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A vaccination programme against the HPV virus was introduced in England in 2008 for girls aged 12-13 along with catch-up programme for those aged 14-18. Many people expected cervical cancer rates in women aged 20-24 to fall by 2014, when the vaccinated cohorts entered their twenties. However, we do not think that is reasonable.
As can be seen in Table 1 below, the first vaccinated women (those aged 17-18 in 2008) would have been invited for screening for the first time in September 2015. Less than 50% of these women received the vaccine.
The HPV vaccine works by preventing infection with HPV types 16 and 18. Therefore women already infected by the time they are vaccinated, have little or no benefit of the vaccine. It is estimated that in 2008 between 17% and 20% of women aged 16-18yrs in England were infected with the HPV 16/18 virus. Therefore, at most only 40% of those invited for screening in September 2015 (80% of the 50% vaccinated is 40%) will have benefited from the vaccine. The vaccine should prevent about 70% of cervical cancer (maybe slightly more in young women, particularly, as seems to be the case, if the vaccine has partial protection against other types of the virus). Thus, we might expect to see a fall in cervical cancer in 25 year old women in 2016 of about nearly 30% (70% of 40% is 28%).
It will not be until September 2018 that a substantial proportion of the population coming for screening will have been vaccinated before being exposed to the HPV virus.
|Table 1. Summary of the school-based HPV vaccine programme in 2008|
|Vaccine age||Birth cohort||Coverage||Year enter screening|
|12/13||01 Sept 95 – 31 Aug 97||86%||2020-2022|
|14/15||01 Sept 94 – 31 Aug 95||68.5%||2019|
|15/16||01 Sept 93 – 31 Aug 94||68.6%||2018|
|16/17||01 Sept 92 – 31 Aug 93||41.7%||2017|
|17/18||01 Sept 91 – 31 Aug 92||38.9%||2016|
|17/18||01 Sept 90 – 31 Aug 91||47.4%||2015|
Interestingly, national cancer statistics for 2016 do show a (20%) reduction in cervical cancer diagnosis at age 20-24 (73 in 2015 and 58 in 2016). Even more surprisingly the number of cancers diagnosed at ages 25-29 also fell from 401 in 2014 to 341 in 2016. These statistics on their own are not sufficient to conclude that the decrease in cervical cancer is directly related to vaccination, particularly since the number of women attending for cervical screening is at a 20-year low and many cancers in women in their early 20s are picked up on screening.
The impact of vaccinating 17-18 year old women on cancer aged 20-24 is likely to be less because it typically takes at least 8 years from HPV infection to cancer, so a women with cancer at aged 22 is likely to have been infected with HPV before she was 17.
Unfortunately, we only have data for women aged 25-29. If the decrease in cancers in women aged 25-29 is all in women aged 25, then the fall would be rather more impressive and would suggest an early impact of HPV vaccination.
Assessing the effect of vaccination on cervical disease and on cervical cancer by looking at routinely reported cervical cancer statistics is going to be further complicated by the upcoming change in the cervical screening programme. At the end of 2019 testing for the HPV virus will replace cytology as the primary screening test in England. HPV testing is a more sensitive test which will mean that we will be detecting more disease at screening. In Australia, where vaccination began in 2007 and where they have achieved higher coverage among their catch-up cohort, a recent data modelling study showed that the introduction of primary HPV testing will lead to an increase in diagnosis of cervical cancers from 2016 to 2018 before rates begin to fall. Hence, in England, just when we would expect the vaccination effect to be most apparent we may instead see an increase in the number of cancers diagnosed due to the introduction of primary HPV testing.
A similar phenomenon was observed in 2009 when the English reality TV star Jade Goody was diagnosed and subsequently died from cervical cancer. The screening programme expected an increase in women attending cervical screening, what they did not foresee was the huge impact it had on cervical cancer diagnosis. In England in 2008 there were 1808 cases of cervical cancer diagnosed in women aged 20-64, however in 2009 there were 2207; 399 more cancers than the year before! By 2010 diagnosis had decreased to 1846. This phenomenon was short lived and was most pronounced in women under age 40.
Therefore, in writing this I hope to encourage those of you who are watching cervical cancer trends for proof of vaccine efficacy to be patient. Almost certainly just when a decrease in the incidence of cervical cancer should be seen we will see a steep increase. Hence routinely reported cancer incidence statistics will not be an appropriate source to evaluate the effectiveness of vaccination unless cancer incidence is reported by year of birth or vaccination status.