More than five million Americans could be eligible for a booster dose of the Covid-19 vaccine by the end of September as part of the Biden administration’s plan to combat the Delta variant of the coronavirus through additional doses eight months after the initial vaccinations.

However, the plan depends on several crucial steps that will take place over the coming weeks. Most importantly, the Food and Drug Administration would have to decide that third vaccinations would be safe and effective for Pfizer-BioNTech and Moderna, the two vaccines that were first introduced and most widely used.

Pfizer is further advanced in submitting data to the FDA that it says supports the use of boosters. Moderna and the National Institutes of Health are still investigating whether a half or a full dose for a third shot would work better, but are expecting results soon. Moderna CEO Stephane Bancel said the company plans to file its data with the FDA in September.

Government officials are due to announce the strategy at a White House briefing on Wednesday. Nursing home residents, nurses, and rescue workers would likely come first, as with the first few shots. Other older people would follow next, followed by the rest of the general population.

Officials envision giving people the same vaccine they were originally given and using pharmacies as key distribution points.

Administration officials discourage people from checking for booster doses on their own, noting that the FDA has yet to decide about their safety and effectiveness. They hope to distribute extra shots in an orderly manner so that people can get a booster shot when advised, and not just based on their own fears.

Dr. Danny Avula, the vaccine coordinator for the state of Virginia, said his state already has thousands of vaccine providers and can likely manage booster vaccinations without major changes. “What caused so much of the urgency and frenzy of January through April was the delivery bottleneck,” he said.

Now the government has more than 100 million doses in stock that could be used for boosters, along with tens of millions more doses that have already been shipped to pharmacies and other places. Even more supplies are to be delivered in autumn.

In interviews on Tuesday, hospital officials and doctors generally supported calls for a booster vaccination.

“I think we’re running out of second chances,” said Dr. Matthew Harris, the medical director of the coronavirus vaccination program at Northwell Health, New York’s largest hospital system. “What keeps me up at night is the inevitability of a variant that doesn’t respond to the vaccine. So if we have that head start, I fully support it.”

Understand the state of vaccination and masking requirements in the United States

    • Mask rules. The Centers for Disease Control and Prevention in July recommended that all Americans, regardless of vaccination status, wear masks in public places indoors in areas with outbreaks, reversing the guidelines offered in May. See where the CDC guidelines would apply and where states have implemented their own mask guidelines. The battle over masks is controversial in some states, with some local leaders defying state bans.
    • Vaccination regulations. . . and B.Factories. Private companies are increasingly demanding corona vaccines for employees with different approaches. Such mandates are legally permissible and have been confirmed in legal challenges.
    • College and Universities. More than 400 colleges and universities require a vaccination against Covid-19. Almost all of them are in states that voted for President Biden.
    • schools. On August 11, California announced that teachers and staff at both public and private schools would have to get vaccinated or have regular tests, the first state in the nation to do so. A survey published in August found that many American parents of school-age children are against mandatory vaccines for students, but are more supportive of masking requirements for students, teachers and staff who do not have a vaccination.
    • Hospitals and medical centers. Many hospitals and large health systems require their employees to receive a Covid-19 vaccine, due to rising case numbers due to the Delta variant and persistently low vaccination rates in their communities, even within their workforce.
    • new York. On August 3, New York City Mayor Bill de Blasio announced that workers and customers will be required to provide proof of vaccination when dining indoors, gyms, performances, and other indoor situations. City hospital staff must also be vaccinated or have weekly tests. Similar rules apply to employees in New York State.
    • At the federal level. The Pentagon announced that it would make coronavirus vaccinations compulsory for the country’s 1.3 million active soldiers “by mid-September at the latest. President Biden announced that all civil federal employees would need to be vaccinated against the coronavirus or undergo regular tests, social distancing, mask requirements and travel restrictions.

Federal officials envision offering additional vaccinations to recipients of Johnson & Johnson’s single-dose vaccine, as well as those who received Moderna or Pfizer. But the government didn’t start offering this vaccine until March, and only 14 million people have received it. For comparison: 155 million people were fully vaccinated with Pfizer or Moderna.

Data from a Johnson & Johnson clinical trial in which participants were given two doses is likely to be submitted to the FDA later this month and is intended to guide the government’s recommendation on the vaccine.

At the Wednesday briefing, administration officials plan to point out that a booster strategy is essential, even if it needs to be changed as more data comes in. They are expected to provide data showing the overall effectiveness of the vaccine against viral infections is declining, although unvaccinated people still make up the vast majority of people who become seriously ill or hospitalized due to Covid.

The government fears that without boosters, more vaccinated people could get serious Covid disease in the coming months, as there is a double trend: the stronger effect of the delta variant and the decreasing protection of the vaccines.

Data from the Israeli Ministry of Health are seen as a warning sign. It suggests that protection from viral infections fell in June and July relative to the time since a person was vaccinated. According to a data set, the effectiveness of the Pfizer vaccine against serious illnesses in people 65 and over who were vaccinated in January fell to 55 percent. But the margin of error was wide and experts said other data appeared less worrying.