He said religious exemptions for the vaccine go against church teachings, and priests should not provide them to parishioners.
In the letter, Cupich said the Catholic Church supports the vaccine, adding it promotes the common good.
RELATED: Cardinal Cupich leads COVID vaccine push after Catholic Church controversy
Pope Francis joined with cardinals from the Americas Wednesday in a public service announcement, urging everyone to get vaccinated.
"In this time, the best response is to be vaccinated, as a sign of charity, as a true expression of how we live out that great commandment, not only to love God but to love our neighbor," said Bishop Robert Casey, vicar general of the Archdiocese of Chicago.