• U.S. agencies are preparing to offer booster shots to all eligible Americans beginning the week of Sept. 20.
• It's now "very clear" that immunity starts to fall after the initial two doses, top U.S. health officials said,

The United States will begin widely distributing Covid-19 booster shots next month as new data shows that vaccine protection wanes over time, top U.S. health officials announced Wednesday.

It's now "very clear" that immunity starts to fall after the initial two doses, and with the dominance of the delta variant, "we are starting to see evidence of reduced protection against mild and moderate disease," according to the statement signed by CDC Director Dr. Rochelle Walensky, acting FDA Commissioner Dr. Janet Woodcock, White House chief medical advisor Dr. Anthony Fauci and other U.S. health leaders.

"Based on our latest assessment, the current protection against severe disease, hospitalization, and death could diminish in the months ahead, especially among those who are at higher risk or were vaccinated during the earlier phases of the vaccination rollout."

As a result, U.S. agencies are preparing to offer booster shots to all eligible Americans beginning the week of Sept. 20, starting eight months after their second dose of Pfizer or Moderna's vaccines, officials said. While they said recipients of Johnson & Johnson's single-shot vaccine will likely need boosters, they are awaiting more data in the next few weeks before making a formal recommendation.

"With those data in hand, we will keep the public informed with a timely plan for J&J booster shots as well," the officials said.

In a statement late Wednesday, J&J said: "We are engaging with the U.S. FDA, CDC and other health authorities and will share new data shortly regarding boosting with the Johnson & Johnson COVID-19 vaccine."

The statement added: "In July, Johnson & Johnson shared data demonstrating that our single-shot COVID-19 vaccine generated strong, persistent immune activity against the rapidly spreading Delta variant and other highly prevalent SARS-CoV-2 viral variants. Interim results from a Phase 1/2a study published in the New England Journal of Medicine also showed that the durability of the immune response was strong, with no waning for at least eight months, the length of time that had been evaluated to date."

Ensuring long-term and durable protection against hospitalization and death are critical in curbing the COVID-19 pandemic.

The plan is subject to a formal recommendation from a CDC vaccine advisory committee and approval from the FDA, also a formality.

The announcement came ahead of a White House Covid press briefing Wednesday, where federal health officials further outlined their plan for boosters.

The booster shot "will boost your immune response," President Joe Biden said later Wednesday in a speech at the White House. "It will increase your protection from Covid-19."

Biden also addressed criticism from some health advocates who say the U.S. should focus on sending vaccine doses to countries facing shortages, rather than prioritizing booster shots for Americans.

"I disagree," Biden said. "We can take care of America and help the world at he same time."

The decision to recommend booster shots comes as the public becomes increasingly concerned about the delta variant and a rise in breakthrough cases — infections in fully vaccinated individuals. It marks a shift from previous comments made by U.S. health officials, who said in recent months that fully vaccinated Americans did not need booster shots at this time.

U.S. officials changed their message on boosters in recent days as cases continued to rise. Fauci said Thursday that everybody will "likely" need a booster shot at some point. On Friday, federal officials approved administering booster shots to Americans with weakened immune systems, which includes cancer and HIV patients and people who have had organ transplants.

National Institutes of Health Director Dr. Francis Collins, who also signed the statement, said Tuesday that new Covid data, including from Israeli health officials, caused U.S. health leaders to rethink their position on vaccine boosters. Israel released new data Monday showing a reduction in the effectiveness of Pfizer's Covid vaccine against severe illness among people 65 and older who were fully vaccinated in January or February.

The U.S. is beginning to see similar trends in vaccine effectiveness as well, Collins said. He said the rise in breakthrough cases may be due to a combination of the fast-spreading delta variant and Covid vaccine protection waning over time.

The effectiveness of Pfizer's Covid vaccine steadily declines over time, dropping to about 84% for vaccinated people about four to six months after getting their second dose, according to CEO Albert Bourla. Moderna said its vaccine remained 93% effective in the first six months after the second dose but expects that protection to fall and boosters will be necessary.

During a press briefing Wednesday, Walensky said officials based their decision on studies that showed immunity from Pfizer and Moderna's vaccines diminished over several months. One study in New York from May 3 through July 25 showed that the vaccine's effectiveness in protection against infection dropped from around 92% to 80%. Another study by the Mayo Clinic showed that Pfizer's vaccine efficacy fell from around 76% to 42% while Moderna's declined from 86% to 76%.

"Right now, it's still as if our vaccine protection is working really well," Collins said. "But we don't want to wait until it's like oh, too late."

The move to recommend boosters is likely to spark criticism, especially as a large portion of the global population has yet to receive even one dose of a Covid vaccine.

Earlier this month, the World Health Organization asked wealthy nations to stop the distribution of booster shots until at least the end of September to give poorer countries the chance to vaccinate their populations with the first rounds of shots. The request is part of WHO Director-General Tedros Adhanom Ghebreyesus' plan to vaccinate 40% of the world by December.

The U.S. released the statement minutes after the WHO condemned wealthy nations that support boosters for the general public.

"We believe clearly that the data to date does not indicate that boosters are needed," Dr. Soumya Swaminathan, the WHO's chief scientist, said during a press briefing. "And we need to know which groups at what period after the vaccination and which particular vaccines people have received in their primary course."

Lawrence Gostin, director of the WHO's Collaborating Center on National and Global Health Law, called the U.S. booster shot plan "a slap in the face" to the international health agency.

"There's a better way to create a win-win," he said in a phone interview. "We should boost only our health workers and vulnerable people. At the same time, Biden should pledge a bold campaign to vaccinate the world, including vastly increased donations and a surge in vaccine production."

"That way we do good to America and do good for the world. It's in our national interests to stop the development of even more dangerous variants," he added.

During a White House briefing Tuesday, press secretary Jen Psaki said the administration believes it can boost the American population while ensuring the rest of the world gets vaccinated.

"We believe that is a false choice. We can do both," Psaki said. "The United States is far and away the biggest contributor to the global fight against Covid. We will continue to be the arsenal for vaccines around the world. We also have enough supply and had long planned enough supply should a booster be needed for the eligible population."

Administering third shots appears to be safe. Early data from small studies of the effects of booster doses in immunocompromised patients didn't show any severe adverse effects from a third shot of an mRNA vaccine, nor did recipients develop side effects beyond those already identified after the initial two-dose regimen.

Once booster shots are approved, nursing home residents, health-care providers and the elderly — the first groups to get vaccinated in December and January — are likely to be prioritized to get extra shots, Collins said Tuesday. He said "ideally" people should stick with the same manufacturer they got their first two doses from.

"But if for some reason you don't have access to it, well, then get the other one," he said. "Again, I'd feel more comfortable as a scientist fixing our plans on real data, and that means sticking to the same kind of vaccine that you got to begin with."

–CNBC's Rich Mendez and Robert Towey contributed to this report.