New York City (NYC) introduced a universal prekindergarten program in 2014 that mandated influenza vaccination for enrollment. We conducted a difference-in-difference-in-differences study to evaluate the program using 2012 to 2019 MarketScan claims data. After the introduction of the program, influenza vaccine uptake among four-year-old children in NYC during the subsequent seasons increased by 6.3 to 9.8 percentage points compared with the rest of New York State. (author abstract)