By Peter Loftus, Melanie Grayce West and Jared S. Hopkins

The first U.S. Covid-19 vaccinations outside of clinical trials began Monday, kicking off the most urgent mass immunization campaign since polio shots were rolled out in the 1950s.

A nurse in New York was among the first to receive the shot, and health workers throughout the U.S. were also set to receive the newly authorized vaccine developed by Pfizer Inc. and BioNTech SE. Pfizer shipped vaccine vials out Sunday, and hospitals and health departments across the country received them early Monday.

A total of 55 sites nationwide had received vaccine shipments by around noon on Monday, said Gen. Gustave Perna, chief operation officer for Operation Warp Speed, the U.S. government's coronavirus-response program. He said at a news conference that plans remain on track for a total of 636 locations to receive vaccines by Wednesday and an additional 581 between Thursday and Sunday, completing distribution of an initial 2.9 million doses. The vaccines are given in two doses several weeks apart.

The government has a reserve of 500,000 doses in case any problems arise, he added.

The largest past immunization campaigns in the U.S. were launched during smaller outbreaks or as preventive measures over many years to try to eradicate a persistent pathogen. The Covid-19 effort "will be logistically and socially and medically the largest unfolding of a vaccination program ever conducted," said Dr. Howard Markel, professor of medical history at the University of Michigan. "We've never had such a massive campaign in the middle of a pandemic."

Public-health officials have been counting on a vaccine's arrival to help bring an end to the deadly pandemic, as its death toll surpassed 300,000 in the U.S. on Monday, according to Johns Hopkins University. The average number of daily deaths over a seven-day period has grown from 824 on Nov. 1 to over 2,400 as of Sunday, prompting new restrictions by many states in an attempt to slow the spread.

The first Covid-19 vaccination in New York City was administered shortly after 9 a.m. Monday morning to Sandra Lindsay, a critical-care nurse at Long Island Jewish Medical Center in Queens.

"I am very proud to be a health-care worker, and I'm also very proud to be in this position to promote public confidence in the safety of the vaccine," said Ms. Lindsay.

Health and Human Services Secretary Alex Azar and Surgeon General Jerome Adams visited George Washington University Hospital in Washington, D.C., Monday afternoon to watch as the first five health workers there received vaccination shots.

Dr. Adams said the vaccination push will require overcoming deeply rooted mistrust of the medical system by many Black Americans, who as a group have been disproportionately affected by Covid-19. "That lack of trust is not without good reason," he said, mentioning the infamous Tuskegee study.

California Gov. Gavin Newsom and Los Angeles Mayor Eric Garcetti watched Monday as workers at Kaiser Permanente Los Angeles Medical Center received some of the first Covid-19 vaccine doses administered in America's most populous state.

"Today is hopeful and there's reason to be optimistic, but let's be mindful of where we really are in terms of this pandemic," said Mr. Newsom.

The Democrat noted that San Joaquin County in the Central Valley had run out of ICU capacity over the weekend and that he expected other counties to follow suit in the coming weeks. He said the state received about the same number of vaccine doses Monday as the number of new Covid-19 cases it reported.

Hospitals in many other states, from Alaska to Michigan and Texas to Florida, also received their first vaccine doses Monday. Some were set to begin injecting patients immediately, while others needed some time for logistics such as dispersing the vials and training staff.

In Washington, D.C., a bipartisan group of lawmakers has put together a $908 billion economic aid proposal that includes $16 billion for vaccine distribution, as well as testing and tracing. Supporters have said the aid is vital to the vaccination campaign, but the current talks must overcome differences on state and local aid and a liability shield.

"Even with vaccines on the way, many are reporting they don't have the funds to hire enough trained workers or purchase enough PPE to safely administer them as fast as possible," Senate Majority Leader Mitch McConnell (R., Ky.), who backs vaccines funding but opposes broader assistance for states, said on the Senate floor Monday. "This is the support that state and local governments need most urgently."

Medical staff have been advised to monitor patients who receive the vaccine for at least 15 minutes to make sure they don't have adverse reactions. Two people who received the Pfizer vaccine in the U.K. last week had severe allergic reactions. The additional time required for monitoring and some logistical hiccups have slowed the vaccine rollout in that country.

Only a small percentage of the U.S. population will get access to the shots initially, as early supplies are limited. Pfizer is shipping a total of nearly three million doses in this first wave. They are going to states based on their populations, with most receiving enough doses to inoculate a little less than 1% of their populations.

The company has said 25 million doses will be available in the U.S. by the end of the month.

Mr. Azar said at the Monday press briefing that the federal government was withholding some doses of the Pfizer vaccine to ensure there was enough supply to make sure people get their second shot. He also said supply will depend on Pfizer.

"We need to make sure that if we vaccinate someone a first time that we expect there to be a vaccine for a booster on the 21st day or the 28th day either from a held back supply at the federal level or ongoing new production," Mr. Azar said.

Pfizer's second shot is given three weeks later; another Covid-19 vaccine, from Moderna Inc., is also given in two doses, but four weeks apart.

If Moderna's Covid-19 vaccine is authorized later this week it could add to the supply of doses this month. Plans call for it to be shipped to 3,285 sites, more than five times as many as those receiving the Pfizer vaccine, Gen. Perna said, because the government has had more time to plan for its rollout.

The federal government is using contractors to ship Moderna's vaccine, while Pfizer is being distributed by the drugmaker itself.

As the initial supplies are limited, the first doses are being reserved for doctors, nurses and other front-line health workers who might be exposed to the coronavirus while caring for infected patients.

Residents of nursing homes and other long-term care facilities also are expected to get early doses, because they are more vulnerable to severe Covid-19 disease and death.

Eventually, as supplies increase, more priority groups, such as essential workers and the elderly, will be vaccinated.

Federal officials expect about 100 million Americans to be immunized against Covid-19 by February or March. The general public could be inoculated in the spring or summer.

The U.S. Food and Drug Administration authorized the Pfizer-BioNTech vaccine on Friday last week, citing its 95% effectiveness at preventing symptomatic Covid-19 in a large clinical trial. On Saturday, an advisory committee to the U.S. Centers for Disease Control and Prevention voted to recommend that the vaccine be used for people 16 years and older.

Long-term care facilities for the elderly are scheduled to begin receiving vaccinations next week through a federal partnership with drugstore chains CVS Health Corp. and Walgreens Boots Alliance Inc. But a few staffers and residents around the country are getting their shots this week including Andy Cartmill, director of environmental services at the Miller Pointe nursing home in Mandan, N.D., who got his on Monday.

Mr. Cartmill, 33 years old, was hospitalized in September with Covid-19 and said he wanted to avoid reinfection and protect residents of the facility, as well as set an example for fellow staffers, some of whom say they don't want the shots.

"It's the only thing that will get us back to normal," he said.

--Scott Calvert, Jared Hopkins, Christine Mai-Duc, Anna Wilde Matthews, Joe Barrett, Melanie Evans and Kris Maher contributed to this article.

Write to Peter Loftus at peter.loftus@wsj.com, Melanie Grayce West at melanie.west@wsj.com and Jared S. Hopkins at jared.hopkins@wsj.com

(END) Dow Jones Newswires

12-14-20 1840ET