ACELLULAR PERTUSSIS VACCINES ARE NOW LICENSED FOR USE IN CANADA

Acellular pertussis vaccines have finally been licensed for use in Canada, after many years of anticipation. The acellular pertussis vaccines are combined with diphtheria and tetanus (DTaP) and have been licensed for the fourth and fifth booster doses, for children 15 months of age or older. The license for these vaccines has not yet been released for the primary series of vaccinations in infants (2 ,4 and 6 months of age) but this will likely occur within a year or so. The whole-cell pertussis vaccine which has been used for over 50 years will ultimately be completely replaced with the acellular vaccine.

How do the acellular vaccines differ from the whole cell vaccine?

Acellular vaccines are a more purified product containing only specific proteins. This specificity has reduced the reactogenicity of the vaccine which has been one of the major concerns regarding the whole cell pertussis vaccines. All of the acellular pertussis vaccines contain pertussis toxoid (PT) with or without the addition of other pertussis specific protein components. These include FHA (filamentous hemagglutinin), 69K (69KDA) protein, or FIM (fimbriae). Of all the studies that have been done with DTaP vaccines there has been a striking similarity in the efficacy and safety of these products which is in marked contrast to the wide range of efficacy demonstrated with the whole cell pertussis vaccines.

What are the advantages of the acellular pertussis vaccines?

The main advantage of the DTaP vaccines are that they are less reactive than the whole cell vaccine, causing fewer side effects. In addition they have been found to be of equal efficacy, produce a longer lasting immunity (there may ultimately only be a requirement for 4 injections in childhood as opposed to the current 5), and will improve adult protection (because of its longer lasting immunity and because it may ultimately be able to be administered to adults, for whom the current whole cell vaccine is contraindicated).

Could the acellular vaccines change the epidemiology of pertussis disease?

With an improved vaccine that provides long lasting immunity and that can be given to adults, pertussis could eventually be eradicated. Currently the protection conferred by the whole cell vaccine is incomplete without booster protection at 15-18 months and 5 years of age. Because reactions are thought to be more severe in older children and adults, current whole cell pertussis vaccines are not recommended for anyone older than 7 years. Due to the longer lasting protection with the acellular vaccines, eventually physicians may be able to eliminate one dose in infancy and perhaps the dose currently given in the second year of life. A fourth dose could be given at school entry and a fifth dose about 10 years later. There are clinical trials currently in progress assessing the safety and efficacy of acellular vaccines among adults.

How common is pertussis in adults?

Pertussis is quite common in adults but is often missed by physicians because they tend to think of it as a disease of childhood and not adulthood, and it is not commonly considered in the differential diagnosis of a chronic cough or a cough unresponsive to standard therapy. The best way to ultimately control the spread of pertussis is to vaccinate both children and adults. But, having a safe and effective pertussis vaccine licensed for adult use is only part of the battle. The real challenge is being able to vaccinate adults, since it is well known that it is extremely difficult to vaccinate adults with any of the currently available licensed vaccines.

What are the disadvantages of the new acellular vaccines?

1.The initial DTaP vaccines will not be combined with Haemophilus Influenzae type B or polio vaccines. This means that children will require additional injections.

2. Another drawback may be the price of the vaccine which is higher than that for whole cell pertussis vaccines. This may effect distribution by budget conscious governments.

3. While the DTaP vaccines are associated with a significantly lower frequency of mild adverse reactions than whole cell vaccines there is not enough experience to evaluate the risk of more severe adverse reactions such as anaphylactic shock and acute encephalopathy.

4.Convulsions are significantly fewer among the recipients of acellular vaccine but fever equal to or more than 40o C, hyporesponsive/hypotonic episodes and persistent crying are less firmly associated with DTaP than with whole cell vaccines.

5. It is unclear whether acellular vaccines would be more effective than whole cell vaccines in controlling disease.

6. In developing countries, there is no clear evidence that the advantages of DTaP vaccines over whole cell products, justify their additional cost. How should physicians vaccinate children right now? In the interim, until any formal changes are recommended, health care providers should continue to immunize children against pertussis according to current guidelines and with currently available whole-cell vaccines. Immunization against pertussis should not be deferred while waiting for acellular vaccines to be approved. The whole-cell vaccines continue to provide protection against pertussis in infants and young children.