After struggling for months to persuade Americans to get the COVID-19 vaccine, U.S. health officials could soon face a fresh challenge: talking vaccinated people into getting booster shots to gain longer-lasting protection as the delta variant sends infections soaring again.

As early as this week, U.S. health authorities are expected to recommend an extra dose of the vaccine for all Americans eight months after they get their second shot, according to two people who spoke to The Associated Press on condition of anonymity to discuss internal deliberations.

That means the biggest vaccination drive in U.S. history is about to get even more extensive.

The move is being driven by both the highly contagious variant and preliminary evidence that the vaccine’s protective effect starts dropping within months.

Last week, U.S. health officials recommended boosters for some people with weakened immune systems, such as cancer patients and organ transplant recipients. If the shots are expanded as expected to other Americans, among the first to receive them could be health care workers, nursing home residents and other older people.

Some experts have expressed concern that a new campaign calling for boosters could muddle the public health message and undercut the continuing drive to win over the tens of millions of Americans who are skeptical or hesitant to get their first shots.

“We have to really make sure that while we’re spending a lot of time and effort on third doses that we don’t undermine our campaign for first vaccinations,” Lawrence Gostin, a public health specialist at Georgetown University, said Tuesday. “That’s truly the existential crisis in the United States.”

Calling for third doses could discourage people who had been skeptical of the shot’s effectiveness in the first place, Gostin warned.

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The step could also cause ill will toward the United States: Global health officials, including the World Health Organization, have called on wealthier nations to hold off on booster shots to ensure poor countries have enough vaccine for the initial doses.

Dr. Tlaleng Mofokeng, a South African expert advising the United Nations, condemned the move toward booster shots in the U.S., saying it will have the effect of “advancing and deepening the existing inequities” when “there are people who are yet to receive a single shot.”

Gostin likewise called the plan “tone-deaf to the rest of the world that can’t even get any doses.”

Booster shots would only begin to be administered widely once the U.S. Food and Drug Administration formally approves the vaccines, which are being dispensed for now under what is known as emergency use authorization. Full approval of the Pfizer shot is expected in the coming weeks.

More than 198 million Americans have received at least one dose of a COVID-19 vaccine, or 70% of those who are eligible, according to the Centers for Disease Control and Prevention. Just under 60% of Americans 12 and older are fully vaccinated.

The vaccination drive has been slower than the Biden administration had hoped for. At the same time, the variant is spreading aggressively through unvaccinated communities and also causing an increasing number of “breakthrough infections” of fully inoculated people.

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Studies show the vaccine remains highly protective against severe COVID-19, but results from Israel released last month suggest its effect wanes. Its effectiveness against symptomatic infection peaked at 96% two months after study participants got their second dose. Four months later, it was down to 90%. By six months, it was about 84%.

Israel, which exclusively administered the Pfizer shot, has been offering a booster to people over 50 to control its delta surge. Researchers are still trying to understand to what extent the breakthrough infections are due to waning immunity or vulnerability to the delta variant.

On Tuesday, European medical regulators said they are talking with vaccine developers about the need for boosters but haven’t made any decisions.

The new urgency from U.S. officials reflects how quickly the variant has knocked the country back on its heels.

On July Fourth, President Joe Biden proclaimed that the nation was declaring its independence from the virus. But since then, infections, hospitalizations and deaths have increased nationwide, overloading emergency rooms across parts of the South and West.

On Monday, Pfizer and its partner BioNTech announced they submitted data to the FDA to support authorizing a booster shot for the general public. Pfizer said a small study showed people who received a third dose had higher levels of antibodies against several versions of the coronavirus, including the delta variant. The company is working on a larger study.

Americans who received the earliest doses of Pfizer’s vaccine — mainly health care workers and nursing home residents — are approaching the eight-month mark from when they received their second dose.

“There is a concern that the vaccine may start to wane in its effectiveness,” the director of the National Institutes of Health, Dr. Francis Collins, said over the weekend. “And delta is a nasty one for us to try to deal with. The combination of those two means we may need boosters, maybe beginning first with health care providers, as well as people in nursing homes, and then gradually moving forward” with others.

The Pfizer and Moderna vaccines are administered in two doses. Officials are continuing to collect information as well about the one-shot Johnson & Johnson vaccine, which was only approved in the U.S. in late February, to determine when to recommend boosters.

The White House has said that even though the U.S. has begun sharing more than 110 million vaccine doses with the rest of the world, the nation has enough to deliver boosters to Americans.