Can a prison official claim qualified immunity from liability in a civil case?

A prison official, however, may claim qualified immunity from suit in certain circumstances. This affirmative defense shields public officials from liability for their discretionary acts that do "not violate clearly established statutory or constitutional rights of which a reasonable person would have known." Harlow v. Fitzgerald, 457 U.S. 800, 818, 102 S.Ct. 2727, 2738, 73 L.Ed.2d 396 (1982). Even where, as here, a plaintiff's federal rights are well-established, qualified immunity is still available to an official if it was "objectively reasonable for the public official to believe that his acts did not violate those rights." Kaminsky v. Rosenblum, 929 F.2d 922, 925 (2d Cir.1991) (citations omitted).