The Biden administration is planning to announce that most Americans who have received the coronavirus vaccine will need booster shots to combat waning immunity and the highly transmissible delta variant that is sparking a surge in covid-19 cases, according to four people familiar with the decision.

The administration’s health and science experts are coalescing around the view that people will need the boosters eight months after being fully vaccinated, according to the people, who spoke on the condition of anonymity to discuss a decision not yet public. The move is likely to be announced as soon as this week.

Administration of boosters would not occur until mid- or late September, after an application from Pfizer-BioNTech for the additional shots is cleared by the Food and Drug Administration, the individuals said.

The conclusion that boosters will be broadly needed was reached after intense discussions last weekend involving high-ranking officials who scrutinized the latest data from the United States and other countries on the effectiveness of the shots.

It is a striking change from public statements by senior officials in recent months who had said it was far too soon to conclude that Americans would need booster shots. In July, the Centers for Disease Control and Prevention and the FDA put out an unusual statement that said, “Americans who have been fully vaccinated do not need a booster shot at this time.” Officials have repeatedly said it was not clear whether boosters would be needed.

But in recent days, the messaging has started to change. As data from other countries and the United States showed waning immunity, health officials moderated their language, hinting that booster shots would be likely. Last week, Anthony S. Fauci, the White House chief medical adviser, said it was “likely” everyone will need a coronavirus booster at some point.

The White House on Monday night declined to comment.

The timing of the announcement remains in flux. It had been tentatively planned for Wednesday, but it was not clear whether the schedule would change. The individuals said the statement was likely to provide additional details on who would get the extra shots and when.

The question of boosters has become increasingly fraught as the pandemic has unfolded, with the ferocity of the delta variant surprising scientists. Data continues to accumulate suggesting that vaccines lose some anti-virus potency over time, but officials have been reluctant to highlight that fact because they are still trying to persuade broad swaths of Americans to get vaccinated — which is considered the best way to exit the pandemic. They are also not sure how much of the reduction in protection is from the passage of time and how much is attributable to the variant.

At the same time, the World Health Organization has criticized more affluent nations for moving ahead with plans to provide boosters. The WHO has called for a halt to booster shots at least through September, saying it would be better — more ethical and more effective in taming the pandemic in the long run — to use the shots to inoculate people overseas.

But U.S. officials have become increasingly concerned as data, some of it not yet peer reviewed, emerges showing a decrease in effectiveness amid a growing number of cases caused by the highly transmissible delta variant. At least 1 million Americans have gone ahead and received additional shots on their own, even though that practice has not been officially greenlit.

“I think delta changed everything,” said one of the people familiar with the decision.

Israel just released data showing that for people vaccinated in January who are 65 years and older, the Pfizer vaccine was less than 55 percent effective against severe disease and hospitalization. The decline in effectiveness, which has been showing up in Israeli data for several weeks, has prompted that nation to begin administering a booster shot to people age 50 and above.

Asked about the effect of the Israeli information on U.S. decision-making, a federal official said: “It’s very relevant and potentially important. … Other data are tending to corroborate what they are seeing, so we have to take it seriously.”

Some officials also are scrutinizing data released last week from the Mayo Clinic, which found that the effectiveness of the Pfizer vaccine had fallen to 42 percent in July. The vaccine remained highly effective against severe cases that could result in hospitalization.

The booster decision comes during an ongoing struggle to persuade a significant portion of the U.S. population to get vaccinated at all against the coronavirus.

Just over 50 percent of Americans are fully vaccinated against the coronavirus, and although vaccinations have been increasing in recent weeks, millions of Americans remain firmly opposed. For months, administration officials had worried that conversations about boosters would undermine confidence in the vaccine and dissuade people from getting immunized. Focus groups of vaccine-hesitant Trump voters confirmed officials’ fears, but health officials ultimately decided to move forward with boosters amid concerns about the delta variant and studies showing the waning effectiveness of the vaccines.

On “Fox News Sunday,” Francis S. Collins, director of the National Institutes of Health, said the combination of waning effectiveness and the “nasty” delta variant “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, such as older Americans who were among the first to get vaccinated.

The FDA last week authorized a third dose of the Pfizer-BioNTech and Moderna vaccines for people who are immunocompromised, including patients taking immunosuppressive drugs because of organ transplants or those receiving cancer treatment.

The booster decision comes before the FDA has granted emergency authorization for children younger than 12 to receive the vaccines. The delay in vaccines for children has worried parents and teachers as more children have been hospitalized in recent weeks because of the delta variant and schools are starting to reopen.