The HPV Vaccine: What It Prevents and the Questions Parents Ask
Very few vaccines prevent cancer. The HPV vaccine does, and the evidence for it has strengthened considerably since it was introduced.
What HPV is
Human papillomavirus is a family of more than 200 related viruses, around 40 of which infect the genital area. It is extremely common; the large majority of sexually active people acquire at least one type at some point.
Most infections clear on their own within two years without causing any problem. A minority persist, and persistent infection with a high-risk type can, over years to decades, cause cellular changes that progress to cancer.
High-risk types, particularly 16 and 18, cause most HPV-related cancers. Low-risk types, particularly 6 and 11, cause genital warts.
What it causes
- Cervical cancer. Essentially all cases are caused by HPV
- Anal cancer, the large majority
- Oropharyngeal cancer, affecting the back of the throat, base of tongue and tonsils. HPV-related throat cancer has risen sharply and is now more common in men than cervical cancer is in women in several countries
- Vulvar, vaginal and penile cancers
- Genital warts
The throat cancer point is worth emphasising, because the vaccine is still often perceived as a girls’ vaccine for a women’s cancer. It is not.
What the evidence shows
Long-term population data from countries with high coverage show substantial reductions in:
- HPV infection with vaccine-covered types
- Genital warts, with sharp declines
- High-grade cervical precancerous lesions
- Cervical cancer itself. Studies following vaccinated cohorts into the age range where cancer occurs have shown large reductions, with the greatest effect in those vaccinated youngest
That last point moved the vaccine from “prevents infection and precancer” to “prevents cancer”, which is a different order of claim.
Why it is given at 11 to 12
Three reasons.
It works best before exposure. The vaccine prevents infection; it does not treat existing infection. Giving it before any sexual activity maximises benefit.
The immune response is stronger in early adolescence. Antibody levels after vaccination are higher at 9 to 14 than at older ages.
Two doses suffice under 15. Those starting at 9 to 14 need only two doses, six to twelve months apart. Starting at 15 or older requires three doses. Vaccinating earlier is therefore simpler and cheaper.
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It can be given from age 9.
The question parents actually ask
“Does this encourage earlier sexual activity?”
This has been studied directly and repeatedly, comparing vaccinated and unvaccinated adolescents on age of first intercourse, number of partners, contraceptive use and rates of sexually transmitted infections and pregnancy. No increase in sexual activity has been found.
The framing is also worth examining. The vaccine prevents a virus that causes cancer. Hepatitis B vaccine prevents a sexually and blood-transmitted virus that causes liver cancer, and is given at birth without controversy. The mode of transmission does not change what the vaccine does.
Safety
The HPV vaccine has been given hundreds of millions of times and is among the most intensively monitored vaccines in existence.
Common side effects: injection site pain, redness and swelling; headache; fever; occasionally nausea.
Fainting occurs more often than with other vaccines, which is attributed to adolescents receiving an injection rather than to the vaccine itself. Observing for 15 minutes afterwards prevents injury from a fall.
Extensive investigations by multiple national and international safety bodies have not found a causal link with chronic fatigue syndrome, postural orthostatic tachycardia syndrome, complex regional pain syndrome, autoimmune disease or infertility. Countries where coverage dropped after safety scares and later recovered have provided useful comparative data, and the pattern of claimed harms did not follow vaccination rates.
Can adults still get it
Yes, with a more individualised benefit.
Routine catch-up is generally recommended up to about 26. From 27 to 45, it is a shared decision: benefit is lower because many adults have already encountered some HPV types, but it is not zero, since few people have been exposed to all vaccine-covered types.
Having had an abnormal cervical screening result or a known HPV infection does not make vaccination pointless, since it can still protect against the other covered types.
For boys and men
Gender-neutral programmes are now standard in many countries. Reasons:
- Men get HPV-related cancers, particularly throat and anal cancer, for which there is no screening programme at all
- Men who have sex with men gain no benefit from herd protection through vaccinating girls
- Higher overall coverage reduces transmission for everyone
Screening is still needed
Vaccination does not cover every oncogenic type, and many women were vaccinated after some exposure. Cervical screening continues on the recommended schedule regardless of vaccination status, though in heavily vaccinated populations intervals are being reviewed as evidence accumulates.
There is no screening test for HPV-related throat cancer, which makes prevention the only tool available for it.
The short version
The HPV vaccine prevents several cancers, works best given before exposure which is why it is scheduled at 11 to 12, requires only two doses if started before 15, and has an extensive safety record. It is for boys as much as girls, since HPV-related throat cancer has no screening programme. And cervical screening continues regardless.