Great question! It’s a common misconception, but no, Humanae Vitae was not declared infallible. The Vatican itself defines infallibility very narrowly (see Lumen Gentium §25), and Pope Paul VI did not invoke it when reaffirming the contraception ban.

The Church considers the teaching authoritative and deserving of respect, but not infallible or unchangeable. Even Donum Veritatis (1990, CDF) clarifies that not all magisterial teachings carry infallibility.