The delayed doses amount to a three-day supply, and this week’s winter storms have slowed the arrival of vaccine in all 50 states, according to Andy Slavitt, the White House senior adviser on the government’s response to the coronavirus. President Biden, speaking at a Pfizer plant in Kalamazoo, Mich., that manufacturers the coronavirus vaccine, said the weather is “slowing up the distribution” of vaccine.

News of the magnitude of the slowdown came after thousands of Americans already lost their appointments for second doses because they never arrived or the vaccination site was closed. The ripple effects are expected to stretch into next week as states await delayed shipments and scramble to get their vaccination efforts back on track.

At a White House briefing, Slavitt said vaccine shippers — FedEx, UPS and the drug distributor McKesson — “have all faced challenges as workers have been snowed in and unable to get to work.” Road closures in some areas have held up deliveries. And more than 2,000 vaccination sites are in places where power was knocked out.

Because the two vaccines allowed for emergency use — manufactured by Pfizer and its German partner BioNTech, and by Moderna — require various degrees of cold storage, it has been important not to risk vaccines arriving in places where scarce doses could be wasted because they could not be properly stored due to storms or power outages.

“The vaccines are sitting safe and sound in our factories and hubs,” Slavitt said.

Slavitt said the government is asking states and vaccination sites to extend their hours — to reschedule appointments lost because of the storms and to prepare to handle additional vaccine supplies expected in the next weeks and months.

Oscar Alleyne, chief of programs and services at the National Association of County and City Health Officials, said he was confident localities will be able to quickly catch up. He said they will most likely prioritize the people who missed their second-dose appointment while providing more vaccines to a growing pool of eligible recipients.

“I have no doubt once folks get back into the saddle the ability to kick up vaccines will improve,” Alleyne said in an interview. “We will see this kind of run on the market with people looking to get vaccinated.”

The extent of the interruptions have been uneven, with some states announcing minimal disruption because they had reserves and others struggling to reschedule appointments.

Storm-ravaged Texas, where 14 million people are still experiencing water service disruptions, is not expecting to receive its shipments of vaccine for this week until early next week, a spokeswoman for the state health department said. This would also push back the arrival of vaccines that were originally scheduled for next week.

But the problems spread beyond Texas and other states pummeled by storms because major distribution centers in Louisville and Memphis experienced bad weather.

That means places spared from punishing weather, like California, have paused vaccinations while waiting for supplies from hard-hit states. San Francisco Bay area jurisdictions announced major vaccination delays because of conditions in the Midwest, the San Francisco Chronicle reported.

“It’s definitely wreaking some havoc. Even if you were in sunny Arizona, your shipment has been delayed,” said Claire Hannan, executive director of the Association of Immunization Managers. “It came at a bad time because we had really ramped things up, and we were doing really well in improving efficiencies and getting vaccines to more sites. Hopefully, we can pick up right where we left off.”

Los Angeles Mayor Eric Garcetti (D) postponed more than 12,000 vaccinations at city-run sites scheduled Friday after two shipments of the Moderna vaccine were delayed because planes were grounded. He said Thursday that the city is automatically rescheduling many canceled slots and prioritizing second-dose appointments.

“It’s a nationwide problem,” Garcetti said at a news conference. “We are in a race against time, a race between infections and injections, and anything that slows down our progress is unacceptable.”

Confusion followed when hundreds showed up to Dodger Stadium for vaccine appointments scheduled Friday morning. Some told local television station KTLA they never received a cancellation notice. Southern California has been especially hard-hit by the coronavirus, with a massive winter surge that stretched hospitals to their limits and exhausted oxygen supplies.

In New York City, officials were unable to schedule more than 30,000 appointments this week and delayed the opening of new vaccination sites in Queens and Staten Island. Mayor Bill de Blasio (D) said the weather delays aggravated an already stressed distribution system.

“It’s been too hand-to-mouth in general, and then it’s been made even worse by the storm,” de Blasio said at a Thursday news conference. “There are so many things that we could be doing right now to get tens of thousands more people vaccinated, but unfortunately, Mother Nature now is causing us the most immediate problem with these supply delays, and we, of course, will overcome them and keep moving forward.”

Some states are trying to calculate the toll of weather delays on vaccine distribution because of decentralized systems. For example, Maryland expected 130,000 first and second doses to arrive Tuesday and Wednesday, but officials have not finished checking with providers to see what arrived, a spokesman for Gov. Larry Hogan (R) said.

There have been few reports of vaccines spoiled because of power outages. Houston garnered some attention after officials rushed to administer more than 5,000 doses of the Moderna vaccine in one day, but Moderna later assured city leaders that the thawing vials could be safely refrozen.

Despite the storm-related hiccups, vaccine providers say they are well-positioned to bounce back to normalcy.

The federal government is aiming to “get the backlog of vaccines out next week,” White House press secretary Jen Psaki said Friday.

“We anticipate we cannot only get the backlog out, but we can stay on pace with what we are planning to distribute to states next week,” Psaki said. “So we are expecting we are going to be able to catch up next week.”

A spokeswoman for FedEx said the shipping company is responding to the disruptions at its Memphis hub by rerouting shipments to its second-largest facility in Indianapolis, as well as regional centers in Oakland, Calif., and Newark.

Slavitt, the White House official, sought to reassure people who recently lost appointments for a second shot, which both vaccines require: 21 days after the first shot for Pfizer and 28 days for Moderna. The Centers for Disease Control and Prevention previously said people can receive their second shot up to six weeks after the first and from a different maker in “exceptional circumstances.”

“It is not a problem,” Slavitt said. “That will be accommodated completely.”

Matt Viser and Erin Cox contributed to this report.