Background
Human papillomaviruses (HPV) are sexually transmitted and are common in young people. Usually they are cleared by the immune system. However, when high-risk (hr) types persist, they can cause the development of abnormal cervical cells, which are referred to as cervical precancer if at least two thirds of the surface layer of the cervix is affected. Precancer can develop into cervical cancer after several years. Not everyone who has cervical precancer goes on to develop cervical cancer, but predicting who will is difficult. There are a number of different hrHPV types which can cause cervical precancer and cancer. HPV16 and 18 are the most important high-risk types, since they cause about 70% of cervical cancers worldwide. Preventive vaccination, by injection of HPV virus-like particles in the muscle, triggers the production of antibodies which protect against future HPV infections.

Review question
Does HPV vaccination prevent the development of cervical precancer or cancer and what are the harms?

Main results
We included 26 studies involving 73,428 adolescent girls and women. All trials evaluated vaccine safety over a period 0.5 to 7 years and ten trials, with follow-up 3.5 to 8 years, addressed protection against precancer. Cervical cancer outcomes are not available. Most participants enrolled were younger than 26 years of age. Three trials recruited women between 25 to 45 years. The studies compared HPV vaccine with a dummy vaccine.

We assessed protection against precancer in individuals who were free of hrHPV, free of HPV16/18 or those with or without HPV infection at the time of vaccination. We separately assessed precancer associated with HPV16/18 and any precancer.

Protection against cervical precancer

1) Women free of hrHPV

Outcomes were only measured in the younger age group for this comparison (15 to 25 years). HPV vaccines reduce the risk of cervical precancer associated with HPV16/18 from 164 to 2/10,000 women (high certainty). They reduce also any precancer from 287 to 106/10,000 (high certainty).

2) Women free of HPV16/18

The effect of HPV vaccines on risk of precancer differ by age group. In younger women, HPV vaccines reduce the risk of precancer associated with HPV16/18 from 113 to 6/10,000 women (high certainty). HPV vaccines lower the number of women with any precancer from 231 to 95/10,000 (high certainty). In women older than 25, the vaccines reduce the number with precancer associated with HPV16/18 from 45 to 14/10,000 (moderate certainty).

3) All women with or without HPV infection

In those vaccinated between 15 to 26 years of age, HPV vaccination reduces the risk of precancer associated with HPV16/18 from 341 to 157/10,000 (high certainty) and any precancer from 559 to 391/10,000 (high certainty).

In older women, vaccinated between 25 to 45 years of age, the effects of HPV vaccine on precancer are smaller, which may be due to previous exposure to HPV. The risk of precancer associated with HPV16/18 is probably reduced from 145/10,000 in unvaccinated women to 107/10,000 women following HPV vaccination (moderate certainty). The risk of any precancer is probably similar between unvaccinated and vaccinated women (343 versus 356/10,000, moderate certainty).

Adverse effects

The risk of serious adverse events is similar in HPV and control vaccines (placebo or vaccine against another infection than HPV (high certainty). The rate of death is similar overall (11/10,000 in control group, 14/10,000 in HPV vaccine group) (low certainty). The number of deaths overall is low although a higher number of deaths in older women was observed. No pattern in the cause or timing of death has been established.

Pregnancy outcomes

HPV vaccines did not increase the risk of miscarriage or termination of pregnancy. We do not have enough data to be certain about the risk of stillbirths and babies born with malformations (moderate certainty).

Conclusion
There is high-certainty evidence that HPV vaccines protect against cervical precancer in adolescent girls and women who are vaccinated between 15 and 26 years of age. The protection is lower when a part of the population is already infected with HPV. Longer-term follow-up is needed to assess the impact on cervical cancer. The vaccines do not increase the risk of serious adverse events, miscarriage or pregnancy termination. There are limited data from trials on the effect of vaccines on deaths, stillbirth and babies born with malformations.