Human papillomavirus vaccination economics

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Modelling studies suggest that it will be cost effective to vaccinate all girls aged up to age 18 with against human papillomavirus (HPV).

The studies assumed a cost similar to the NHS list cost of the vaccine - approximately £80 per dose, or £240 per course.

Questions have arisen about when girls should be vaccinated within the programme, and without it. The department of health has advised that people should only "exceptionally" be vaccinated outside the programme. But what are the underlying economics?

The two vaccines available (Gardasil and Cervarix) both protect against cervical cancer. Gardasil also protects against genital warts (GWs).

To be more precise, they both protect against virus types 16 and 18, which cause about [check details] 60% and 10% of cases of cervical cancer respectively; and Gardasil also protects against virus types 6 and 11, which between them cause over 90% of GWs.

The prevalence of the viruses is also significant, especially early on, before herd immunity starts to affect it; and type 16 is the most common, the most persistent, and the most infectious virus type, as well as the most oncogenic. Preventing HPV type 16 is therefore much more valuable than preventing type 18.

Studies get citation show that infection rates start to rise from the age of 14 (and that type 16 is the commonest type, and also the one most worth protecting against).

When making decisions about vaccine programmes, we are making decisions on behalf of populations, based on the average rates. In the consulting room we are dealing with individuals. So how does this affect our decisions?

In theory you could calculate the value of protecting an individual based on her current status (infected with 1 or more of the vaccine-preventable virus types, depending on which ones she's been infected with), and her likelihood of becoming infected.

Lets use some illustrative figures. Let's assume that the value of vaccination on average exceeds the cost of vaccination in girls aged up to 18, at population level. For the sake of argument, let's assume that this is based on a vaccine cost of £240, and that the value is £250.

Girls up to the age of 18 will include many who have been infected already. Obviously, at a population level, the younger the girl, the less likely she is to have been infected, and the greater the value of vaccinating her. But on an individual level, the value will depend on the amount of sex she has had, the number of sexual partners she's had, and the number of sexual partners they have had. There is no value at all in vaccinating somebody who has already been infected with all the virus types prevented by a vaccine.

The cost of the vaccine negotiated for the vaccination programme is a secret; but we can assume that it exceeds the value of preventing genital warts, as the DH would otherwise have chosen Gardasil rather than Cervarix. It would be reasonable to assume that the price difference is of the order of £20 per dose, or £60 per course.

So how should we apply these facts in the course of a consultation?

If we assume that the average value of vaccinating a girl aged under 18, the value would be very much higher for a girl who hasn't been infected (i.e. for most virgins) than for girls who have acquired one or more vaccine-preventable virus types. Given that the rate of infection rises quite steeply from age 14 (x% of 18 year olds are infected with at least one virus type - Jit et al from memory), the value of vaccination will range from zero in a few cases, to a value very much greater than the average value for girls who have not been infected. Let's assume it's £500 for the latter.

If a girl has had no sexual experience, we can assume that she's not been infected; but as the likelihood of her having been infected increases, the likely value of vaccinating her will fall.

So, if we see a girl who has had no sexual experience now, but who tells us she has a boyfriend, and is very likely to have had considerable experience by the time she will be vaccinated by the programme, what will be the best use of public money: telling her to wait until she gets the jabs through the programme, or vaccinating her now on an FP10?

If we assume that the value of vaccinating her now is £500 (as above); but that the efficacy of the vaccine by the time she's had had that much sexual experience will be only 60% of its efficacy now, its value have fallen by the same amount. So, instead of the vaccine providing an outcome worth £500, it will only give a payout of £300. So it's worth £200 less then than it would be worth now. (And that's not taking into consideration the additional value of protecting against GWs if we use Gardasil, as we can do at no extra cost.) This would mean that we will have saved £60, by using the cheaper vaccine available to the programme; but at the expense of buying outcomes worth £200 less - a net loss to the tax-payer of £140.

Of course, these figures are only illustrative, and it is not easy to know how much the value of vaccination will fall with the amount of sexual experience girls will get between their consultation now, and the time they get the vaccine through the programme. It is true that girls who are concerned about getting protection against GWs might have an incentive to exaggerate the likelihood of their having been sexual active before they would have been offered the vaccine. Nevertheless, there is a reasonable chance that giving the more expensive vaccine (especially Gardasil) without delay will be considerably more cost-effective than waiting to use the cheaper vaccine through the programme.

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