As kids go back to school this week, more of them are fully immunized against infectious diseases, a new study shows.

While the number of fully vaccinated kindergarten students has always been high — with more than 90% getting most vaccines — fewer 11- and 12-year-olds get all their recommended shots.

A new report from the Centers for Disease Control and Prevention, however, shows progress among this age group.

• Tetanus, diphtheria and pertussis. About 85% of teens in 2012 received the Tdap booster shot, which protects against tetanus, diphtheria and pertussis, also known as whooping cough, according to a national survey of teens ages 13 to 17. That's up from 78% in 2011.

Widespread whooping cough outbreaks in recent years have highlighted the need for teens to get boosters. There have been nearly 15,000 cases of whooping cough so far this year, just under half the number by this time last year, according to the CDC.

• Meningitis. About 74% of teens in 2012 were vaccinated against meningitis, up from 71% in 2011. This vaccine protects against meningococcal meningitis, a rare but sometimes deadly form of bacterial meningitis, which causes an inflammation of the lining of the brain and spinal cord.

• HPV. About 21% of boys in 2012 received at least one dose of vaccine against HPV, or human papillomavirus, the family of viruses that causes genital warts and cervical cancer, in addition to several other tumor types. That rate is up from 8% in 2011. The CDC and American Academy of Pediatrics recommend three doses of HPV vaccine for both boys and girls at age 11 or 12.

About 54% of girls received at least one dose of HPV vaccine in 2012, the same as in 2011. Only 33% of girls received all three recommended HPV shots.

The CDC first recommended HPV shots for girls seven years ago, but expanded that recommendation for boys only in 2011, based on studies showing that the shots prevent genital warts in both sexes. Doctors also concluded that vaccinating boys against HPV could help protect their partners from cervical cancer.

• Chickenpox. About 75% of teens received the recommended two doses of chickenpox vaccine in 2012, up from 68% in 2011. Although chickenpox shots are supposed to be given in early childhood — a first dose at ages 12 to 15 months, and a second dose at ages 4 to 6 — the CDC recommends "catch-up" doses for older kids who missed the vaccine as toddlers.

In spite of those increases, vaccine coverage for teens still falls below national goals. Only 36 states meet the CDC's goals for Tdap coverage, while 12 meet the goals for meningitis vaccination and nine meet the goal for chickenpox, according to the new report.

While all states require kindergarten kids be vaccinated to attend school, fewer are as strict about middle-school children. Improving teen vaccination rates could be due to stricter school-entry laws, says the CDC's Jason McDonald. In the 2012-2013 school year, 40 states required Tdap vaccination for middle school students, while 13 required meningitis shots.

Health officials are concerned that the percentage of girls getting HPV vaccinations has flattened out, without any recent increase, says Kevin Ault, a professor of obstetrics and gynecology at Emory University School of Medicine in Atlanta. CDC director Thomas Frieden has estimated that raising the percentage of girls vaccinated against HPV to 80% could save 50,000 lives from cervical cancer.

Pediatricians need to be as serious "about giving vaccines to teens as we are about giving vaccines to infants," says Claire McCarthy, a pediatrician at Boston Children's Hospital. "We lose steam in adolescence and that isn't good for our patients. We routinely give multiple vaccines at once in the early years, but let ourselves be talked out of it when kids are older. We also need to work harder at convincing parents of the merits of the HPV vaccine. I think we aren't trying as hard as we could."