The growing consensus within the administration that at least some Americans will need a booster is tied in part to research suggesting that the Pfizer vaccine is less effective against the coronavirus after about six months. More than half of those fully vaccinated in the United States so far have received Pfizer’s vaccine, in two doses administered three weeks apart.

Pfizer’s continuing global study of its clinical trial participants shows that four to six months after the second dose, the vaccine’s effectiveness against symptomatic infection drops from a high of 95 percent to 84 percent, according to the company.

Data from the Israeli government, which has fully vaccinated more than half of its population with Pfizer doses since January, also points to a downward trend in effectiveness over time, although administration officials are viewing that data cautiously because of wide margins for error.