A month and a half after announcing that it will switch to a nonbinding early action policy this fall, the Yale admissions office filled 43 percent of the class of 2007 with early decision applicants.

Yale accepted 557 students from a pool of 2,611, yielding a 21.3 percent early acceptance rate. While the number of applicants increased by 23 percent from last year, the number of students accepted increased by less than one and a half percent.

The percent of the class that Yale filled early reflects similar numbers from other schools, including Stanford University, which admitted 37 percent of its incoming class under early decision. Stanford officials also announced in October that they would switch to early action beginning this fall.

The University of Pennsylvania filled 47 percent of its incoming freshman class with early applicants, while Columbia University filled 43 percent of its class early. Dartmouth College and Cornell University filled 37 and 36 percent of their incoming classes, respectively, through early decision.

Yale President Richard Levin said the University may admit slightly more students early next year because of the switch to early action and concerns about yield — the percent of accepted students who enroll.

“It’s likely that we’ll make even more offers,” Levin said. “We may have to be a little cautious the first year until we get a sense of how many students will accept our offer.”

Dean of Admissions Richard Shaw said it is hard to say now what next year’s numbers will look like, but early action will present new challenges for the admissions office. He said the wait list may be more active next year as a result.

Shaw said he expects the number of applicants to increase next year but is unsure whether more exceptional students will apply early, triggering more acceptances.

“Because early [action] is nonbinding, the question will be whether the candidates will be as strong or that more kids will be taking a shot at it,” he said. “The kids we admit are extraordinarily powerful, but the 23 percent increase didn’t result in a 23 percent increase in the most powerful — the overall quality of the pool did not increase with the size of the pool.”

Shaw said next year will certainly be more unpredictable than this year.

“My sense is that we’ll probably lose some percentage of the kids we admit and that will impact the overall yield, that will make the spring somewhat more unknown,” he said.

This year, 48 percent of applicants applied for financial aid, which Shaw said is about the same as last year.

Yale accepted 29 international students, also in line with last year’s figure. Last year, the number of international applicants rose dramatically after Yale extended need-blind admission to students outside the United States.

The University deferred about 1,000 students, who will be reconsidered with the main pool of applicants, of which approximately one in 10 will be offered admission in the spring, Shaw said. The number of applicants rejected was about equal to the number deferred.

“We want to not lead them on,” Shaw said. “We think that’s an important policy, that we let kids know if we’re sure it’s not going to happen, rather than just rolling them forward as other institutions do.”

The admissions office offered online notification to early applicants, following a practice that the University began last year. This spring, Princeton admissions officers illegally accessed Yale’s admissions Web by using students’ Social Security numbers. To avoid such problems this year, students were assigned personal identification numbers to access their decisions online.

Shaw said the online notification was “perfect” and that 2,344 students — about 90 percent of all applicants — had accessed the site by Dec. 16, three days after the site went live.

“The personal identifiers worked very well,” Shaw said. “That level of security was very high, so we’re quite happy.”

Shaw said international applicants benefited most from the online system because mail to other countries can be very unpredictable, but that all applicants seemed pleased by it.

“What tells the story is, in years past on that Monday or Tuesday after a mailing, the phones would be endlessly ringing and now it’s almost quiet because a lot of that information has been relayed already,” he said. “Frankly, it suggests that that’s it, that’s the new way to do business.”