A low dose of the Pfizer-BioNTech coronavirus vaccine did not produce a potent immune response in children aged 2 to just under 5 years of age, the companies announced Friday, a discouraging setback that threatens to keep the vaccine from younger children for longer than many parents had hoped.

In ongoing clinical trials, the companies tested 3 micrograms of the vaccine — one-tenth of the adult dose — in children 6 months to less than 5 years of age. After two doses, children between 6 months and 2 years produced an immune response that was comparable to that of people aged 16 to 25 years, the companies said. But children between 2 and 5 years old did not.

The companies said they now would test a third low dose of the coronavirus vaccine in children 6 months to under 5 years of age. But they do not plan to test a higher dose of the vaccine in children between 2 and 5 years of age.

“The goal here is to understand the potential of protection of the third dose,” said Jerica Pitts, a spokeswoman for Pfizer. “We are studying 3 micrograms at this time.”

In a conference call with investors and analysts Friday, Kathrin Jansen, Pfizer’s head of vaccine research, said the company planned to seek authorization for a “three-dose series” in children, instead of the originally planned two doses. If the revised strategy works, “we would have a consistent three-dose vaccine approach for all ages,” she said.

The announcement underscores the emerging idea among many experts that people should be considered “fully vaccinated” only after receiving what are now viewed as booster shots: three doses of the mRNA vaccines made by Pfizer-BioNTech or Moderna, or after receiving at least one additional shot following a single dose of the Johnson & Johnson vaccine.

Early studies indicate that three doses of the mRNA vaccines may provide a stronger bulwark against the omicron variant than two doses.

In the ongoing trial, children younger than 5 years of age will receive 3 micrograms of the vaccine at least two months after their second dose. Pfizer and BioNTech also plan to evaluate a third dose of 10 micrograms — one-third of the adult dose — in children 5 through 12 years of age.

The changes have been cleared by the Food and Drug Administration and the European Medicines Agency, according to a statement released by the companies Friday. The companies say they would seek FDA authorization for a three-dose series in younger children in the first half of 2022.

The FDA last week authorized booster shots of the Pfizer-BioNTech vaccine for adolescents ages 16 to 17. Pfizer did not provide an update on when it planned to ask the agency to authorize booster shots for children ages 5 to 15 years of age.

The FDA has authorized the full adult dose for immunizations of adolescents 12 through 17 years of age. The companies plan to test booster doses of 10 or 30 micrograms to assess the safety and potency of both doses in 600 children in that age group.

The mRNA vaccines have been linked in some rare cases to heart problems in boys and young men, prompting some experts to suggest a lower dose for adolescents.

This article originally appeared in The New York Times.

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