But this summer, Ms. Lorenzin’s successor in the new populist government, Giulia Grillo of the Five Star Movement, sought to loosen the requirements, at one point offering something she called “flexible obligation.” Ms. Grillo, who declined to comment for this article, finally settled on the extension for students without proof of vaccination.

“Nothing has changed; it’s exactly like it was,” Rocco Casalino, the spokesman for the Five Star Movement, said after the law was approved on Thursday — raising the question of why, then, it was passed at all.

On Thursday, Five Star senators said they would next seek to undo the vaccine law altogether.

“Unfortunately, the state has diffused ambiguous information that can confuse the parents who are trying to decide whether or not to vaccinate their kids,” said Roberto Burioni, a leading immunologist at Università Vita-Salute San Raffaele and a bête noire of the anti-vaccine movement. (One mother posted a Facebook message saying she hoped he’d drowned on his summer vacation.)

“The state should be clear, and be clearly on the part of science,” Mr. Burioni said.

The measles vaccine, which over the past 50 years has helped eliminate the disease in the United States, was introduced in Italy in 1976. The percentage of coverage climbed steadily to more than 90 percent in 2003. But the obligation was eased in 1999, Ms. Lorenzin said, because Italy had mistakenly believed it had ingrained the belief in science and vaccines into the culture.

Instead, she said, the opposite happened. She attributed the surge of distrust in vaccines to the fact that the diseases had become so rare as to seem unreal, to the debacle of a since-rejected and retracted report in the scientific journal The Lancet linking vaccines to autism, and to what she called “Doctor Google.”

By 2015, coverage had declined to about 85 percent. Cases of measles, which can cause blindness, brain inflammation, pneumonia and in some cases death, have been rising.