HBS does not subscribe to Nielsen data directly. However Nielsen data may be incorporated into other subscription databases. 

MBAs and Harvard students:
You may wish to check the eMarketer, Statista, and analyst reports in Refinitiv Workspace; these resources sometimes include Nielsen data in their reports.

HBS faculty and doctoral students: 
Harvard does license Nielsen data through The Kilts Center for Marketing at Chicago Booth.  However, the terms are quite restrictive: access to the data is available only to faculty and PhD students under the supervision of a faculty member.  In addition, only certain Nielsen data sets are included: consumer panel, retail scanner, and ad intel data.