Diphtheria is a bacterial infection transmitted from person to person through close physical and respiratory contact. It can cause infection of the throat that may lead to fatal obstruction of breathing. Like other respiratory infections, transmission is increased in overcrowded and poor socio-economic conditions. Large epidemics occurred in Europe in the 1940s involving over one million cases and 50 000 deaths.

Tetanus is the only vaccine-preventable disease that is acquired through environmental exposure. The disease is caused by a potent neurotoxin produced during anaerobic bacterial growth in necrosed tissues, such as dirty wounds. Clinical symptoms of tetanus are muscle spasms which may be fatal unless treatment is rapidly initiated. Neonatal tetanus is the most common form of the disease in developing countries and is caused by contamination of the umbilical stump with spores following childbirth.

Diphtheria and tetanus vaccines are amongst the most successfully used worldwide and are considered an essential component of the Expanded Programme on Immunization (EPI) schedule. Their use has resulted in a significant decrease in the incidence of these diseases in developed and developing countries.

The quality control of vaccines has always relied on three components: control of the starting materials; control of the production process; and control of the final product. For traditional vaccines such as tetanus and diphtheria toxoids, there is a vast production experience and a long history of use. In this case, considerable emphasis is placed on a bioassay for potency testing of the final product in animals.

However, fundamental problems still exist in the standardization and control of the potencies of these toxoids globally, even when international standards are used, since different potency tests are being used in different regions of the world. Until a universally accepted method is developed, problems will continue to be encountered in the international movement and licensing of diphtheria and tetanus vaccines. In recent years, efforts have been made to simplify the current tests and reduce the number of animals used in control testing.

The standard design of the D- and T-potency test developed by WHO and the European Pharmacopoeia involves titrating the test vaccine in groups of animals and comparing the dose response curves in a probit analysis to calculate the potency of the test vaccine in international units against the reference.

However, problems observed with this system include:

• the use of large numbers of experimental animals;

• potencies estimated in one species of animal may differ from those obtained in another;

• inconsistent data are available on the predictive value of the potency test for efficacy in the target population;

• The reference and test vaccine cannot be considered as unknown dilutions of each other;

• Dilution of the vaccine results in a dilution of the D- and T-toxoid, as well as other components present in the vaccine. This may alter the effect of the vaccine;

• the testing of booster injections is not covered; and

• the probit analysis is not sensitive enough to demonstrate small differences in immunological characteristics of test and reference vaccine.

Another common approach is that used by the United States Food and Drug Administration, where the test vaccine is injected into a small group of guinea-pigs and the induced antibody titer in serum determined in a prescribed toxin neutralization assay in vivo against equine hyperimmune reference serum. The problem with this system is that no reference vaccine is included in the assay and no statistical analysis of the data is possible because the toxin neutralization titer is estimated in pooled sera. External factors may also influence the antibody titer.

A working group has now been set up by WHO to work towards the harmonization of potency measurements of these vaccines. The group met at the Rijksinstituut voor Volksgerondheid en Milieu-Hygiene (RIVM), Netherlands, in 1999, and agreed that WHO should continue to evaluate the value and limits of the current potency assays and explore alternative approaches. As a result, a small group of experts was established to coordinate the development of a simple, robust and standardized assay suitable for demonstrating consistency of immunological characteristics for batch release. Also, the relevance of the minimum potency expressed in an international unit (IU) of toxoid determined in animals to human immune responses should be re-evaluated based on clinical efficacy and safety data. In addition, the working group also urged WHO to develop guidelines on the antigen content and quality control of diphtheria and tetanus vaccines used as boosters in adults and adolescents.

Further discussion of these activities will take place at the forthcoming International Symposium on Tetanus Vaccine for Human Use to be held in Strasbourg, France, from 22-23 June 2000. The meeting is organized by the European Directorate for the Quality of Medicine in collaboration with WHO.