccine in children – and hopefully alleviate any parents’ fears. A third of adults in the United States said they have no plans to immunize their children against the coronavirus, according to a recent survey by Verywell Health.

Given the low risk of Covid-19 in children, some parents may be skeptical of the urgency to vaccinate their children with a brand new burst, said Dr. Offit. “Because of this, the vaccine should be kept on a very high safety standard,” he said.

To date, more than 42 million people in the US have been vaccinated with few permanent side effects. And the FDA has several systems in place to carefully monitor serious reactions to the vaccine.

“You’re really, really looking at the data,” said Dr. Oliver. “As a pediatrician and a mother, I have really good faith that these systems will work.”

Once a vaccine is available for children, schools can reintroduce extracurricular activities that involve close contact, such as band exercises, team sports, and choirs. However, in the meantime, there is ample evidence that schools can reopen with other precautions, said Dr. Oliver.

“I don’t think we have to expect a vaccine to open schools in the fall,” she said. “We should now plan to open schools.”

Dr. Oliver also urged parents to ensure that children are immunized against other diseases. According to the CDC, orders for vaccines for children without the flu under the Childrens Vaccines program fell by a total of around 10.3 million doses.

“Now is the time to really catch up on missed doses of these vaccines,” she said. “Measles, HPV, tetanus boosters, pertussis boosters – all of these are really important.”