Federal health officials announced plans to offer Americans COVID-19 booster shots beginning the week of Sept. 20, eight months after their second dose of the Pfizer or Moderna vaccines, pending approval from the FDA and CDC.

“Having reviewed the most current data, it is now our clinical judgment that the time to lay out a plan for COVID-19 boosters is now,” said Surgeon General Vivek Murthy in a briefing Wednesday.

“Recent data makes it clear that protection against mild and moderate disease has decreased over time. This is likely due to both the waning immunity and the strength of the widespread delta variant,” Dr. Murthy added. 

The boosters will be a third dose of the Pfizer-BioNTech or Moderna mRNA vaccines – federal health officials say people who have received the Johnson & Johnson vaccine will likely need a booster dose, but they are still reviewing data from the single-shot inoculation.

"We expect more data on J&J in the next few weeks," a group of federal health officials wrote in a statement. "With those data in hand, we will keep the public informed with a timely plan for J&J booster shots as well."

The boosters are expected for Americans 18 and older, officials said, and they will first go to people vaccinated earliest in the process, such as in December or January.

“This will boost your immune response, increase your protection from COVID-19, and is the best way to protect ourselves from new variants that could arise,” President Joe Biden said of the booster shots in a speech from the White House on Wednesday evening. 

Hours after the announcement, President Biden told ABC News in an exclusive interview that he and First Lady Dr. Jill Biden will get a booster shot, noting that they were some of the earliest people to get vaccinated.

"We're gonna get the booster shots," Biden said, adding: "We got our shots all the way back in, I think, December, so it's past time."

Biden went on to say that the administration has been preparing for the possibility of booster shots for several months, assuring listeners there is enough stockpile of both the Moderna and Pfizer vaccines in order for every eligible adult American to get their third dose. 

The president also pushed back at criticism that the United States should not be giving out booster shots until other countries have received their first dose, saying he thinks it is possible  to “take care of America and help the world at the same time.”

Top scientists at the World Health Organization bitterly objected to the U.S. plan, noting that poor countries are not getting enough vaccines for their initial rounds of shots.

So far, the U.S. has donated 115 million doses to 80 countries, more than all other nations combined.

Top health officials stressed on Wednesday that the vaccines are still protective for Americans who got the shot earlier this year, and they remain effective to fight the most severe effects of the delta variant. 

“One thing is very clear: getting vaccinated can keep you out of the hospital. Getting vaccinated can save your life,” said Centers for Disease Control and Prevention Director Rochelle Walensky.

But Walensky also noted worrisome data about the shots’ strength among vulnerable populations, noting a CDC study that showed their effectiveness dropped from 75% to 53% in nursing homes over the summer, as the delta variant spread.

The White House COVID-19 response coordinator, Jeff Zients, said the U.S. has enough supply and vaccination sites to give out boosters in the fall, pointing to 80,000 existing sites around the country.

“Importantly, boosters will be free, regardless of immigration or health insurance status. No ID or insurance required,” Zients said. 

Earlier Wednesday, the World Health Organization reiterated that it does not see boosters as necessary, especially with millions of people around the world still waiting for their first shot.

"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 Dr. Mike Ryan, who oversees health emergencies for the WHO. "That's the reality. Science is not certain on this. There are clearly more data to collect."

President Biden pushed back on that assertion.

“Are you comfortable with Americans getting a third shot when so many millions around the world haven't had their first?” ABC's Stephanopoulos asked Biden.

“Absolutely, because we’re provided more to the rest of the world than all the rest of the world combined,” Biden said. “Before we get to the middle of next year, we're gonna provide a half a billion shots to the rest of the world. We're keeping our part of the bargain. We’re doing more than anybody.”

Dr. Soumya Swaminathan, the WHO's chief scientist, warned of "even more dire situations" if wealthier countries start giving boosters ahead of poorer nations which lack vaccines.

"We believe clearly that the data does not indicate that boosters are needed,” Swaminathan said in Geneva. 

Swaminathan expressed concern that leaving billions of people in the developing world unvaccinated could foster the emergence of new variants, like the highly contagious delta variant which is driving the surge of cases in the United States.

Zients responded to those comments Wednesday, saying that the United States would both help vaccinate the world and provide booster shots at home: “We have to protect the American people, and we have to continue to do more and more to vaccinate the world. Both are critical.”

Dr. Anthony Fauci, the president’s chief medical adviser, also explained why the announcement was made this week, before FDA approval.

“If you wait for something bad to happen before you respond to it, you find yourselves … considerably behind your real full capability,” Dr. Fauci said. “You don't want to find yourself behind, playing catch up.”

The Associated Press contributed to this report.