Experts who carried out a review of research on the human papillomavirus vaccine have hit out at “unjustified claims” that it can harm women for fear these could prevent them taking up the jab.
A Cochrane review, which examined 26 studies carried out across the world over the past eight years, concluded the vaccine was effective in preventing pre-cancerous cervical cells in young women, although it does not work as well in those aged 25 to 45.
“The findings of this review should be viewed within the context of multiple global surveillance studies”
Meanwhile, evidence from robust studies carried out to date does not appear to suggest the vaccine increases the risk of harmful side effects as some others have claimed, said those behind the review.
Campaigns against the vaccine in some countries, including Ireland, Japan and Denmark – where it has been blamed for seizures, walking problems and neurological issues – have seen vaccination rates plummet.
Yet the team of Cochrane researchers, writing for the highly respected library of systematic reviews, said they “did not find an increased risk of serious adverse effects”.
The team carried out a detailed analysis of the findings from the studies, which involved more than 73,400 women in total – most under the age of 26. their findings have been published in the Cochrane Database of Systematic Reviews 2018.
“Long-term follow-up studies are needed to find out the effects of HPV vaccination on cervical cancer rates”
The review looked at evidence for two types of HPV vaccine – the bivalent vaccine targeting HPV16 and 18, and the quadrivalent vaccine targeting HPV16 and 18 as well as two other “low risk” types of HPV that cause genital warts.
A new vaccine that targets nine types of HPV was not included in the review, because it has yet to be compared to a placebo in a randomised controlled trial.
The review explored the effect of the vaccine on all women, whether or not they had a form of HPV at the time they were vaccinated. It also looked at the impact on those who were clear of a “high risk” form of the virus when they were immunised.
The researchers were able to look at whether the vaccine had prevented the development of abnormal cervical cells – known as cervical lesions or cervical “pre-cancer”.
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But they could not judge whether the vaccine had helped prevent cervical cancer as it could take years to develop following HPV infection, and none of the trials they looked at had been running for long enough.
In young women who did not carry HPV, they found the vaccine significantly reduced the risk of developing cervical pre-cancer.
About 164 per 10,000 women who received a placebo went on to develop pre-cancerous cells, compared with just two in 10,000 women who received the vaccine.
The results for all women aged 15 to 26 showed the vaccines reduced the risk of cervical pre-cancer linked to HPV16 and 18 from 341 to 157 per 10,000 women. Meanwhile, vaccination reduced the risk of any type of pre-cancer lesions from 559 to 391 per 10,000.
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Of the 26 studies, just three recruited women aged between 25 and 45 and there was evidence the vaccines did not work as well in this older age group, probably because they were more likely to have been exposed to the virus already.
When it came to side effects, the reviewers said the evidence showed the vaccines did not appear to increase the risk of “serious adverse events”, which was about 7% among both those who had been vaccinated and controls.
Looking at the impact on women who became pregnant during the trials, the researchers did not find an increased risk of miscarriage or termination.
However, they said more research was needed to provide greater certainty on very rare side effects, such as stillbirth and babies born with abnormalities.
Lead review author, Dr Marc Arbyn, from the Unit of Cancer Epidemiology at the Belgian Cancer Centre, said the findings should be seen in the context of widespread safety monitoring across the globe.
“The findings of this review should be viewed within the context of multiple global surveillance studies, which have been conducted by the Global Advisory Committee on Vaccine Safety from the World Health Organization since the vaccinations were licensed,” he said.
“The committee concluded that the risk-benefit profile of prophylactic HPV vaccines remains favourable and expressed its concerns about unjustified claims of harm that lack biological and epidemiological evidence, and which may affect the confidence of the public,” he said.
He added: “At the same time, the committee encouraged health authorities to continue surveillance and examination for potential adverse events.”
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Dr Jo Morrison, consultant in gynaecological oncology at the Musgrove Park Hospital in Somerset, said it was probable the HPV vaccine would lead to a drop in cervical cancer rates.
“Vaccination aims to prime the immune system to produce antibodies that can block subsequent natural HPV infection,” she said.
“This data shows that immunising against HPV infection protects against cervical pre-cancer, and it is very likely that this will reduce cervical cancer rates in the future,” she noted.
“However, it cannot prevent all cervical cancer and it is still important to have regular screening, even if you have been vaccinated,” she added.
Dr Morrison agreed that more research was needed to assess the impact of the vaccines over time.
“Cervical cancer can take many years to develop following HPV infection and development of pre-cancer lesions, therefore, long-term follow-up studies are needed to find out the effects of HPV vaccination on cervical cancer rates,” she said.