Immunising against tetanus - a continuing problem.
The conquest of infectious diseases demands either a means of effective prevention or total eradication of the causative agent. In tetanus only the first proposition can be entertained.
Despite two decades of intensive studies on tetanus in India and the availability of a highly effective toxoid, immunization against tetanus remains a challenge to physicians and public health authorities alike. There is very little concerted effort for massive immunization notwithstanding the fact that nearly 90% of the Indian population at large is inadequately protected against tetanus as a result of either ignorance or prejudice. "Epidemics" of tetanus leading to closure of hospital wards and operation theatres continue to occur.
This article investigates the present immunization problem and discusses new data which may lead to a simplified immunization schedule.
Production of tetanus antibody
Tetanus toxin is a protein with a molecular weight of 148,000 Daltons. The molecule possesses at least three active sites of which one is related to antibody binding. The site for toxic action has been investigated by altering amino acid sequences in the toxin molecule. This has indicated tyrosyl residues to be of critical importance to toxicity. When toxin is toxoided with formaldehyde, methylene bridges are created between tyrosyl and lysyl residues, thus detoxifying the molecule whilst preserving its immunogenic activity.
Tetanus toxoid is a potent antigen which is effective even if administered together with antitetanus serum. The addition of aluminum salts as adjuvant enhances antigenic potency, probably by delaying the release of antigen and causing an inflammatory reaction at the site of injection. Antibodies that form in response to toxoid are derived from immunoglobulins secreted by B lymphocytes. The mediation of T helper lymphocytes is necessary for antibody production.
The size of the antibody response to immunization is limited in two ways. As antibody accumulates in the circulation it covers antigenic sites on toxoid molecules and thus exerts negative feedback control. In addition, it is now recognized that a subset of T lymphocytes known as T suppressor cells limit antibody responses, although as yet there is no direct evidence for their participation in the response to tetanus toxoid.
In any population immunized with tetanus toxoid a small number of individuals will prove to be "poor responders". Here the mechanism is different from that just mentioned and recent evidence shows a relationship to the HL-A typing of the individual and to the specific immune response (IR) genes that he possesses.
The basis for three injections for immunity
A first dose of toxoid elicits a response mainly of IgM antibody. IgG antibody appears much later in this primary response and assumes low levels. However, following a second injection of toxoid after four to six weeks the IgG fraction boosts considerably with a ten fold or even a hundred fold rise in titre. A third and fourth injection given after minimal intervals of four to six months further increases the antibody titre, following which the fall off of titre with time is extremely slow. A titre of 0.01 IU/ml affords protection against tetanus.
Immunization studies to demonstrate effectiveness of tetanus toxoid
Active immunization with tetanus toxoid when correctly carried out is almost 100 per cent effective and ensures protection against tetanus for a number of years. It can be best carried out by three intramuscular injections of absorbed tetanus toxoid (5 to 10 Lf units per dose). For maximum effect, the second dose should be given after an interval of four to six weeks and the third dose after an interval of six to twelve months. A booster dose every five to ten years or after an injury reinforces the immunity immediately.
In a study by `Peebles et al in the United States initial immunization in infancy and reinforcing boosters at 16 to 18 months and at about five years were routinely carried out with 0.5 ml of D.T.P. Vaccine, aluminum phosphate precipitated. Tetanus Toxoid content of D.T.P. was about 4 Lf units per 0.5 ml. Titrations were carried out by the standard toxin neutralization test in mice. Data showed that the fourth injection induced roughly an eight fold rise to about 1.32 IU/ ml. Titre after the basic course (three doses) was 0.16 IU/ml. A fifth and sixth injection induced a further two fold rise to about 2.64 N/ml. However, additional injections showed no further increase. Full protection was maintained for about 12 years after four injections or more. Three injections also gave a substantial amount of immunity but were inadequate to 'maintain as much as four or more injections.
In India, Gaitonde et al have found that after a three dose -schedule of adsorbed tetanus-toxoid (5 Lf per dose) in pregnant women, spaced at monthly intervals, only 90% achieved protective titres. Varma et al investigated the effect of booster dose (5 Lf) in 705 army personnel who had received primary immunization /boosters and showed that all subjects had titres only between 0.1 to 2.1 IU/ml even when blood samples were taken within one year of the last booster.
Of great interest is a recent study by the Haffkine Institute, Bombay in a rural unimmunized Adivasi community where it was found that approximately one third of individuals demonstrated a protective antitoxin titre whilst a similar number excreted Clostridium tetani in their stools. The majority of these organisms, however, were non-toxigenic. Following culture they were subjected to sonication when it was found that the cytoplasmic fraction on injection into laboratory animals, conferred immunity against toxin challenge, whilst, extracts of the cell debris failed to protect (S. Mukerji-Personal communication). These data if confirmed, suggest the mode of production of naturally acquired antitoxin to be through the gastrointestinal tract and also suggest that the latter exists in a sizable section of the population. A high potency one dose vaccine could thus prove sufficient to boost antitoxin titres to immune levels and to offer a practical means by which to reduce tetanus in this country.
Studies with single dose tetanus immunization
Our own experience with a single tetanus booster injection has shown a four fold increase in titre with 5 Lf, a ten fold increase with 100 Lf and a twenty fold increase with 250 Lf, irrespective of the value of the individual's basal titre. These results are noteworthy for the limited rise in titre even when using highly potent vaccines.
Figure 1 shows the response of 90 patients with a basal titre of < 0.005 IU/ ml to a single dose of 100 Lf adsorbed tetanus toxoid. Protective titres developed in all 72 patients who were followed up at one month. The titres remained unaltered in the 17 patients who returned for follow up at one year and in the 28 patients who were seen after two years. Our study with a 250 Lf dose is still in progress. It is notable that no side effects have been observed. It is not possible to state how long protection will be provided by a single dose of this vaccine but one could expect a more durable immunity than a 100 Lf dose. All titres in these studies originally estimated by the passive hemagglutination method were later confirmed by the mouse neutralization test.
Why does Indian experience not parallel that of Western Countries?
I. The middle and upper social classes in India today are conscious of the need for tetanus immunization . This is almost invariably given as triple vaccine (DPT) from the third to the sixth month of life. The same consciousness for the need of booster doses is not present in our population. Thus only 15 per cent of a batch of newly admitted medical students were found to have a protective titre (0.01 IU/ ml), although the majority gave a history of receiving triple vaccine in childhood. Insufficient exposure to tetanus toxoid may thus account for a rapid fall off of protective titres.
II. Host Factors
(a) Tetanus in India is concentrated amongst the lower socio-economic classes. When the immune responses of 30 tetanus patients together with their relatives were examined, nearly 50 per cent showed impairment of the inflammatory response and cell mediated immunity. In some of them, there was evidence of malnourishment and iron deficiency anemia which are known to impair immune responses. However, when the same population was immunized with tetanus toxoid protective titres were achieved although the levels were inferior to those developed by healthy medical students.
(b) The poor response of healthy subjects to tetanus immunization may be linked with the individual's histocompatibility antigens (HL-A). These are known to be situated on chromosome No. 6 in close proximity to genes controlling immune responses. HL-A typing of the individual may therefore indicate his ability to respond to various antigens. Typing, of a hundred Indian patients with tetanus has so far, not revealed any distinctive HL-A pattern in them to suggest an increased susceptibility to the disease.
III. Persistent Antigen Stimulation
No Indian is found to possess a normal histological pattern of the jejunum according to Western criteria. That this is not genetically determined has been shown by the demonstration of identical changes in American Peace Corps Workers living in India for a period exceeding three months.
A body of evidence has now accumulated to attribute the abnormalities to regular bacterial contamination of the small bowel. It has been shown that clostridia' organisms can excite B and T cell responses in Peyer's patches and that such sensitized cells induce systemic immunity following their transport to the spleen and mesenteric lymph nodes.,  If Clostridium tetani chronically contaminate the small bowel, then this could be a means of inducing continuous low grade antitoxin production by the patient. Such individuals when given the conventional course of toxoid immunization would be unable to develop a high degree of antibody response as they are partially desensitized. This could explain the low titres seen in Indians in response to conventional tetanus immunization and to subsequent booster doses.
A simplified schedule of tetanus immunisation
At present our data on high potency single dose tetanus vaccines (100 Lf and 250 Lf) suggests that irrespective of basal titres, long lasting protective immunity is maintained for at least two years even in unimmunized persons who have not had contact with tetanus vaccine before. Armed with this data, public health authorities in India can evolve a simplified scheme of tetanus immunization with a single dose for select groups like pregnant women and school entering children, and for mass immunization campaigns where it may not be possible to give the ideal three dose schedule. The impact of such a scheme should not be underestimated as the majority of tetanus cases and tetanus deaths occur in the young. Thus a need expressed by many doctors working in developing countries for a tetanus toxoid that can be given confidently in one dose to provide effective protection against tetanus will be met.
We are thankful to Glaxo Laboratories (India) Limited for their gift of special tetanus toxoid.