Study: Girls Still Get Protection When Shots Are Given Months Later Than Recommended
By Brenda Goodman
WebMD Health News
Latest Sexual Health News
Reviewed by Laura J. Martin, MD
April 12, 2011 -- Delaying doses of a vaccine that protects against cervical cancer doesn't appear to make it any less safe or effective, a new study shows.
The vaccine against human papilloma virus (HPV) is given in three shots over a period of six months.
Research has shown that the vaccine is highly effective at blocking the strains of HPV responsible for causing about 70% of all cervical cancer cases.
But several recent studies have shown that most women and girls who start the shots don't get them on time, if they finish the series at all.
"This study should be very reassuring," says Kathleen M. Neuzil, MD, MPH, clinical associate professor of allergy and infectious diseases in the department of global health at the University of Washington in Seattle. Neuzil is also the senior advisor for immunizations at the international nonprofit organization PATH, which is also based in Seattle.
"Certainly clinicians and parents can be reassured that if there are delays, as we know occur, this vaccine still works very well," says Neuzil.
Other experts who have studied the problem of HPV vaccine compliance agree.
"About half of those who start the HPV series actually complete it, and really, only a quarter are completing it on time," says Emmanuel B. Walter, MD, MPH, professor of pediatrics at Duke University in Durham, N.C.
"This gives us hope that it's OK if girls get their doses late," says Walter, who published a study on HPV vaccine compliance in the March 2011 issue of Vaccine. "I say that with the caveat that we don't know exactly what protection is or how effective the vaccine is only after two doses or one dose of the vaccine."
Comparing HPV Vaccine Schedules
For the study, Neuzil and her team enrolled 903 girls between the ages of 11 and 13 at 21 different schools in rural Vietnam.
The schools were randomly assigned to give three doses of the HPV vaccine to the girls participating in the study on one of four different dosing schedules:
- The recommended schedule at 0, 2, and 6 months.
- A timetable where the shots were spaced over the school year: 0, 3, and 9 months.
- A shot every six months for one year: 0, 6, and 12 months.
- A shot every 12 months for two years: 0, 12, and 24 months.
More than 800 girls completed all three doses, and researchers gave them blood tests after each shot to measure levels of antibodies against two cancer-causing HPV strains.
Compared to girls who got their doses on the recommended six-month timetable, researchers found that girls on the 9- and 12-month dosing schedules had only slight dips in their antibody levels, which weren't expected to be clinically meaningful.
Girls who got their shots over two years had significantly lower antibody levels than the six-month group. But Neuzil points out that even those levels were still higher than have been seen in other studies of older teens and college-aged women. That suggests that spreading the shots over years may still shield girls from the cancer-causing virus.
Neuzil acknowledges, however, that nobody knows the magic number for antibodies against HPV. "We don't know what level of antibody protects."
Side effects in the study were mostly mild, with many girls complaining that their arms were sore after the shots. About 1% complained of more serious reactions, including weakness, nausea, and vomiting.
The study was funded by the Bill and Melinda Gates Foundation. Drugmaker Merck provided the vaccine doses.
The study is published in TheJournal of the American Medical Association.
A Case for Flexible HPV Vaccine Schedules
"We know we have trouble getting adolescents vaccinated," says Lauri Markowitz, MD, team lead for epidemiology research in the division of STD prevention at the CDC. "They don't go to the doctor's office as often as young children do. It's challenging to get them to finish on time."
Though the official schedule is still a shot at 0, 2, and 6 months, public health authorities have acknowledged that there's some flexibility with that time frame.
"Right now, we recommend that if someone's late for a vaccine dose, it doesn't need to be repeated, you just complete the schedule," says Markowitz.
And there are two studies, one funded by the CDC and the other funded by National Institutes of Health, that are testing later dosing schedules to see how far the timeline can be stretched, especially between the second and third shots.
"We're starting to build a case for more flexible schedules," Neuzil says.
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