The Vatican said Wednesday that there has been a “slight improvement” in Pope Francis’ condition but that “the prognosis remains reserved,” as he remains in a Rome hospital for treatment of pneumonia in both lungs.

The pontiff’s CT chest scan Tuesday evening showed “normal progression of the pulmonary inflammatory condition,” the Vatican reported.

A Tuesday evening update described the pope’s condition as critical but stable.

The 88-year-old was admitted to the hospital on Feb. 14. The hospital stay is his longest during his time as pope. He is prone to lung infections after having had part of a lung removed when he was a young man.

His illness has prompted Catholics to gather in Vatican City, his native Argentina and other areas of the world to hold prayer vigils.

Francis has been pope since 2013, when his predecessor, Pope Benedict, resigned.

Some information for this story was provided by The Associated Press, Agence France-Presse and Reuters.