A new meta-analysis of 26 studies from around the world has found human papilloma virus (HPV) vaccines protect against cervical lesions in young women, particularly in those who are vaccinated between the ages of 15 and 26.

Most people who have sexual contact at some point in their life will be exposed to the human papilloma virus (HPV). In the majority of women, HPV infection will be cleared by the immune system without intervention. When the immune system does not clear the virus, persistent HPV infection can cause abnormal cervical cells. These lesions are known as cervical ‘precancer’ because over time they can progress to cervical cancer, if left untreated.

There are many different types of HPV. Some are associated with the development of cervical lesions that can become cancerous; these are considered high-risk HPV strains. Two of these high-risk types (HPV16 and HPV18) account for about 70% all cases of cervical cancer worldwide. Vaccines have been developed that help the immune system recognize certain HPV types, fight them off and prevent them from infecting cells.

A team of Cochrane researchers has summarized results of 26 studies in 73,428 women conducted across all continents over the last eight years. Most women in the studies were under the age of 26, although three trials recruited women between 25 and 45 years. The studies were well-designed, randomizing the women to either HPV vaccine or a placebo. The review evaluates evidence for two vaccines: the bivalent vaccine targeting HPV16 and 18, and the quadrivalent vaccine targeting HPV16/18 as well as two low-risk HPV types causing genital warts. The newer vaccine that targets nine HPV types was not included in the review since it has not been compared against a placebo in a randomized controlled trial.

The review looked at two groups of people: women who are free of high-risk HPV at the time of vaccination and all women regardless of HPV status at vaccination. The effects of the vaccine were measured as precancerous lesions associated with HPV16/18 and precancerous lesions irrespective of HPV type. Because cervical cancer can take several years to develop, regulatory bodies and international health agencies such as the World Health Organization (WHO) regard cervical lesions as the preferred outcome measure for HPV vaccine trials. The review looked at data from 10 trials assessing cervical lesion data at between three and a half to eight years after vaccination.

The studies found that in young women who did not carry HPV, vaccination reduced the risk of developing precancerous lesion. About 164 per 10,000 women who got placebo and 2 per 10,000 women who got the vaccine went on to develop cervical precancer.

The researchers also looked at data from all enrolled women regardless whether they were free of high-risk HPV at vaccination or not. Among women aged 15 to 26 years, vaccines reduced the risk of cervical precancer associated with HPV16/18 from 341 to 157 per 10,000. HPV vaccination reduced also the risk for any precancer lesions from 559 to 391 per 10,000.

In older women vaccinated between 25 to 45 years the HPV vaccine does not work as well. This might be because older women are more likely to have been exposed already.

The evidence also shows that the vaccines do not appear to increase the risk of serious side effects, which occur in about 7% in both HPV vaccinated or control groups. The researchers did not find increased risk of miscarriage in women who became pregnant after vaccination. However, they emphasize that more data are required to provide greater certainty about very rare side effects and the effect vaccines have on rates of stillbirth, and babies born with abnormalities in those who became pregnant around the time of vaccination.

The analysis shows the benefits of HPV vaccines outweigh any potential risks, says Cochrane lead author, Dr. Marc Arbyn, of the unit cancer epidemiology, Belgian Cancer Centre, Sciensano.

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“Vaccination aims to prime the immune system to produce antibodies that can block subsequent natural HPV infection,” says Dr. Jo Morrison, consultant in gynaecological oncology at the Musgrove Park Hospital, Somerset, UK. “These data show that immunizing against HPV infection protects against cervical precancer, and it is very likely that this will reduce cervical cancer rates in the future. However, it cannot prevent all cervical cancer and it is still important to have regular screening, even if you have been vaccinated.”

This study is the latest in a recent wave of research that has shown the HPV vaccine’s efficacy in reducing cervical cancer rates. Sadly, while individual states in India have recently included the vaccine in public health programs, the vaccine was left out of the national immunization programme again this year.