The U.S. plans to offer booster shots next month. Some health experts are wary.

The decision to hand out booster shots plunges the government into a new and more complicated stage of its yearlong vaccination campaign. Health officials are racing to head off a resurgence of infections even as they struggle to convince millions of unvaccinated Americans to get their first shots. The booster policy has also sparked tensions inside and outside the administration, with some health experts arguing that available data does not show a clear-cut need to give additional doses to all adults — especially when large portions of the globe remain unvaccinated.

The policy will apply to people over 18 who have received the Moderna or Pfizer vaccines, and is contingent on authorization from the FDA and a review by CDC’s vaccine advisory committee. Administration officials said they also anticipate offering booster shots to people who received the one-shot Johnson & Johnson vaccine, but are waiting for further data before officially making that recommendation.

Americans would be eligible for booster shots eight months after receiving their second vaccine under the emerging plan, an approach that would put health care workers and nursing home residents first in line.

“It will be just as easy and convenient to get a booster shot as it is to get a first shot today,” White House Covid-19 Response Coordinator Jeff Zients vowed. “We are prepared for boosters and we will hit the ground running.”

The shots will be available at roughly 80,000 vaccination sites nationwide, Biden said during his speech, and will be available to any adult with a vaccine card that shows they received their initial shots at least eight months before.

The call for boosters is a turning point in the federal government’s vaccine strategy. The summer began with Biden stressing a plan to return the country to normal by July Fourth. But over the course of the last month, administration officials have slowly retreated from that message as Delta has roared across the country — notably by asking vaccinated Americans to once again mask up indoors.

The decision to recommend boosters is likely to post a messaging challenge for an administration already grappling with hardening resistance to Covid-19 vaccines.

Health officials on Wednesday emphasized that the vaccines remain highly protective against severe disease that could lead to hospitalization and death, but that they were eager to preempt any potential decline in that effectiveness.

“If you wait for something bad to happen before you respond to it, you find yourselves considerably behind,” top infectious disease doctor Anthony Fauci said. “Better to stay ahead of it than chasing after it.”

The booster decision came after weeks of intense debate inside the top echelons of the administration’s Covid-19 task force about whether new data on vaccine effectiveness over time suggested Americans needed a second round of shots. Those deliberations coincided with a push by the administration to continue donating doses to lower- and middle-income countries that are still struggling to complete a first round of vaccinations.

In a meeting Sunday, Biden administration officials reviewed vaccine data collected by the CDC that showed protection from vaccines declined in recent months as the Delta variant took hold and infections began to rise across the country. A study tracking adults in New York found that vaccine effectiveness against infection declined from 91.7 percent in early May to 79.8 percent by late July, according to a private administration briefing held for public health experts on Wednesday morning. During the same period, the state saw a rise in infections attributable to the more transmissible Delta variant.

The agency released the analysis, which included information on vaccinated and unvaccinated people gathered by city and state reporting systems, on Wednesday.

CDC Director Rochelle Walensky also cited data collected by the Mayo Clinic on the decreased effectiveness of the Pfizer and Moderna vaccines against infection from the Delta variant. The effectiveness of the Pfizer vaccine dropped from 76 percent against earlier versions of Covid-19 to 42 percent against Delta. The Moderna vaccine’s effectiveness dropped from 86 percent to 76 percent against Delta.

Officials have also studied similar data from Israel which showed vaccine effectiveness against infection declined from about 95 percent in June to 64 percent in July as the Delta variant took over.

While the New York study found that vaccines still work well to prevent severe disease and hospitalizations, the data from the U.S. and abroad convinced Biden officials that Americans need boosters beginning this fall to complement other public health measures, such as masking and social distancing.

“I am strongly in favor of this booster strategy, and hope that what it will herald is a phase in the pandemic where we will empower patients, in consultation with their physicians, to make the best choices for themselves,” said Leana Wen, an emergency physician and public health professor at George Washington University. “Some may be comfortable with the level of protection against severe disease afforded by the existing vaccines; others will want to reduce symptomatic infection by getting a boost. People need to consider their own medical circumstances, exposures, and risk tolerance when it comes to booster doses.”

But the push to distribute boosters within a matter of weeks is sure to spur international backlash, and risks sowing confusion among Americans who were told for months that vaccines were strong protection against infection.

The World Health Organization has pushed back against the plan given the intense unmet need for Covid-19 vaccines in much of the world. The WHO this month urged wealthy countries to refrain from offering boosters until late September, to help meet its goal of vaccinating at least 10 percent of people in every country. But several nations, including Israel, France and Hungary, have started offering boosters to portions of their population or plan to do so soon.

“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,” said Mike Ryan, executive director of the WHO Emergencies Programme, during a briefing Wednesday.

During the private administration briefing on Wednesday, several outside public health experts sharply questioned the administration’s strategy, according to multiple people on the call, telling officials they were unconvinced the scientific data justified distributing boosters.

They also aired concerns about the potential impact it could have on the global vaccination effort, while criticizing the abrupt rollout of the administration’s plan after weeks of top health officials downplaying the prospect that boosters would be needed imminently.

“I still don’t understand why the administration is moving forward with extra doses of vaccine for the general population,” Céline Gounder, an infectious disease specialist and former Covid-19 adviser to the Biden transition, tweeted Wednesday, in a reflection of the private frustrations many health experts have directed toward the Biden team in recent days.

Biden and his top health advisers on Wednesday dismissed suggestions the booster effort would dent the U.S.’s vaccination efforts abroad.

“We’re going to be the arsenal of vaccines to beat this pandemic as we were the arsenal of democracy to win World War II,” the president said, touting the United States’ planned donation of more than 600 million vaccine doses to nations in need. That figure dwarfed the donations of all other countries put together, Biden added. Yet it is still only a fraction of the vaccine supply needed to give initial doses to the entire world.

Surgeon General Vivek Murthy said the United States would intensify its efforts to meet the global need for shots.

“I do not accept the idea that we have to choose between America and the world,” he said. “We will also continue to accelerate our efforts to vaccinate the rest of the world. We take that responsibility very seriously.”

But even as the administration lays out plans for a second round of shots, more than 40 percent percent of the American population over the age of 12 is still not vaccinated. It’s not clear how the federal government will convince large portions of the country to not only sign up for their first set of shots but to return for a booster.

Biden officials did not provide specific details about how the booster shots would be distributed, but suggested they would rely heavily on the sprawling combination of federal and state sites and pharmacies used to distribute the first round of vaccines. They also stressed that vulnerable populations, including frontline health care workers and nursing home residents, should be at the front of the line.

A second CDC study released Wednesday showed vaccine effectiveness among residents of nursing homes who received the Pfizer or Moderna vaccine was around 75 percent before the emergence of Delta and 53 percent during Delta’s spread.

A total of 100 million doses of the Pfizer and Moderna vaccines are available now, according to two senior officials with knowledge of the situation. The government has contracted for an additional 400 million to be distributed as needed.

Carmen Paun contributed to this report.