Pfizer (PFE  ) and BioNTech (BNTX  ) announced late last week that they will be delaying their request for the U.S. Food and Drug Administration (FDA) to authorize their COVID-19 vaccine for children under 5-years-old until early April, as the companies wait on more data on the effectiveness of a third dose regimen.

"Given that the study is advancing at a rapid pace, the companies will wait for the three-dose data as Pfizer and BioNTech continue to believe it may provide a higher level of protection in this age group," the companies said in a joint press statement on Friday. "This [delay] is also supported by recent observations of three dose booster data in several other age groups that seems to meaningfully augment neutralizing antibody levels and real world vaccine protection for Omicron compared to the two-dose regimen."

The pair expect to have three-dose protection data available for review in early April. The companies' extension allows the FDA to have more time to receive updated data on the two- and three-dose regimen for that age group.

The FDA said over the weekend that the agency is postponing a review meeting scheduled for Tuesday to examine vaccine data for the youngest Americans. Children under 5-years-old are the last age group in the United States that is not eligible for vaccination.

Dr. Peter Marks, head of the FDA's Center for Biologics Evaluation and Research, which handles vaccine safety, told reporters on a call on Friday that data from Pfizer and BioNTech currently indicates it is best to wait for results on a third dose regimen, CNBC reports. Marks said the decision to delay the vaccine's authorization should reassure parents that the FDA is making sure the vaccine is safe and effective for young children.

"Rather than having any issue of causing anyone to question the process, I hope this reassures people that the process has a standard, that the process is one that we follow, and we follow the science in making sure that anything that we authorize has the safety and efficacy that people have come to expect from our regulatory review of medical products," Marks told reporters, quoted by CNBC.

Pfizer had amended its clinical trial for the youngest American in December to study a three-dose regimen after the first two doses did not produce a strong immune response in children ages 2- to 4-years-old. The companies are testing a smaller, 3-microgram dose, on young children, compared to the 30-microgram dose approved for adults.

Rival Moderna (MRNA  ) said prior to Pfizer-BioNTech's announcement on Friday that it plans to submit data to the FDA in March for its vaccine efficiency in younger children.