According to a new study, the HPV vaccine, which protects against viruses causing cervical cancer and genital warts is effective for young teens but not for women in their 20s.

Reported in New England Journal of Medicine, the study examined the expensive vaccine Gardasil and studied is value as a public health investment.

Gardasil, made by Merck, was licensed in 2006 for use in girls and women from ages 9 to 26 years. The Centers for Disease Control and Prevention recommends the vaccine being given to girls between the ages of 11-12 years and some doctors feel women in their 20s can get vaccinated in "catch-up" vaccination campaigns.

The new study by researchers at the Harvard School of Public Health has found the vaccine is most cost effective and is a good financial investment in public health when it is given to the age groups of 11-21 years.

The virus HPV, or human papillomavirus, is spread through sexual contact and Gardasil targets the two types of HPV said to cause about 70 % of the cervical cancer, as well as the two types that that mostly cause genital warts. Given in three doses spread over a period of six months, Gardasil costs about $ 375.

Co-author of the study, Jane Kim of the Harvard School of Public Health, feels the vaccine is most effective if given before the girls are exposed to HPV.

Carol Baker, professor of pediatrics at Baylor College of Medicine and a member of the CDC's Advisory Committee on Immunization Practices, feels that more than half of the girls have already been exposed to the HPV virus by the time they complete high school. As some women could still be protected the “catch up” shots to women up to the age of 21 years makes sense says Kim but not to women who are older.

In 2006 the vaccine was approved for women up to the age of 26 years. Dale Morse, chairman of the advisory committee says the CDC has no plans to revise its recommended age guidelines. Not just a vaccine's cost, but also its safety and potential to prevent disease, is taken into consideration by the CDC, he says.

The American Cancer Society estimated 11,000 Americans contract cervical cancer each year with nearly 4000 deaths as a result of it. The researchers in an attempt to determine the cost effectiveness of Gardasil worked out how much society is willing to pay to extend the life of a person by a year. The Gardasil vaccine was set at $43,600 for each 12 year old girl that was well below the $100,000 mark worked out as an upper range of cost effectiveness.

According to Merck about 40 % of 11-12 year olds have received the Gardasil vaccine.

Charlotte Haug of the Journal of the Norwegian Medical Association, who wrote an accompanying editorial in the New England Journal, feels that if the vaccines immunity reduces with time, cervical cancer screening, which has shown benefits might prove to be more cost effective. Spokeswoman Amy Rose says studies have shown Gardasil’s immunity to last for at least five years. Doctors plan to follow patients to see how long the immunity lasts as it’s a new vaccine.

The study was funded by the CDC, the cancer society, the National Cancer Institutes and the Bill and Melinda Gates Foundation.