WASHINGTON — Almost a month ago, President Biden announced a plan to make coronavirus booster shots available to most adults in the United States eight months after they received their second dose. But a week before the plan is to roll out, its contours are up in the air amid a chorus of dissent inside and outside the government.

The White House has already been forced to delay offering boosters to recipients of the Moderna vaccine, and for now it is planning third shots only for those who received the Pfizer-BioNTech vaccine. Depending on what two public health agencies decide in the coming days, the administration may have to change course again, perhaps restricting extra shots to older Americans and others who are particularly vulnerable to serious illness.

A series of dueling reviews this week illustrated the fierce argument among scientists about whether boosters are needed, and if so, for whom. A study released on Wednesday in The New England Journal of Medicine appears to bolster the case made by the White House and its senior health advisers, stating that those who received a third shot of the Pfizer vaccine in Israel were far less likely to develop severe Covid than those who received two injections.