(CNN) The human papillomavirus or HPV spreads through intimate contact during sex with someone who is infected. In some women, an HPV infection will persist and lead to cervical cancer. HPV vaccines protect against cervical cancer in young women, especially when the women are vaccinated between the ages of 15 and 26, a new Cochrane Report finds.
The report's authors, who examined evidence from 26 previously published studies of more than 70,000 women, also found no serious side effect risks associated with the vaccines.
Most people who have sexual contact will be exposed to HPV at some time. Even if exposed, most women will clear the viral infection naturally. However, a persistent infection could lead to abnormal cervical cells, called cervical "precancer" since these cells can slowly progress to cancer if not treated.
"Cervical cancer is the fourth most frequent cancer in women in the world," said lead author Dr. Marc Arbyn of the Unit of Cancer Epidemiology at the Belgian Cancer Centre in Brussels. More than half a million cases are diagnosed each year, and about half of these women will die of the disease, he said.
Though there are many types of HPV, only some strains raise a woman's risk for developing cervical cancer; HPV16 and HPV18 account for about 70% of all cases of cervical cancer worldwide.
For the new review, Arbyn and a team of researchers evaluated study evidence for two commercially available HPV vaccines: Cervarix, which targets HPV16 and HPV18 only, and Gardasil, which takes aim against those strains plus two HPV types that cause genital warts.
The World Health Organization recommends HPV vaccination for both girls and boys between the ages of 9 and 14. The United States, the UK, Australia and nations in the European Union all have similar if slightly different guidelines, though only some include boys. Since not all the studies in the review included boys, the researchers did not examine evidence of HPV vaccine effectiveness pertaining to men.
None of the studies reviewed by Arbyn and his team tracked the women for more than eight years -- not long enough for cervical cancer to develop. So, to measure the effectiveness of each vaccine, the researchers looked at rates of precancer.
In women between the ages of 15 and 25 who tested negative for HPV before receiving the vaccine, the risk of precancer associated with HPV16 or HPV 18 was 164 for every 10,000 unvaccinated women, compared with just two per 10,000 vaccinated women. The vaccines also reduced the risk of any cervical precancer (whether caused by the two HPV types or not) from 287 to 106 for every 10,000 women.
Among the same age women, who tested either positive or negative for HPV, the vaccines reduced their risk of cervical precancer associated with HPV16 or HPV 18 from 341 to 157 for every 10,000 women, according to the new report. HPV inoculation also reduced the risk of any precancer from 559 to 391 per 10,000 in this group.
In older women -- those vaccinated between the ages 25 and 45 -- the effects of HPV vaccine were smaller, the authors estimate. Unvaccinated versus vaccinated, the risk of precancer associated with HPV16 and HPV18 dwindled from 145 to 107 for every 10,000 women. This is probably due to the women having been previously exposed to HPV, the authors said, adding that the vaccine did not provide protection against any cervical precancer.
Despite claims and reports that the vaccine causes neurological issues -- including seizures -- Arbyn and his colleagues found local reactions, such as a swollen arm, when receiving either of these two vaccines but "no increased incidence of serious adverse effects." However, the reviewers added that "evidence on rare potential harms ... are difficult to capture" in the types of studies reviewed.
Dr. Jo Morrison, a consultant in gynecological oncology at Musgrove Park Hospital in Somerset in the UK, said the claims that the vaccine damages young girls "are not substantiated by the evidence." Morrison, who was not involved in the study, added, "going down this road risks causing harm by reducing vaccination rates."
Overall, Morrison believes "the review reassures people that HPV vaccination is effective. They should be encouraged to vaccinate their daughters, as per the government recommendation."
Helen Bedford, a professor at University College London's Institute of Child Health, noted that "HPV vaccine was introduced 10 years ago for 13- to 14-year-old girls to prevent infection with HPV, which can lead to cancer of the cervix."
"This, together with early evidence of reduction in cervical cancer in Finland, confirms the groundbreaking value of this cancer-preventing vaccine," said Bedford, who was not involved in the study. "It also provides reassuring evidence of the safety of HPV vaccines."