Pfizer and BioNTech on Friday announced plans to add a third dose of the COVID-19 vaccine in an ongoing trial with young children, as the current two-dose regimen didn't appear to be strong enough for some of them.

The third shot, for children ages 2 to 5, will be administered at least two months after their second dose.

For kids younger than 5, Pfizer is using a dose of 3 micrograms — or one-tenth of the adult dose. However, an analysis conducted one month after the second dose found that kids between 2 and 4 didn't have a very strong immune response.

The vaccine did generate an adequate immune response in children between 6 months to 2 years of age.

“The study will now include evaluating a third dose of 3 (micrograms) at least two months after the second dose of the two-dose series to provide high levels of protection in this young age group,” the companies said in a joint press release.

This reflects the “companies' commitment to carefully select the right dose to maximize the risk-benefit profile,” the statement added.

The changes have been endorsed by and agreed upon by the U.S. Food and Drug Administration and the European Medicines Agency.

If the three-dose study is successful, Pfizer and BioNTech expect to submit data to regulators to support an emergency use authorization for children 6 months to under 5 years of age in the first half of 2022, the companies said.