The most recent issue of the Catholic Medical Quarterly (UK) is dedicated to the 45th anniversary of Humanae Vitae. Included as an editorial is the last known letter by Padre Pio, written to Pope Paul VI on September 12, 1968:
… I know that your heart is suffering much these days … for the lack of obedience of some, even Catholics, to the high teaching that you, assisted by the Holy Spirit and in the name of God, are giving us. I offer you my prayers and daily sufferings as a small but sincere contribution on the part of the least of your sons in order that God may give you comfort with his Grace to follow the straight and painful way in the defense of eternal truth, which never changes with the passing of the years. Also, in the name of my spiritual children and the Prayer Groups, I thank you for your clear and decisive words that you especially pronounced in the last encyclical “Humanae Vitae”; and I reaffirm my faith, my unconditional obedience to your illuminated directions.
Read the entire letter. Among the other many excellent articles is an essay, “The infallible teaching of Humanae Vitae”, by Fr Thomas Crean O.P. Fr. Crean argues that the encyclical meets the criteria for infallibility articulated by Vatican I: