Demand for vaccines is skyrocketing as the United States grapples with a record death toll from COVID-19 and the threat of new, more contagious variants. After a slow start in December, many states and cities have quickly ramped up vaccine delivery, widening access to larger groups of people and setting up mass testing sites.

But now there's a new wrinkle: Some mayors and governors say they have run out of available vaccines and have had to cancel appointments. The Biden administration has promised to overhaul the country's faltering vaccine effort, but there's only so much it can do to increase the available supply.

Here's what you need to know.

Q: How many doses are available?

A: There are simply not enough doses of authorized vaccines to meet the enormous demand. And that is not likely to change for the next few months.

The two companies with authorized vaccines, Moderna and Pfizer, have each promised to provide the United States with 100 million vaccine doses by the end of March, or enough for 100 million people to get the necessary two shots.

Both companies are manufacturing the doses at full capacity and are collectively releasing about 12 million doses each week, a number expected to gradually increase.

Q: President Joe Biden said he would use the Defense Production Act to increase supply. Will that help?

A: Vaccine experts and the companies themselves have said that at least in the short term, using the Defense Production Act will not significantly increase supply, although every little bit could help. That's because manufacturing facilities are already at or near capacity, and there is a worldwide race to develop vaccines that use a finite amount of resources.

In a plan released Thursday, the Biden administration indicated it would continue to use the act to boost supplies needed for vaccine manufacturing.

Q: What about the federal stockpile of vaccines?

A: There is no significant reserve of vaccines to speak of. For the most part, vaccines are being shipped out each week as they are manufactured.

Last week, Alex Azar, the outgoing secretary of health and human services, stirred confusion when he announced that the federal government would be releasing a reserve of vaccine doses. Many states said they were told that this meant an influx of vaccines was on the way, which could be used to inoculate more people.

In his news conference, Azar urged states to open up their immunization policies and said they had been moving too slowly to use the doses they had already been sent. As a result, several governors, including Andrew Cuomo in New York, changed eligibility rules to allow people 65 and older to get the vaccine.

Q: Will there be enough vaccine supply to give everyone a second dose?

A: Federal officials have previously said they were working with states to track who has gotten a vaccine and when they are due for their booster shots, which is three weeks later for the Pfizer vaccine and four weeks later for the Moderna shot.

The incoming Biden administration has vowed to overhaul distribution to the states, providing more transparency to local officials about how much vaccine they can expect, in the hopes of allowing states to better plan.

Q: Are we going to get more vaccines anytime soon?

A: Yes, most likely. At least three other vaccines are in late-stage clinical trials, and the success of any one of them could mean millions more doses by this spring.

Johnson & Johnson is expected to announce the results of its vaccine trial any day now, and if it is successful, the first doses could become available in the United States by February.

By March and April, results from trials testing two-dose vaccines by AstraZeneca and Novavax could also be made public. AstraZeneca has an arrangement with the U.S. government to provide 300 million doses and Novavax to provide 110 million.

What's more, both Pfizer and Moderna say their factories are ramping up and expanding capacity each week. They have signed deals to supply an additional 100 million doses each of their vaccines in the second quarter of this year.