WHEN Virginia Bottomley, the Secretary of State for Health, addresses the Conservative Party conference tomorrow, she is expected to extol the benefits of a new pounds 20m vaccination campaign, designed to prevent a predicted measles epidemic.

But already the campaign to inoculate 7 million children has run into problems. In some schools, consent forms were given out before the official launch, and parents threw them away, thinking that they did not apply to them.

In addition, family doctors were not warned about the volume of inquiries to expect or briefed about the campaign, and many were caught on the hop. The British Medical Association last week protested to Dr Kenneth Calman, the Government's Chief Medical Officer, about the handling of the issue.

Finally, the campaign was officially launched the day after the Baltic Sea ferry disaster and attracted relatively little space in newspapers, so that anxious and perplexed parents were not told the reasons for the campaign.

Many parents are experiencing vaccination fatigue. I, for one, have held my 13-year-old daughter's hand (metaphorically speaking) through seven or eight jabs for diphtheria, pertussis (whooping cough), tetanus, measles, rubella and tuberculosis, and know only too well how much children hate them. Mothers of younger children also have to cope with shots for mumps and meningitis. Now we discover that yet another vaccination is necessary.

'How many times are parents going to be faced with these decisions?' asks Jackie Fletcher, founder and co-ordinator of Jabs, a self-help support group for parents of vaccine-damaged children.

What mystifies most parents about the campaign are two questions: if our children have already been vaccinated against measles, why should they go through it again? And if most children have been vaccinated, why are we now expecting a measles epidemic?

On one level, these questions are simple to answer. First, the reason for a second jab is that one in 10 children failed to gain immunity the first time round. Your child might have been one of the unlucky ones. (It is too expensive to carry out blood tests to discover which children have antibodies and which do not).

Second, the reason we are expecting a measles epidemic is that an estimated 1.2 million children are still at risk of catching it. These fall into two categories: the one in 10 who did not obtain immunity the first time, and the large numbers who have never been vaccinated. Take-up of measles vaccine in the early Eighties was less than 60 per cent. With the introduction of the measles, mumps and rubella vaccine (MMR) in 1988, take-up has soared to more than 90 per cent, but there is still a backlog of non- immune children.

But these questions mask more complex issues. Why, for example, given that the vast majority of children have been vaccinated, do we not have enough immunity in the population to make the virus no longer a threat? And if this cohort of schoolchildren requires a second measles jab, will future generations also need two?

The answer to the first question lies in the high infectiousness of the measles virus. Dr Elizabeth Miller, head of the immunisation division of the Communicable Diseases Surveillance Centre, says: 'The virus is like a heat-seeking missile that seeks out and infects those who are unprotected. The short term aim is to stop an epidemic, and the long-term aim is to ensure 95 per cent of the population is immune.'

Doctors know that an epidemic is on its way because there were 8,700 cases in the first half of this year, compared to 9,600 in the whole of 1993. The epidemic was only temporarily halted in the summer, when the schools broke up, and the Department of Health estimates that we could see as many as 200,000 measles cases next year, and possibly 50 deaths, if we do not vaccinate everyone at risk.

In answer to the second question, it does seem likely that a two-injection programme might be necessary. Experts in the US have recommended a two-shot regime and many states are gradually introducing it. The two vaccinations do not have to be years apart in time. The second is not given because the first one is wearing off, as some parents imagine. In fact, those in the very first cohort of children to have been given the measles injection in 1966 have as good immunity now, 28 years later, as they did at the start. The second shot is administered simply to confer immunity on those who did not manage to gain it the first time round - the 10 per cent who got away.

But other issues have been worrying parents. Some of them remember that certain makes of MMR vaccine were withdrawn in 1992 because of side effects. This is irrelevant to the current campaign, however, because it was the mumps component that was the problem, having given rise, in a few cases, to a mild form of meningitis.

Others fear that, if their child is given a vaccination too soon after another vaccine of the same sort (rubella is included in the vaccination programme, and some girls will recently have had a separate rubella jab), they will get a severe local reaction. 'Not true,' says Dr Miller. 'That can happen with bacterial vaccinations, such as diphtheria and tetanus, but does not apply to live viral vaccines, such as measles and rubella.'

It will be a tragedy if the hiccups in the programme are not resolved. The US suffered a measles epidemic in 1989-90, in which 130 children died. Many parents are unaware of how serious measles can be. It is not just a new snuffles and spots, but can cause severe ear infections, broncho-pneumonia and brain damage. More than 1,000 children died of the disease in Britain in 1940.

Samuel Butler, the Victorian writer, called vaccination 'the medical sacrament corresponding to baptism'. Dr Calman wants to see all children 'medically baptised'.