In a joint letter to President Joe Biden, Gov. Jared Polis and his Massachusetts counterpart, Charlie Baker, urged regulators to quickly authorize the COVID-19 vaccine for children under the age of 5.

“Parents have been told, over and over again, that COVID-19 vaccines are on the horizon for their children,” the governors wrote in a letter released by Polis’ office Friday. “They deserve the peace of mind that comes with being able to choose to protect their children through vaccination.”

Frustration with the Food and Drug Administration has built in recent months over perceived delays in issuing approval for young Americans. Vaccines for children between the ages of 5 and 11 have been approved for more than six months, but younger children – three-quarters of whom are projected to have already been infected with the virus – remain unprotected.

Moderna submitted data to the Food and Drug Administration that it hopes will prove two low-dose shots can protect babies, toddlers and preschoolers – albeit not as effectively during the omicron surge as earlier in the pandemic.

Moderna’s vaccine isn’t the only one in the race. Pfizer is soon expected to announce if three of its even smaller-dose shots work for the littlest kids, months after the disappointing discovery that two doses weren’t quite strong enough.

While questions are swirling about what’s taking so long, Marks pointedly told lawmakers earlier this week that the FDA can’t evaluate a product until a manufacturer completes its application. In a statement Thursday, the FDA said it will schedule a meeting to publicly debate Moderna’s evidence with its independent scientific advisers but that the company still must submit some additional data. Moderna expects to do so within weeks.

Diane Herrero, a deputy director of the Colorado Department of Public Health and Environment, told a panel of medical experts here this week that the state could begin seeing distribution of Moderna’s vaccine for children by mid-June. She said Colorado has more than 290,000 children under the age of 5.

In their letter, Polis and Baker urged the FDA to approve Moderna’s application quickly, without waiting for Pfizer’s. They said it would be “ill advised” to wait to consider both together and approve them simultaneously. They noted that Pfizer was given approval to vaccinate 5-to-11 year olds just three weeks after submitting its application.

The slow vaccine authorization for America’s youngest children, Polis and Baker said, is delaying states’ ability to prepare for the next surge. 

“Many parents of young children feel left behind,” they wrote, “and are angry and dismayed.”

The Associated Press contributed to this report.