Now, regulators are leaning toward postponing any action until the early summer, arguing that it would be simpler and less confusing to simultaneously authorize and promote two vaccines to the public, rather than green-lighting one on a faster timetable and the other down the road.

The FDA’s plan is tentative and could still change, the people cautioned, especially if the current Covid surge accelerates. In a statement, an agency spokesperson said its decision making is based on “when the data are provided to FDA and fully support a regulatory action.”

But the deliberations represent the latest instance where the administration has struggled to align scientific considerations with political realities.

The pressure on President Joe Biden to get an under-five vaccine online is acute and growing as deeply frustrated parents are either seeking to resume a wider range of everyday activities or living in communities where others are dispensing with protective measures. An extended delay threatens to further dent his political standing with those voters who question how they can get back to normal with unvaccinated children at a time when the White House itself urges much of the nation to move past the crisis.

The administration is also under rising pressure to move on a vaccine for the youngest children from members of its own party, including Colorado Gov. Jared Polis, who on Tuesday called for “more urgency and action” on the issue.

“It is immoral to wait any longer,” he wrote in a letter sent to Biden. “Many parents of young children feel left behind, and are rightfully displeased that the FDA’s lack of action and urgency has left them unable to protect their children and loved ones like everyone else.”

Inside some quarters of the administration, there is similar uneasiness about the appearance of the FDA sitting on a vaccine that it otherwise expects to authorize, particularly if cases continue to increase, two of the people with knowledge of the matter said. The U.S. is now averaging roughly 40,000 new Covid cases per day, up from about 25,000 at the beginning of April, according to the Centers for Disease Control and Prevention.

Yet FDA officials have argued it’s a more complicated calculation. They worry that authorizing a single vaccine and then, soon thereafter, another one might make it harder for the administration to promote the shots and undermine confidence in their effectiveness. Fewer than 30 percent of children ages five to 11 are fully vaccinated so far, CDC data shows, and polling indicates there could be similar hesitation among parents with very young kids.

The situation presents more complexities than the process the agency went through when it authorized different vaccine shots for elder populations on a non-simultaneous schedule.

The drug company Moderna plans to formally request authorization for its vaccine for children under six by the end of the month, meaning regulators could conceivably clear it for use by mid-May. But the two-dose vaccine’s ability to prevent symptomatic Covid cases underwhelmed some outside experts, splitting the public health community over whether the FDA should wait for data on a prospective third shot or just authorize the vaccine in an effort to get young children some level of protection.

Further complicating the situation is that the FDA already rebuffed a competing two-shot vaccine regimen for young people from Pfizer, telling the company to test a three-shot regimen instead. Pfizer is now unlikely to officially seek authorization until June.

That leaves the FDA with the prospect of green-lighting Moderna’s vaccine, only to potentially find out several weeks later that Pfizer’s vaccine performs far better.

Health officials worry that scenario would spark backlash from parents who had rushed out to get the Moderna shot — compounding the existing confusion inherent in explaining why one vaccine is two shots and the other is three, and worsening the misinformation swirling around the broader vaccination campaign.

“At the very least, this is going to be messy,” one senior Biden official said. “Try to explain that to parents.”

In response to a request for comment on its deliberations over the vaccine, the FDA spokesperson said the agency’s “approach has always been to conduct a regulatory review that’s responsive to the urgent public health needs created by the pandemic, while adhering to our rigorous standards for safety and effectiveness.”

The White House has largely left the final decision to the FDA, wary of violating the “follow the science” pledge Biden made at the outset of his presidency. The White House referred questions for this story to the FDA. Still, the delays have vexed officials who see the vaccine as key to convincing Americans that the administration has successfully reined in the pandemic.

Biden and his top advisers have insisted that the nation now has the tools needed to live safely with Covid, pointing to the availability of vaccines and therapeutics. But the roughly 19 million children under five have access to neither, while other precautions like widespread masking and social distancing are quickly evaporating.

“The actions have not supported the language,” Kavita Patel, a primary care physician and former senior Obama-era health official, said of the administration’s vaccine messaging. “It is hard to imagine how any parent can continue to have blind faith in the administration as each day passes without any options.”

The concerns inside and outside the administration ratcheted up this week after a federal judge invalidated the mask mandate for public transportation, raising the odds of Covid exposure for children too young to properly mask themselves on planes, subways and buses. The CDC, on Wednesday, said that it believed the mandate should be kept in place and the Department of Justice subsequently announced that it would be appealing the judge’s ruling.

A day after the judge’s ruling, new data from the CDC found unvaccinated children ages five to 11 were twice as likely to be hospitalized as those who were vaccinated during the recent Omicron wave.

In a sign of the urgency on the ground, a Wednesday meeting of outside advisers to the CDC was dominated by parents eager for a vaccine, with one mother tearfully describing her child to the panel as a “sitting duck” for infection.

“We’ve seen, with the more highly transmissible variants, the more kids get infected and hospitalized,” said Peter Hotez, a virologist at the Baylor College of Medicine. “Now you’ve got all the masks coming off everywhere.”

Lauren Gardner contributed to this report.