WASHINGTON — COVID-19 vaccine booster shots will be made widely available to Americans starting on Sept. 20, U.S. health officials said on Wednesday, citing data showing diminishing protection from the initial vaccinations as infections rise from the delta variant.

U.S. officials will offer a third shot to Americans who received their initial inoculation of two-dose COVID-19 vaccines made by Moderna and by Pfizer/BioNTech at least eight months earlier, the Department of Health and Human Services said in a statement.

The U.S. government expects to give out 100 million booster shots for free at around 80,000 locations nationwide, White House coronavirus response coordinator Jeff Zients said.

Initial booster doses will be given to Americans who received two-dose vaccines, but officials said they anticipate that people who received the single-dose Johnson & Johnson shot, authorized in the United States in February, will also need boosters.

"You want to get out ahead of the virus," Anthony Fauci, President Joe Biden's chief medical adviser, told reporters. "If you wait for something bad to happen before you respond to it, you find you're considerably behind your real full capability of being responsive."

The booster shots, officials said, initially will focus upon health care workers, nursing home residents and older people — among the first groups to be vaccinated in late 2020 and early 2021.

Shares of Pfizer rose around 1.65% on the news in midday trading. Moderna shares dropped around 2.8% and J&J fell 0.5%.

There is mounting evidence that protection from COVID-19 vaccines wanes after six or more months, particularly in older people with underlying health conditions. The officials cited this in their decision on boosters but stressed that the U.S.-authorized shots have proven "remarkably effective" in reducing the risk of severe disease, hospitalizations and deaths.

More than a million Americans sought an extra vaccine dose before the official decision on boosters was announced, according to federal data. U.S. officials previously authorized a third dose of the Pfizer/BioNTech and Moderna vaccines for people with weak immune systems.

The U.S. announcement defies guidance from the World Health Organization, whose chief scientist said current data does not indicate the need for booster shots.

In recent weeks, several other countries have decided to offer booster shots to older adults and people with weak immune systems, including Israel, France and Germany. European Union officials said on Wednesday they do not yet see a need to give booster shots to the general population.

U.S. officials do not expect a serious uptick in hospitalizations and deaths among vaccinated people until at least eight months after their initial inoculation, U.S. Surgeon General Vivek Murthy said.

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The amended emergency use authorization paves the way for people who have had an organ transplant, or those with a similar level of weakened immune system, to get an extra dose of the same shot they have initially received.

A new study conducted in New York and cited by U.S. Centers for Disease Control and Prevention chief Rochelle Walensky found that COVID-19 vaccine effectiveness dropped from around 92% in May to around 80% more recently.

Vaccines are generally less effective against the delta variant now dominant in the United States, Walensky added, citing a study of U.S. nursing homes showing a 53% dropoff against delta.

Delta variant concerns

The United States leads the world in reported COVID-19 cases and deaths, with around 500 Americans dying daily. Daily U.S. cases soared from fewer than 10,000 in early July to more than 150,000 in August as the delta variant took hold.

The new cases include vaccinated people, though they are far less likely to experience severe disease or death than the unvaccinated.

A new study by an Israeli health care provider found that a third dose of the Pfizer/BioNTech shot is 86% effective in people over age 60. It followed another Israeli study that showed evidence of waning immunity from COVID-19 vaccines in the months after inoculation, raising the risk of serious infection among the elderly and people with compromised immune systems.

"The surprise factor is the vaccine vulnerability — people who are vaccinated became vulnerable much earlier than expected," Dr. Eric Topol, director of the Scripps Research Translational Institute in California, said about the study.

World Health Organization officials contend that vaccine doses should first go to people in poorer countries who have not yet received an initial inoculation.

"We are planning to hand out extra life jackets to people who already have life jackets, while we are leaving other people to drown without a single life jacket. That's the reality," Mike Ryan, WHO's chief emergency expert, told a news conference, referring to nations proceeding with booster shots.

Zients said the U.S. boosters decision will not diminish its vaccine donations abroad, amounting to about 200 million shots over the coming months.

The U.S. booster plan is dependent on the FDA determining that a third dose of the two-dose vaccines is safe and effective, and a favorable recommendation from CDC advisers.

Contributing: Ahmed Aboulenein, Jarrett Renshaw and Francesco Guarascio

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