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Preparing for the Hib top-up

A Hib catch-up campaign starts on May 12, Dr David Baxter explains what it will entail

small but worrying increase in invasive Haemophilus influenzae b disease has led the Government to launch a fourth round of Hib vaccination1. Details are being

finalised but we know an additional dose will be introduced for children under four in England and Wales starting on May 12.

Why is the programme being introduced?

Conjugate Hib vaccination was introduced in 1992 as part of the primary immunisation programme. Three doses, given at two, three and four months combined with diphtheria, tetanus and whole-cell pertussis (with a catch-up programme in the under-fours) led within a year to a fall in invasive Hib disease from more than 1,000 cases annually to fewer than 50.

Since 1999, there has been a modest increase in invasive disease (meningitis, septicaemia and epiglottitis), particularly in the one to four age group.

Last year, 122 cases were reported in this age group.

What is the microbiology of invasive Haemophilus influenzae disease?

Haemophilus influenzae (Hi) is a gram-negative rod that occurs in capsulate and non-capsulate forms. Capsulates are generally associated with invasive disease since the thick carbohydrate capsule is

anti-phagocytic.

Most people carry Hi organisms (often the

non-capsulate form) as commensals in their upper respiratory tract. Capsular organisms are divided into six subgroups (a-f) based on differences in the capsular polysaccharide.

Before the Hib vaccine, around 95 per cent of cases were caused by group b.

Why is Hib a conjugate subunit vaccine?

One of the body's defence responses on encountering a polysaccharide antigen is for the B cell to produce specific antibody without requiring T cell help; the

so-called T-independent response.

This response is poorly developed in infants so young children and polysaccharide vaccines are poorly immunogenic among this group. Covalently linking the polysaccharide antigen to a very small amount of a carrier protein converts it to a T-dependent vaccine that is highly immunogenic even in the first year of life.

Why the increase in invasive Hib disease?

Two possible reasons are mooted. When the programme was introduced it was given to every child under four. This possibly reduced Hi carriage among older children and adults, resulting in less exposure among infants and very young children. Some infants and children would have been expected to develop invasive disease since they were primary vaccine failures. But because no mass catch-up occurred after 1992/3 this effect on community spread was lost, so Hi circulation among older people has recurred over recent years leading to the observed increase of cases among 'primary vaccine failures' or those whose immunity has declined to non-protective levels.

Another explanation is based on the observation that infants who received two doses of vaccines containing acellular pertussis in their primary schedule seemed to be at higher risk of invasive disease than children who received two doses of vaccine containing whole-cell pertussis. This effect may result from experimental data showing Hib antibody titres depend on which pertussis vaccine was given with the Hib vaccine. When Hib vaccine was given with whole-cell pertussis the resulting antibody titres were higher than with acellular pertussis.

This is due to the adjuvant effect of the whole-cell pertussis vaccine and is the reason for recommending infants to have this vaccine in their primary programme. Reduced immunogenicity leads to more primary vaccine failures.

What are the aims of the programme?

The Department of Health programme will target children aged six months to under four years (on April 1, 2003), who will be offered one dose of Hib vaccine. This can be given with other vaccines as appropriate to the individual's age and status or on its own, with MMR or with diphtheria, tetanus and pertussis (if given opportunistically for children who have not completed the schedule). An additional dose will also be given to children who reach six months of age during the programme.

What are the likely side-effects?

These should be the same as when giving Hib as part of the two-, three- and four-month schedule: local reactions at the site of immunisation and fever. These come on in about 5 per cent of recipients within a few hours of vaccination, last 24 to 48 hours and are treatable with paracetamol. Contraindications to Hib are an acutely ill child or a child who has had an anaphylactic reaction to a previous Hib vaccine.

Will there be enough vaccine?

The department is buying 2.5 million doses which will be distributed to each practice on a per capita basis through your usual arrangements. It is confident there will be enough.

How will the campaign be organised?

Shortly before the start your local child health system will send you a list of all children aged between six months and under four years on April 1, 2003, with appointment cards giving you full autonomy to organise the programme. It is is expected the programme will be completed within four months.

For a practice with 8,000 patients, this means about 290 immunisations ­ around 20 a week. This is quite a commitment but is achievable, especially as it is supported by additional funding.

Is there a patient group directive?

Not from the department, but your district immunisation co-ordinator will provide one.

How will it be funded?

Funding is still under discussion with the GPC, but it is likely to be paid as an item of service at B rate (£6.80). The department has recognised the extra workload and each PCT may also get a sum for practice administrative costs.

How will parents learn about the programme?

Information will be made available through surgeries, clinics and NHS Direct. National press advertising begins on April 25. Leaflets and factsheets will be sent to practices ­ further supplies will be available free through Prologe.

Which licensed Hib vaccines are available?

Two are currently licensed in England and Wales.

Act Hib comes in a combination pack: lyophilised Hib conjugate vaccine (Act Hib) with adsorbed diphtheria, tetanus and whole-cell pertussis used to reconstitute the Act-Hib instead of ordinary diluent. This vaccine is recommended for primary immunisation at two, three and four months.

Hiberix is a lyophilised vaccine given on its own, or mixed with Infanrix, which contains diphtheria, tetanus and acellular pertussis. Infanrix Hib should not be used for primary infant immunisation.

Both vaccines use tetanus protein as the carrier.

Will a fourth Hib become part of the national programme?

This isn't known at the moment, but a decision will be made in the near future.

The DoH is confident there will be enough vaccine distributed for the campaign~

Funding is still under discussion but is likely to be an item-of-service payment~

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