Parents will have to wait for a coronavirus vaccine for their young children after Pfizer and BioNTech announced Friday that they are modifying their clinical trial to include a third shot at least two months after the initial two-dose regimen for children under age 5.

The companies reported that two doses of the pediatric vaccine failed in 2-, 3- and 4-year-olds to trigger an immune response comparable to what was generated in teens and older adults. The vaccine did generate an adequate immune response in children 6 months to 2 years old.

If three doses are successful at triggering a protective immune response, the companies expect to submit the data to regulators in the first half of next year.

“It is important to note that this adjustment is not anticipated to meaningfully change our expectations that we would file for emergency use authorization and conditional approvals in the second quarter of 2022,” said Kathrin Jansen, head of vaccine research and development for Pfizer in a call with investors.

Coronavirus vaccines have been authorized for everyone age 5 and older.

The companies have been testing two shots of a 3-microgram dose in children under 5, a small fraction of the adult dose. Because children’s immune systems are different from adults‘, smaller doses are expected to trigger equivalent immune protection. The trials were designed to test the safety and efficacy of the lower dose.

Many parents and pediatricians hoped that if the results had been positive, Pfizer would be in a position to file for regulatory authorization early next year.

Kawsar Talaat, one of the principal investigators of the Pfizer pediatric trial and a physician at Johns Hopkins Bloomberg School of Public Health, said that while the news may be disappointing, a third dose is expected to work well to provide protection, particularly against omicron.

“I think that a third dose will give a nice boost, and honestly, this is really exciting — as we know from the adult data, three doses is probably better for omicron. And I think it’ll be good to have similar data for children,” Talaat said.

The companies are also testing a third dose in 5- to 11-year-olds and a third low-dose shot for adolescents between 12 and 17.

The decision was informed by comparing the immune response in the youngest children to older teens and young adults, and by emerging data that three doses are most protective against variants such as omicron.