WASHINGTON — The Biden administration is planning to announce that most Americans who have received the coronavirus vaccine will need booster shots to combat waning immunity from the immunizations and the highly transmissible delta variant, which is sparking a surge in COVID-19 cases throughout the nation, 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 they are fully vaccinated, according to the people who spoke on the condition of anonymity to discuss a decision not yet public. The decision is likely to be announced as soon as this week.

The actual administration of the boosters would not occur until mid- or late September, after Pfizer’s application for additional shots for the general public 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.

The statement 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 Food and Drug Administration 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 wasn’t clear whether boosters would be needed.

But in recent days, the messaging has started to change. As data from other countries and in the United States showed waning immunity, health officials moderated their language, hinting booster shots would be likely. Last week, Anthony Fauci, the White House chief medical adviser, said it was “likely” everyone will need a COVID-19 booster shot at some point.

The White House declined Monday night 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 exactly who would get the extra shots and when.

The question of boosters has become increasingly fraught as the pandemic continues to unfurl, 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 – considered the best way to exit from the pandemic. And they are not sure how much of the reduction in protection is from the passage of time and how much is due 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 organization 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 who have not received vaccines.

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. And at least 1 million Americans have gone ahead and received additional shots on their own, even though that practice has not been officially blessed.

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

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

Asked about the impact of the Israeli information on U.S. decision-making, a federal official said, “It’s very relevant and potentially important,” adding, “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% in July. The vaccine remained highly effective against severe cases that could result in hospitalizations.