In this paper by Dave Hawkes, Candice Lea, and myself, we addressed some common questions about the HPV vaccine and its use in preventing cancer:
Q1: How do we know the HPV vaccine will prevent cancer?
A1: The first HPV vaccine was registered for use in Australia in 2006. Because of the long lead time from HPV infection to the development of cancer, we are currently unable to definitively measure the success of HPV vaccination in reducing the incidence of cervical, or other HPV linked cancers. However, HPV vaccination has already been shown to reduce both HPV infections and HPV-associated pre-cancerous cervical lesions.
The reason we expect a reduction in the rate of certain cancers is by understanding how the virus works. The HPV virus triggers a series of genetic changes, specifically changes in the genes that regulate additional cancer-causing genes. Over time these cells replicate, leading to pre-cancerous lesions in some cases and cervical cancer in others. HPV is associated with 99.7% of cervical cancers and is considered a necessary causative factor of cervical cancer. This is despite the knowledge that not every HPV infection progresses to CIN and then to cancer.
Q2: Why is the vaccine being given to boys as well?
A2: HPV vaccination of boys has two major benefits; firstly, it will reduce the transmission of HPV to women and secondly, HPV infection is associated with a number of cancers which males are susceptible to, such as; cancers of the penis (40% HPV association), cancers of the anus (90% HPV association), mouth (3% HPV association) and throat (12% HPV association).
Q3:The vaccine only targets some types of HPV? What about the others?
A3: The two most commonly used vaccines target types16 and 18 for Cervarix® or types, 6, 11, 16, and 18 for Gardasil®, which is the vaccine commonly used in Australia. Strains 16 and 18 are the most common types linked to cervical cancer, while types 6 and 11 are linked to genital warts.
The vaccine has also been shown to reduce infection with some cancer-associated HPV types that are closely related to those in the vaccines types we vaccinate against.
Q4: How effective is the HPV vaccine?
A4: The vaccine provides immunity for HPV types 16/18 in 95% of people who take the recommended course of doses. The current evidence does not show a reduction in protection (as measured by immunoreactivity) over time.
Q5: Is the vaccine safe?
A5: Adverse events have been reported following HPV vaccination, but the overwhelming majority of these reactions are minor and largely local injection site reactions (e.g. redness, swelling, pain at injection site). These reactions do include other minor self-limiting reactions such as syncope (fainting episodes), headache, and nausea. In addition to our paper, the CDC has a useful summary of minor to moderate reactions of short duration for both types of HPV vaccines and other vaccines. These are consistent with other vaccinations.
Matters of general vaccine safety have been previously covered in-depth in the Australian Academy of Science’s booklet, The Science of Immunisation: questions and answers, and the paper Vaccine Components and Constituents: Responding to Consumer Concerns.
Q6: What about reported deaths from the HPV vaccine?
A6: In 2009, a study investigated 32 deaths attributed to Gardasil® reported by the public on VAERS (Vaccine Adverse Event Reporting System). Of the 32 deaths, there was not enough information to identify the person and investigate the cause of death in 12 cases. The cause of the remaining 20 deaths were: 2 due to diabetes, 3 due to pulmonary embolism, 6 were cardiac-related, 2 were idiopathic seizure disorders, 4 were unexplained, 1 was due to juvenile amyotrophic lateral sclerosis, 1 case of meningoencephalitis (inflammation of the brain and surrounding membrane), and the final death was related to prescription drug abuse. The authors concluded that statistically these results were not significantly different from what you would expect from a similar sized un-vaccinated population.
For references and more details please refer to the paper. I appreciate the work of my co-authors on both the paper itself and this blog.