CNN  — 

US health officials and medical experts announced in a joint statement on Wednesday that booster doses of Covid-19 vaccine will be offered this fall, subject to authorization from the US Food and Drug Administration and sign off from the US Centers for Disease Control and Prevention.

“We are prepared to offer booster shots for all Americans beginning the week of September 20 and starting 8 months after an individual’s second dose,” US health officials, including CDC Director Dr. Rochelle Walensky and FDA Acting Commissioner Dr. Janet Woodcock, said in the statement.

“At that time, the individuals who were fully vaccinated earliest in the vaccination rollout, including many health care providers, nursing home residents, and other seniors, will likely be eligible for a booster. We would also begin efforts to deliver booster shots directly to residents of long-term care facilities at that time, given the distribution of vaccines to this population early in the vaccine rollout and the continued increased risk that COVID-19 poses to them,” the statement said.

The officials write that the authorized Covid-19 vaccines are “remarkably effective in reducing risk of severe disease, hospitalization, and death, even against the widely circulating Delta variant,” but it’s clear that protection against the coronavirus begins to decrease over time.

“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,” the statement said. “For that reason, we conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability.”

The statement was signed by Walensky, Woodcock, Surgeon General Dr. Vivek Murthy, National Institutes of Health Director Dr. Francis Collins, National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci, Assistant Secretary for Health Dr. Rachel Levine, Chief Science Officer for the COVID Response Dr. David Kessler and Chair of the COVID Health Equity Task Force Dr. Marcella Nunez-Smith.

While those initial booster doses will be for the Pfizer/BioNTech and Moderna vaccines, the officials noted in the statement that they anticipate booster shots will likely be needed for people who initially received the Johnson & Johnson vaccine. “Administration of the J&J vaccine did not begin in the U.S. until March 2021, and we expect more data on J&J in the next few weeks,” the statement said.

The CDC also said on Wednesday that the FDA will review data on boosters among adolescents to determine when people younger than 18 will be part of the rollout.

The officials ended the statement by noting that they continue to expand efforts to increase the supply of vaccines globally for other countries, “building further on the more than 600 million doses we have already committed to donate globally.”

Booster plans are to ‘stay ahead’ of the virus

The Biden administration’s plans to administer booster doses of Covid-19 vaccine are an effort to “stay ahead” of the coronavirus, Murthy said during a virtual White House Covid-19 briefing on Wednesday.

Murthy emphasized that coronavirus vaccines still appear to be effective in protecting against severe Covid-19, hospitalization and death – but data suggest that protection against mild and moderate disease appears to decline over time, and US health officials want to boost protection before that decline in protection against severe disease.

“We are concerned that this pattern of decline we are seeing will continue in the months ahead, which could lead to reduced protection against severe disease, hospitalization and death,” Murthy said during Wednesday’s briefing.

Three separate studies demonstrate how protection against Covid-19 infection that vaccines provide may wane over time, Walensky said during the briefing.

One of those studies, conducted in New York, found that vaccine effectiveness against new Covid-19 diagnoses declined from 92% to 80% over time from May 3 through July 25, based on the state’s vaccine records.

“This allowed New York to study vaccine effectiveness against infection over time for more than 10 million New Yorkers of all ages,” Walensky said. The data published in the CDC’s Morbidity and Mortality Weekly Report (MMWR) on Wednesday.

Another study, conducted by the Mayo Clinic, analyzed vaccine effectiveness for both the Pfizer and Moderna vaccines among more than 80,000 people across all ages using data through July 16, Walensky said, adding, “Like we saw in the New York data, vaccine effectiveness against infection declined over time.”

The Mayo Clinic study found that effectiveness fell from 76% to 42% among those who received the Pfizer vaccine and from 86% to 76% among those who received the Moderna vaccine.

A third study, published in the CDC’s MMWR, found that vaccine effectiveness against Covid-19 infection among nursing home residents declined from 75% in March to 53% in August, Walensky said.

The researchers, from the CDC and Lantana Consulting Group in Vermont, wrote in the study that an “additional dose of COVID-19 vaccine might be considered for nursing home and long-term care facility residents to optimize a protective immune response.”

In all three studies, “taken together, you can see that while the exact percentage of vaccine effectiveness over time differs depending on the cohort and setting studied, the data consistently demonstrate a reduction of vaccine effectiveness against infection over time,” Walensky said. “Despite waning vaccine effectiveness against infection, data analyzed through July continue to demonstrate a stable and highly effective protection against severe illness and hospitalization for people who are vaccinated.”

It is also “conceivable” that the planned rollout of booster shots could help reduce the spread of disease, Dr. Fauci said Wednesday.

Although, “transmissibility is a bit more tricky than looking at a clinical phenomenon, such as infection, seriousness of disease and hospitalization,” Fauci said during the White House briefing.

But data show that a booster shot prompts the body to produce a new batch of antibodies, Fauci said. These catch the virus as it tries to enter the body, and could stop the virus from multiplying in the nose and throat after someone breathes it in.

“The increase with a boost is really quite striking – multiple-fold increase – that it is conceivable that that would be important in lowering the level of virus in the nasopharynx, which could have an impact on transmission. I certainly hope that’s the case,” Fauci said.

“If it is, then you could really get multiple benefits from doing this. You can get benefits for disease, severity of disease, and then ultimately infection and transmission. But the bottom line with full transparency – we don’t know that right now.”

FDA and CDC to weigh in next

The White House’s plan to offer booster shots to vaccinated adults starting September 20 still depends on whether the FDA authorizes boosters and whether the CDC’s Advisory Committee on Immunization Practices (ACIP) recommends those boosters, Murthy said Wednesday.

ACIP is a group of medical and public health experts that develop recommendations on how to use vaccines in the United States.

Get CNN Health's weekly newsletter

Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.

“I want to be very clear: We are not skipping the very important FDA and ACIP process here. They have an incredibly important role to play in evaluating safety and making recommendations for vaccines. We respect that, we honor that and that will be a part of this process as well,” Murthy told the White House briefing.

White House Coronavirus Response Coordinator Jeff Zients told reporters on Wednesday’s Covid-19 briefing that boosters will be free and available at established vaccine sites.

“It will be just as easy and convenient to get a booster shot as it is to get the first shot today,” Zients said. “We have enough vaccine supply for every American.”

Murthy said they made the announcement now for the sake of transparency, and to ensure boosters can roll out in an orderly way.

“We also want to ensure that people and states and localities and the public more broadly can plan,” Murthy said. “You can’t turn on a booster effort with the flip of a switch. You have to lay the groundwork.”

CNN’s Dr. Sanjay Gupta and Jeremy Diamond contributed to this report.