Bloodshift by Garfield Reeves-Stevens

"In London, crossbow wielding Jesuits and superhuman assassins battle to the death over an unidentified coffin. In Washington, the top secret Nevada Project struggles to hide the truth behind its deadly viral research. And in New York, the members of an ancient cult plot to kill one of their own... to keep him from revealing their apocalyptic secret.

Now, Granger Helman--an ex-mob hitman--is being forced out of retirement. His target is a beautiful woman named Adrienne St. Clair. She is the most dangerous prey he's ever stalked. For she isn't human. She isn't even alive. She's a vampire. And Granger Helman is falling in love with her..." (from back of book)

Paperback Info: ISBN 0770422446, Bantam Books, 1981