NEW YORK — U.S. health officials Wednesday announced plans to dispense COVID-19 booster shots to all Americans to shore up their protection amid the surging delta variant and evidence that the vaccines' effectiveness is falling.

The plan, as outlined by the director of the Centers for Disease Control and Prevention and other top authorities, calls for an extra dose eight months after people get their second shot of the Pfizer or Moderna vaccine. The doses could begin the week of Sept. 20.

Health officials said people who received the single-dose Johnson & Johnson vaccine will also probably need extra shots. But they said they are awaiting more data and have yet to work out the details.

"Our plan is to protect the American people, to stay ahead of this virus," CDC Director Dr. Rochelle Walensky said in a statement.

The overall plan is awaiting a Food and Drug Administration evaluation of the safety and effectiveness of a third dose, the officials said.

In a statement, health officials said it is "very clear" that the vaccines' protection against infection wanes over time, and now, with the highly contagious delta variant spreading rapidly, "we are starting to see evidence of reduced protection against mild and moderate disease."

"Based on our latest assessment, the current protection against severe disease, hospitalization and death could diminish in the months ahead," they said.

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

"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. Michael Ryan, the WHO's emergencies chief.

The organization's top scientist, Dr. Soumya Swaminathan, said: "We believe clearly that the data does not indicate that boosters are needed" for everyone. She warned that leaving billions of people in the developing world unvaccinated could foster the emergence of new variants and result in "even more dire situations."

WHO officials have repeatedly expressed concerns that variants will continue to crop up in areas where the virus goes unchecked through measures like vaccination. They have called for vaccine equity and "solidarity" among countries.

But White House press secretary Jen Psaki this week said the question of whether to distribute boosters or help other nations "is a false choice. We can do both."

She said the U.S. is "far and away" the biggest contributor to the global fight against COVID-19 and will "continue to be the arsenal for vaccines" to the world. She added that the U.S. has enough vaccine for dispense boosters to the American people.

U.S. public health experts have become alarmed in recent weeks over the rapid spread of the highly contagious variant and reports of "breakthrough" cases in vaccinated people.

In making the announcement on boosters, the CDC released three studies conducted during the delta surge that suggest that the COVID-19 vaccines remain highly effective at keeping Americans out of the hospital but that their ability to prevent infection is dropping markedly among nursing home patients and others.

However, the new studies — on their own — fall short of the kind of data that some experts thought would be necessary for a recommendation like that.

Some scientists have been looking for signs that hospitalizations or deaths are increasing, as a necessary indicator that boosters might be needed. The new studies, on the contrary, found no change in vaccine effectiveness against hospitalizations.

To some leading scientists, the studies "would not be sufficient, in and of themselves, to make the case for a booster," said Dr. William Schaffner, an infectious-diseases expert at Vanderbilt University.

Schaffner is a liaison to an expert advisory panel that helps the CDC form its vaccination recommendations. He said members of the committee were surprised when Biden administration officials this week disclosed plans to call for a booster for the general public.

Of the three studies released by CDC on Wednesday, the one that spoke most directly to a possible need for boosters was a look at reported infections in residents of nearly 15,000 nursing homes and other long-term care facilities.

It found that effectiveness of the Pfizer and Moderna vaccines against COVID-19 infections dropped, from about 74% in March, April and early May, but fell to 53% in June and July.

The researchers said it's not clear how much of the decline is attributable to the delta variant and how much might be due to a more general weakening of immunity that could have been occurred against any strain.

The infections studied included all infections, with or without symptoms. The researchers said more work is needed to determine if there was a higher incidence of infections that resulted in severe illness.

Another one of the studies looked at hospitalizations at 21 hospitals. It found vaccine effectiveness against COVID-19-associated hospitalization in inoculated people was 86% at two to 12 weeks after second dose, and 85% at 13 to 24 weeks after.

The third study found that protection against hospitalizations stayed steady at about 95% over the nearly three months studied. And vaccine effectiveness against new laboratory-confirmed infections for all adults in New York state declined from about 92% in early May to about 80% in late July.

The researchers said they are not certain why the decline occurred, but they noted it coincided with the delta variant as well as an easing of social distancing and mask rules.