More than 13 million people in Pennsylvania and New Jersey could need booster shots of a coronavirus vaccine in the coming months, according to a federal plan announced Wednesday, but local health officials said they are confident the rollout will run smoothly.

The third shots of the Pfizer and Moderna vaccines are set to be offered nationwide starting in late September, the White House coronavirus response team said. Since the booster should be given eight months after the second shot, first in line will be health workers, long-term-care facility residents and staff, seniors, and people with underlying health conditions who were vaccinated early in the initial rollout.

Booster shots will likely be needed for people who received the one-dose Johnson & Johnson vaccine, too, federal officials said. However, the J&J shot wasn’t rolled out until March, officials added, and a booster-shot plan for that vaccine won’t be released until more data are reviewed in the coming weeks.

The Biden administration’s decision was made after federal health officials said they reviewed recent data showing that the protection offered by the vaccines against symptomatic infection decreases over time when up against the highly transmissible delta variant. To date, they said, the vaccines continue to largely prevent hospitalizations and death.

Across the country and the region, the vast majority of people being hospitalized are not vaccinated, public health leaders say, and vaccines remain the best defense against the virus.

“We do not believe the general population needs booster shots today,” U.S. Surgeon General Vivek Murthy said at a news briefing. “We are announcing a plan for the future.”

Protection against severe infection has diminished slightly in other countries, such as Israel, that have been hit hard by delta, federal officials said, and they are concerned that without boosters the vaccine’s protection against hospitalizations and deaths — particularly in the immunocompromised, who were also among the first to get vaccinated — could decrease slightly in the months ahead.

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Boosters, however, show a “dramatic increase” in antibody response, said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

By the time the new plan is implemented, which is set for the week of Sept. 20, it will be “just as easy and convenient to get a booster shot as it is to get a first shot today,” said Jeff Zients, President Joe Biden’s coronavirus response coordinator. He added that vaccines are now available at 80,000 providers, including more than 40,000 local pharmacies. “The bottom line is we are prepared for boosters and we will hit the ground running.”

Health officials in Pennsylvania and Philadelphia, which runs its own rollout separate from the state, said they were working on the logistics of a booster-shot rollout.

Commonwealth officials are “confident in the ability of vaccine providers to meet the demand,” spokesperson Maggi Barton said in a brief statement.

Philadelphia will use its existing network of vaccine providers to distribute third doses, acting Health Commissioner Cheryl Bettigole said at a news briefing just before the White House announcement, adding, “We’ll follow the CDC’s lead in terms of the specific groups for whom boosters will be recommended.”

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The state and city are already working with providers to administer boosters to people who are moderately or severely immunocompromised, in line with federal guidance announced last week. Bettigole said residents should contact their doctors to see if they fit into that group, which includes organ-transplant recipients and people being treated for cancers of the blood.

Details about the broader booster rollout were scarce Wednesday, with the White House team stressing that its announcement was meant to give states and vaccine providers time to plan in advance: “You can’t turn on a booster effort with a flip of a switch,” Murthy said.

The Pennsylvania Department of Health did not answer questions about how exactly it plans to balance efforts to increase first- and second-dose administration rates with efforts to give booster shots, how it will ensure residents don’t encounter the same appointment-scheduling difficulties they did earlier in the rollout, and whether measures will be in place to make sure the most vulnerable get shots first.

Nationally, Zients said, “we will be laser-focused on getting boosters to long-term care facilities to make sure residents get their shots and are safe and protected.” In the coming weeks, he said, federal health officials plan to work with states, vaccine providers, pharmacies, and national and community organizations to spread the word about the booster campaign.

The plan is contingent on an independent Food and Drug Administration evaluation of the safety and efficacy of a third dose, as well as a Centers for Disease Control and Prevention committee review and recommendation. The team awaits the guidance of the FDA and CDC to determine whether those between the ages of 12 and 18 should get the third shot, Murthy said. Children under 12 are not yet eligible for the vaccines, and it’s unclear when that authorization may come.

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The formal announcement of a U.S. plan for booster shots comes as the World Health Organization continues to argue that administering boosters will further lead to vaccine inequity worldwide. In dozens of countries, less than 25% of people are fully vaccinated, according to Our World in Data. Ten countries have administered 75% of the global vaccine supply, World Health Organization Director-General Tedros Adhanom said Wednesday, while poor countries have vaccinated only a small percentage of their population.

Another WHO official, health emergencies director Michael Ryan, used a stark analogy for booster-shot campaigns: “We’re planning to hand out extra life jackets to people who already have life jackets, while we’re leaving other people to drown without a single life jacket.”

The United States’ new plan will not affect its efforts to help the rest of the world, Zients said, noting the country has already shipped 115 million doses to other countries and plans to donate an additional 200 million in the coming months.

“To end this pandemic we have to protect the American people and we have to continue to do more and more to vaccinate the world,” he said. “We’re going to do both.”

Federal officials’ response to the growing evidence of waning immunity from coronavirus vaccines also provided a sobering reminder of the questions that remain about the delta variant’s ability to break through vaccinations, once again highlighting the importance of data in virus response.

The Pennsylvania Department of Health — which, unlike New Jersey, Delaware, and Philadelphia, is not tracking all confirmed infections in vaccinated residents — says it has prioritized the collection of more comprehensive breakthrough case data.

“This analysis requires pulling data from multiple sources and will take some time,” the department said in a statement, “but it will provide a more accurate picture.”

It is unclear when the data analysis will be complete.

Staff writer Tom Avril contributed to this article, which also contains information from the Associated Press.