A new vaccine has a '99 per cent success rate' - so why is the NHS not rushing to prescribe it?

To some commentators, it is a no brainer. A vaccine with a "99 per cent success rate" has been available to protect girls against cervical cancer since last year. Despite the UK's national screening programme, cervical cancer still kills 1,100 women a year in this country. Why would any government not choose to introduce such a life-saver? Why would any parent who can afford it not buy it for their daughters?

Next week, the Joint Committee on Vaccination and Immunisation (JCVI), meets to consider the first of these questions again. After months of delay, some accuse it of dragging its feet, allowing 20 women a week to die from what is now a vaccine-preventable disease.

But the issue raises complex medical, ethical and cost issues. And the claim of a 99 per cent success rate, though factually accurate, is misleading - highlighting the difficulty of getting the message clear.

Cervical cancer is the first cancer shown to be caused by an infection, the Human Papilloma Virus (HPV), which is preventable by a vaccine. There are two vaccines - Gardasil, licensed last year and made by Merck, and Cervarix, expected to gain a licence in the next couple of months and made by GlaxoSmithKline. Clinical trial results for both are very encouraging. They provide 99 per cent protection against the two commonest causes of cervical cancer, HPV 16 and 18, which are together responsible for 70 per cent of cases of cervical cancer.

But there are scores of HPV viruses, of which at least 15 cause cancer. A 99 per cent "success rate" against the two commonest causes does not mean 99 per cent protection against cervical cancer. It means, according to the trial results, about 70 per cent protection. Thus any girl who had the vaccination would, once she became sexually active, still have a 30 per cent chance of being infected with another HPV virus which could develop into cancer, and would therefore still need regular screening.

Two critical questions remain. How long is the vaccine effective for? Is it also protective against other HPV viruses (providing so-called "cross-protection")? Early research suggests this could increase its defence against cervical cancer from 70 per cent to close to 80 per cent. Vaccination would then be as effective as the cervical cancer screening programme, which detects about 80 per cent of cervical "cancers" (in their pre-cancerous state), and might ultimately replace it.

Anne Szarewski, the Cancer Research UK clinical trials consultant at the Wolfson Institute of Preventive Medicine, said: "For the current generation of women we will need the screening programme to continue. In the future, 12-year-olds who have been vaccinated probably won't need screening."

But will women who have been vaccinated continue to attend for screening? Some public health specialists are worried that vaccinated women would wrongly believe they were fully protected and "collapse" the screening programme. Mistaken reports of a "99 per cent success rate" suggest this is not an idle warning. Attendance is already falling as cervical cancer has declined - the £150m programme has become a victim of its own success.

Who should get the vaccine? Government programmes to vaccinate girls exist in Scandinavia, the US, Australia and Germany. The JCVI is understood to favour giving it to 12-year-old girls, as in these countries, before they are sexually active, but when they are old enough to understand what it protects against.

If the vaccine were introduced for 12-year-olds, parents of 13-year-olds would protest that their daughters were being neglected and their lives put at risk. The thorny question is who to include in any catch-up programme. The JCVI is understood to want to give it to girls up to the age of 16 - but the government may balk at the additional expense. Wherever the age cut off is set, those just above it are bound to feel aggrieved.

The vaccine costs £80 a shot on the NHS and three shots (£240) are required to give maximum immunity. With bulk buying, it would cost an estimated £120m to cover all 12-year-olds. That would rise by £120m for every year included above the age of 12. The bill to vaccinate all girls from 12 to 16 would be around £600m.

How long will protection last - experience is so far limited to five years - and will booster jabs be needed? Scientists are optimistic protection will be lifelong. They say the immune response provoked by the vaccine is many times more powerful than that from natural exposure to the virus. But some specialists fear the scenario where sexually inactive 12-year-olds are vaccinated only to lose their protection a decade later as they enter their sexually active twenties.

A second unknown is whether the vaccine is protective for older, sexually active women who may already have been infected with HPV. Studies show 25 per cent of women of university age are infected with HPV - implying that at least 75 per cent may gain protection from the vaccine. A study of 20,000 women sponsored by Merck, makers of Gardasil, published in the Lancet this month suggested that it was 44 per cent effective (against the most common cancer causing viruses, HPV 16 and 18). Further research is awaited.

Offering the vaccine to women and young girls also presents difficulties because of the link with sex. Few people realise that cervical cancer is a sexually transmitted disease.

Objections have come from the Catholic church, family groups and the right-wing press, claiming that the vaccine will "encourage promiscuity" and "destroy children's innocence". Yet the virus is ubiquitous - 75 per cent of women become infected at some point during their lives. Some GPs are anxious about how to raise the issue without implying their patients have, or are about to have, a dubious lifestyle.

Cervical cancer specialists say the focus on women in relation to this sexually transmitted virus is unhelpful. The best answer would be to vaccinate boys as well . Although they do not suffer from cervical cancer, they spread the viruses that cause it. This is unlikely to happen, as it would double the cost. However, experience with rubella, the virus that can damage the foetus if caught by pregnant women, showed that only when boys were vaccinated did herd immunity develop. One study of the cervical cancer vaccine showed that unless vaccination rates rise above 75 per cent in girls it would be cost effective to vaccinate boys.

Once the JCVI has made its recommendations, ministers must decide how to respond. One reason for the delay is thought to be that the committee is waiting for the GlaxoSmithKline vaccine, Cervarix, to be licensed in order to trigger a price war between the manufacturers.

In the meantime what should parents of teenage girls do? Private clinics are already offering the jabs at £450-500 a course. Dr Szarewski, who is running trials of Cervarix, has unequivocal advice. "That is what some women spend on a handbag. If you can afford it, it is well worth having, even though there is no guarantee [of protection]. Women will still have to go and get screened."

It's the belt and braces approach. Only long-term experience with the vaccine will determine whether future generations can do away with the belt.