CHICAGO — Many pediatricians and family doctors are not strongly recommending the HPV vaccine for preteens, contributing to low vaccination rates, a survey of nearly 600 doctors suggests.

The vaccine is to protect against the human papillomavirus, which is spread through sex and can cause several kinds of cancer, including cervical cancer. The government wants girls and boys to get it at age 11 or 12 because it works best if children get it before they become sexually active.

While nearly all doctors surveyed discuss the vaccine with at least some patients that young, more than one-third don’t strongly recommend it for those ages. They were most likely to recommend vaccination and to administer the shots to older children and girls. The most common reasons doctors cited for delaying HPV discussions and vaccinations included a belief that patients hadn’t had sex and that parents would object.

The authors, led by University of Colorado researcher Dr. Allison Kempe, surveyed 582 pediatricians and family physicians by mail or online about two years ago.

The doctors were in a nationwide network that participates in similar surveys and whose views have been found to be similar to random samples of U.S. physicians, the researchers said.

The study was published online Monday in Pediatrics.