Ever since the introduction of vaccines against the human papillomavirus (HPV), scientists have been finding indirect evidence of the millions of cancers they will prevent. Now the missing piece of the puzzle has been slotted into place, with research showing vaccinated women have half the rate of cervical cancer as those who never got the injections. Early vaccination creates a much larger benefit.
Certain strains of HPV are considered the cause of almost all cervical cancers, as well as initiating smaller numbers of cancers of the penis, rectum, and vagina. Other HPV strains cause genital warts. The Gardasil and Cervarix vaccines prevent the transmission of the most lethal strains.
It is therefore logical to conclude vaccination would slash the rates of these cancers, saving millions of lives. However, there can be decades between infection with the virus and the cancer's appearance. Mass vaccination programs started worldwide once trials concluded the vaccines were safe and prevented infection, without waiting for confirmation cancer rates would fall.
Now at last that decision has been vindicated. Dr Jiayao Lei of Sweden's Karolinska Institutet has compared outcomes for more than 500,000 women worldwide who were vaccinated against HPV with 1.2 million others of similar ages and demographics who were not.
In the New England Journal of Medicine, Lei and co-authors report the rate of cervical cancer for unvaccinated women was 94 per 100,000 over an 11-year period. For those that were vaccinated, the rate was 47 per 100,000.
A 50 percent reduction, while an immense achievement, is far below what health experts had hoped from the vaccines. However, Li found the reduction among those vaccinated before they turned 17 was 88 percent. Presumably many of those who got the shots later gained little benefit because they had already been exposed to the relevant strains.
The results are very much in line with expectations. Study after study had shown more immediate benefits from the vaccination, including the near-disappearance of genital warts among those vaccinated with Gardasil and dramatic falls in cervical abnormalities that could subsequently turn into cancers. There have even been applications against existing skin cancers.
Nevertheless, contrary to the claims of the anti-vaccination movement, scientists prefer to avoid assumptions on something this important, so final confirmation has been long-awaited.
"This is the first time that we, on a population level, are able to show that HPV vaccination is protective not only against cellular changes that can be precursors to cervical cancer but also against actual invasive cervical cancer," Lei said in a statement.
Vaccination against HPV attracts opposition not only from the usual anti-vaxxers but from those who claim it will “encourage teenage promiscuity”. The authors strongly endorse continued vaccinations early enough to minimize the chance of HPV infection.