WASHINGTON (AP) - 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 signs that the vaccines’ effectiveness is slipping.

The plan, as outlined by the chief of the Centers for Disease Control and Prevention and other top health 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.

“Our plan is to protect the American people, to stay ahead of this virus,” CDC Director Dr. Rochelle Walensky said as the agency cited a raft of studies suggesting that the vaccines are losing ground while the highly contagious variant spreads.

People who received the single-dose Johnson & Johnson vaccine will also probably need extra shots, health officials said. But they said they are waiting for more data.

Officials said that before any booster program starts up, the Food and Drug Administration and a CDC advisory panel would need to evaluate the safety and effectiveness of an extra dose.

“We have a responsibility to give the maximum amount of protection,” President Joe Biden said at the White House. He added that extra doses are also “the best way to protect ourselves from new variants that could arise.”

The announcement came the same day the Biden administration said it would require nursing homes to mandate vaccinations for staffers in order to continue receiving federal funds.

Hundreds of thousands of nursing home workers remain unvaccinated, despite the heightened risk of fatal infections among elderly residents.

Officials said it is “very clear” that the vaccines’ protection against infections wanes over time, and they noted the worsening picture in Israel, which has seen a rise in severe cases, many of them in people already inoculated.

They said the U.S. needs to get out ahead of the problem before it takes a more lethal turn here and starts leading to increasing hospitalizations and deaths among the vaccinated.

Dr. Anthony Fauci, the government’s foremost expert on COVID-19, said one of the key lessons of the coronavirus is that it’s better to “stay ahead of it than chasing after it.”

The first boosters would go to people in high-priority groups that received the initial U.S. vaccinations: nursing home residents, health workers and those with underlying health conditions.

Health officials are likely to recommend that the booster be the same brand of vaccine that people received initially.

Dr. Mark Mulligan of NYU’s Langone Health center welcomed the announcement, saying: “Part of leadership is being able to see around the corner and make hard decisions without having all the data. It seems to me that’s what they’re doing here.”

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.

White House officials noted that the U.S. has donated 115 million doses to 80 countries, more than all other nations combined. They said the U.S. has plenty of vaccine to dispense boosters to its own population.

Israel is already offering booster shots to people over 50. And European regulators are looking into the idea.

Last week, U.S. health officials recommended a third shot for some people with weakened immune systems, such as cancer patients and organ transplant recipients. Offering boosters to all Americans would be a major expansion of what is already the biggest vaccination campaign in U.S. history. Nearly 200 million Americans have received at least one shot.

Some experts have expressed concern that calling for boosters would undermine the public health message - and reinforce opposition to the vaccine - by raising more doubts in the minds of people already skeptical about the shots’ effectiveness.

Officials said the eight-month time frame was a judgment call about when vaccine protection against severe illness might fall, based on the direction of the current data.