Short answer: it's internally rolling but, like other non-rolling internally-rolling schools, everyone is given a score. Only at the end of the season, the top scoring applicants get accepted.
The following passage is adapted from the HST
Connector.
There is one
HST committee, which contains three subcommittees, and one main
committee with four NP subcommittees. There is a separate MD/PhD
subcommittee.
All interviewed candidates are discussed in one of the subcommittees, with consideration given to academic and extracurricular records, letters of recommendation as well as reports from the interview. Approximately half of the interviewed applicants are then advanced to the main committee [this may be different for NP].
This committee then assesses these applicants on the basis of their application and the reports from interviewers and the subcommittee along the model of NIH study sections. A primary and two secondary reviewers presents the applicant, discussing strengths and weaknesses and recommending rough scoring ranges.
Following group discussion, applicants are scored by all members of the group, and these results are used to determine a rank order for admissions.
Subcommittees meet throughout the season, I believe, because I overheard the HST admissions director talking to a faculty member about attending the subcommittee meeting the next week (in December).
This set of recommendations is then presented to the HMS Admissions Executive Committee, which accepts them as proposed, usually.
For HST, last year, 165 students were interviewed, 80 advanced to the HST committee, ~40 were accepted.