Pfizer Inc. booster doses will be available to millions of Americans after Centers for Disease Control and Prevention Director Rochelle Walensky backed the shots for seniors, many adults with underlying health conditions and workers at high risk of Covid-19 exposure.

President Biden said Friday the majority of Americans who are fully vaccinated with Pfizer and partner BioNTech SE’s vaccine will be eligible for boosters, a total of 60 million people and 20 million Americans currently. He said the administration is looking...

Pfizer Inc. booster doses will be available to millions of Americans after Centers for Disease Control and Prevention Director Rochelle Walensky backed the shots for seniors, many adults with underlying health conditions and workers at high risk of Covid-19 exposure.

President Biden said Friday the majority of Americans who are fully vaccinated with Pfizer and partner BioNTech SE’s vaccine will be eligible for boosters, a total of 60 million people and 20 million Americans currently. He said the administration is looking to expand the rules to include more people.

“If you’re fully vaccinated, you’re highly protected from severe illness...we’re doing everything we can to keep it that way, which is where the booster comes in,” he said.

The eligible—people who were fully vaccinated with the Pfizer-BioNTech shot—would get an extra dose at least six months after their second dose.

Dr. Walensky overruled a CDC advisory panel to make frontline workers like nurses, teachers and grocery-store employees eligible for shots, an unusual move capping several weeks of sometimes confusing and contradicting messaging on the U.S. booster policy.

The CDC director normally follows the advisory panel’s lead.

Eric Topol, director of the Scripps Research Translational Institute in La Jolla, Calif., expressed concern the wavering could undermine public confidence in vaccines.

“None of this looks good. It looks like there’s confusion at the top, and it just flows through to the public,” he said.

Dr. Walensky also backed the panel’s decision to recommend boosters to people 65 years and older as well as adults 50 to 64 years with underlying medical conditions. She said the shots should be made available to 18- to 49-year-olds with underlying medical conditions.

Vaccination sites don’t have to follow the CDC’s guidance, but many pharmacies and doctors do in practice.

Many people will qualify for the booster because the CDC’s list of underlying medical conditions is wide-ranging. They include cancer, chronic lung and kidney disease and heart disease, as well as diabetes, obesity, pregnancy and smoking.

“As CDC Director, it is my job to recognize where our actions can have the greatest impact,” Dr. Walensky said in a statement late Thursday. “At CDC, we are tasked with analyzing complex, often imperfect data to make concrete recommendations that optimize health. In a pandemic, even with uncertainty, we must take actions that we anticipate will do the greatest good.”

She said the U.S. vaccination campaign remains focused on inoculating as many people as possible with primary doses and that the CDC would review information on the Moderna Inc. and Johnson & Johnson vaccine boosters as soon as possible.

The decision to go against the recommendation of the Advisory Committee on Immunization Practices marks the latest shift in federal planning for giving boosters.

Food and Drug Administration staff and advisers refused to endorse the Biden administration’s original plan for making boosters widely available beginning this week, saying data so far didn’t support broad use.

The FDA cleared boosters of the Pfizer vaccine for people 65 and older and certain other adults at high risk of severe illness, including because of their jobs or where they live.

Some health experts inside and outside the federal government have expressed concern that the back-and-forth over who should get the extra doses will confuse people and potentially deter some who would benefit from the additional dose.

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The White House has said it wanted to make plans for boosters to make for a smooth rollout, but it always planned to leave the details of authorization to regulators acting on scientific evidence.

Those who qualify for boosters will have to attest to their eligibility but won’t have to provide additional documentation, CDC officials said.

The ACIP is a 15-member panel including pediatricians, infectious-disease doctors and other medical experts.

After the FDA greenlights vaccines, the ACIP advises the CDC on who should receive them, when and under what circumstances. Usually it meets three times a year, though the panel has met many more times during the pandemic.

The CDC advisory panel was worried about supporting a broad recommendation for people such as healthcare workers or others exposed because of their jobs. The panel feared it would essentially allow anyone who wanted a booster to get one without enough safety data to back that up.

“It seems uncharacteristically openly ended for the lack of data of need in any of these groups,” said Sarah Long, a professor of pediatrics at Drexel University College of Medicine.

The Biden administration also said Friday that federal contractors and subcontractors must be vaccinated against Covid-19 by Dec. 8.

The deadline was included in new guidelines issued by The Safer Federal Worker Task Force, a coalition of federal entities that includes the White House’s Covid-19 response team and the Office of Management and Budget.

The guidelines offer exemptions to the vaccination requirement “in limited circumstances where an employee is legally entitled to an accommodation.”

The rules follow an executive order issued this month by Mr. Biden mandating federal workers and contractors be vaccinated against Covid-19. The directive stepped up the requirements for these workers after Mr. Biden earlier said federal workers and contractors who work on-site must be vaccinated or face regular testing and other measures.

The task force had already said federal workers must be fully vaccinated by Nov. 22.

Mr. Biden has said updated vaccine rules covering federal workers and contractors and healthcare settings, along with a forthcoming emergency temporary standard from the Labor Department that will apply vaccine and testing requirements at private employers with 100 or more employees, will affect about 100 million Americans, equivalent to two-thirds of all workers.

—Jared S. Hopkins and Amara Omeokwe contributed to this article.

Write to Felicia Schwartz at felicia.schwartz@wsj.com and Andrew Restuccia at andrew.restuccia@wsj.com