Now, at last, supply of the vaccine has built up, Ms. Nguyen said, and new versions of the shot have come to the market from companies in China, India and Indonesia. Supply is expected to triple by 2025.

Populous countries including Indonesia, Nigeria, India, Ethiopia and Bangladesh are planning to introduce or expand use of the vaccine this year, which may challenge even the expanded supply. But the hope is that there will soon be sufficient doses for countries to be able to vaccinate all girls between 9 and 14, Ms. Nguyen said. Once they are caught up, the vaccine will become routine for 9-year-olds.

“We’ve set the target of 86 million girls by the end of 2025,” she said. “That will be 1.4 million deaths averted.”

Ms. Chengo and her friends were convulsed by giggles at the mere mention of sex, but they said that in fact, many girls in their grade were already sexually active, and that it would be better when Tanzania was able to vaccinate girls at age 9.

“Eleven is too late,” said Restuta Chunja, with a somber shake of her head.

Ms. Chengo, a sparkly-eyed 13-year-old who intends to be a pilot when she finishes school, said that her mother told her the vaccine would protect her from cancer, but that she shouldn’t get any ideas.