Atlanta ? The groundbreaking vaccine that prevents cervical cancer in girls is gaining a reputation as the most painful of childhood shots, health experts say.

Health officials have touted the Gardasil vaccine as an important new protection against a cancer-causing sexually transmitted virus. In recent months, they’ve also noted reports of pain and fainting from the shot.

During its first year of use, reports of girls fainting from vaccinations climbed, but it’s not clear whether the pain of the cervical cancer vaccine was the reason for the reaction.

The pain is short-lived, girls say; many react with little more than a grimace. But some teens say it’s uncomfortable driving with or sleeping on the injected arm for up to a day after getting the shot.

Officials at Merck & Co., which makes the vaccine, acknowledge the sting. They attribute it partly to the virus-like particles in the shot. Pre-marketing studies showed more reports of pain from Gardasil than from dummy shots, and patients reported more pain when given shots with more of the particles.