Which vaccines are available in the U.S. now, and which may be coming?

About 20 percent of the U.S. population has received at least one Covid-19 vaccine shot as the pace of inoculations in the United States sharply climbs. Here is a look at the vaccines that have been authorized by the Food and Drug Administration and where some other vaccine candidates stand.

How many vaccines are authorized in the U.S. now?
Three: from Pfizer-BioNTech, Moderna and Johnson & Johnson. Pfizer’s was the first, in December, with Moderna’s following shortly after; each is given in two shots spaced three to four weeks apart. Johnson & Johnson’s vaccine, authorized last month, is given in one dose.

Is the single-shot Johnson & Johnson vaccine widely available?
Not yet. When it was authorized on Feb. 27, Biden administration officials cautioned that supplies would be limited for the first month, with 3.9 million shots initially and 16 million more by the end of March.

Johnson & Johnson pledged last year to deliver 37 million doses by the end of March and a total of 100 million by the end of June, but it is still working on getting production up to that scale. A recent deal with Merck is meant to increase manufacturing and packaging capacity.

President Biden said last week that the federal government would order another 100 million doses of Johnson & Johnson’s shot.

Which vaccine could the F.D.A. authorize next?
Novavax could apply for emergency use authorization for its two-shot vaccine in late April. It offers robust protection, though it was not as effective against a variant circulating rapidly in South Africa as it was against other versions. Novavax could deliver 110 million doses by the end of June if the F.D.A. clears the vaccine for use.

AstraZeneca, whose vaccine is authorized in more than 70 countries, has not yet reported results from its U.S. clinical trial, nor has it applied for authorization in the U.S.

The AstraZeneca vaccine, developed with Oxford University, has run into some problems. European countries have suspended use of it over concerns about blood clots, although no evidence has been found of any causal link. Some people in Germany are also declining to receive it because of its lower overall efficacy in clinical trials, compared with other vaccines.

When will all Americans be able to get vaccinated?
Mr. Biden said he would direct all states to expand eligibility to include every adult — roughly 260 million people — by May 1. No vaccine is authorized yet for children.