TT OR Td - WHEN AND WHY ?
Dr. Digant D. Shastri*
Consultant Pediatrician and Neonatologist, Surat. *
Introduction
The Expanded Programme on Immunization (EPI) has made major advances in the last two decades in terms of achieving high vaccine coverage among infants against the programme's target diseases (tuberculosis, tetanus, diphtheria, pertussis, polio, measles, yellow fever and, more recently, hepatitis B), on a world-wide basis. The Scientific Advisory Group of Experts (SAGE) to EPI has indicated the need to expand immunization activities beyond infancy, either as part of routine immunization services, or as part of disease elimination or eradication measures. The adolescent age group (10 to 19 years) represents an important additional target group in this respect, since the success of EPI is now being seen to have important long-term effects on the traditional epidemiological patterns of major infectious diseases, often raising the average age of incidence.

In the pre-immunization era, a large proportion of adults had disease-induced immunity to common infections. Many countries are now finding the majority of individuals have vaccine-induced immunity, which may or may not have the same long-term stability. Questions therefore arise as to the policy and strategy implications for postinfancy immunization programmes. Adolescence presents special challenges for immunization in relation to lifestyle and other social issues, whilst also offering special opportunities, such as vaccine delivery in the setting of educational institutions.

So far, a relatively small number of vaccines have had potential for administration to the adolescent age group. For many years, tetanus toxoid has been targeted at women of child-bearing age, many of whom are adolescents. More recently, measles vaccine (M) has been of particular interest, as well as the measles-containing vaccines measles-rubella (MR) and measles-mumps-rubella (MMR), and tetanus-diphtheria (Td) vaccines. Principal barriers that have prevented such vaccines from wide administration to data (and are likely to remain impediments in the future) are:
  1. Lack of awareness by the public
  2. Cost of the interventions
  3. Lack of suitable infrastructure to reach the target group
  4. Country to country difference regarding disease control objectives, financial capacity and political commitment

For any vaccine, the age at which it should be administered is determined by a number of factors, such as the age-specific incidence of target diseases, age-specific responses to vaccines and risks of complications. As well, cost, supply, implications for the cold chain, training, compatibility with other vaccines and other services must all be taken into account. These factors, therefore, have to be considered in evaluating existing or new vaccines that may be appropriate for delivery to adolescents. Dealing with implementation issues in depth is largely beyond the scope of this presentation.
Reasons for Adolescent Immunization
  • To boost immunity that is waning, thereby increasing the duration of effective protection derived from the vaccines already given earlier in life, especially in the absence of "natural" boosting from exposure to the infectious agent.
  • To accelerate control or elimination efforts: Disease control initiatives frequently encompass immunizing across a wide age range, including adolescents, with the aim of increasing herd immunity, interrupting transmission or catching-up on cohorts missed in the past. Special activities may be mounted to respond to, or prevent outbreaks of target diseases.
  • To counter a specific risk: As for all ages, travel represents a special need for adolescent immunization, indeed may be even more important on the basis that adolescent behaviour may place them at increased risk. Special efforts may be appropriate to immunize them before they enter the risk period.

Nine vaccines (tuberculosis, diphtheria, tetanus, pertussis, polio, measles, hepatitis B, Hib and yellow fever) are offered in infant immunization if countries follow the classical EPI recommendations. However, the extent to which it is appropriate to refer to "extra doses" or "booster doses" in adolescence will depend on the how well the infant vaccination programme performed some 10-19 years previously. It may be appropriate in some settings to implement a booster dose policy during adolescence for a fixed duration "catch-up" period only, to cover a cohort of children who were not fully immunized in infancy.
Diphtheria
The duration of immunity against diphtheria may depend on exposure to diphtheria organisms and thus varies geographically. Data on the persistence of immunity in developing countries are scarce. But even where vaccination coverage rates have been high for 5 to 10 years, diphtheria outbreaks have been reported. These are characterized by high case fatality rates, a large proportion of patients with complications and occurrence in both younger and older age groups. Epidemics in previously well-controlled settings in eastern Europe have raised awareness of the need for wider vaccination strategies. Consequently several countries have introduced one or two diphtheria toxoid booster doses at ages beyond infancy. Careful analysis is needed to assess when additional dose(s) should be added to the routine schedule. The distribution of diphtheria cases and serological studies can identify age groups or geographical areas at increased risk of diphtheria. The appearance of diphtheria in older age groups (as happened, for instance, in the former USSR in the mid-1990s) may be an early signal to indicate the need to consider the introduction of booster doses. Diphtheria vaccine is commonly combined with tetanus toxoid. The childhood version of this combined vaccine is referred to as DT. A lower dose of the diphtheria component of the vaccine is recommended for persons over the age of 7 years, to minimize adverse reactions, and the corresponding combined vaccine is known as Td. Since both tetanus toxoid and diphtheria toxoid can reasonably be given on a booster basis around every ten years, there is little reason to use monovalent diphtheria vaccine or tetanus vaccine.
Tetanus
There are two distinct aims of vaccination using tetanus toxoid-containing vaccines (TT, DT, Td):
  1. Preventing neonatal tetanus (NT) and maternal tetanus by vaccinating women before or during pregnancy.
  2. Preventing tetanus in the general population.

For prevention of tetanus in the general population, maintaining protection in the entire population is essential, and has been targeted by EPI through at least three doses of tetanus toxoid during infancy. National immunization schedules as well as maternal and child health (MCH) services have targeted pregnant women with tetanus toxoid, the strategy being:
  • Five doses spread over an adequate time period for previously unimmunized women.
  • Two doses of TT for women who received 3 doses of tetanus toxoid-containing vaccine in infancy (DTP or DT).
  • One dose for women who also received a childhood booster of tetanus toxoid containing vaccine (DTP, DT or Td) in addition to the three primary doses.
  • No further vaccination for women who were also boosted in adolescence with TT or Td.

The primary immunizing course of three doses of DTP is given in the first months of life. At least three more doses of vaccine containing tetanus toxoid (TT or Td) should be given before childbearing age in females for full protection. The optimal time for boosting boys and girls is considered by WHO to be at 6, 7 and 8 years of age (a year between each dose) at school. These are most easily given as Td, but certainly all doses given after the seventh birthday should be Td. If a child enters adolescence without the three booster doses complete, the missing doses should be given as early in adolescence as possible, so maximizing the chance of delivering it before girls become pregnant for the first time. Some countries target school leaving (at around 12 years) for the final booster dose of Td when school attendance is still above 90% for boys and girls. As females who have already received primary immunizing doses of DTP and booster doses of DT or Td reach childbearing years, the global pool of women in the reproductive age group who are not immune to tetanus will diminish dramatically. The same schedule will protect men at risk from tetanus through exposure to infection in the course of their work. School-based programmes for administering booster doses of tetanus toxoid may be the best strategies to cover the gaps in immunity against tetanus created by the respective strategy of primary immunization with three doses of DTP during infancy and TT to women of childbearing age. Some key countries with high-risk populations for unsafe birth practices have high levels of school attendance in urban areas, making this strategy workable. However, any school-based programme depends for its success on high female enrolment and low dropout rates - situations not readily found in high-risk rural areas. As tetanus and diphtheria toxoids are frequently combined in the same vaccine, and since the duration of vaccine-induced immunity is similar in both cases, it makes good sense to use Td rather than either toxoid separately. Additional programmatic advantages include the safety of Td in all age groups including pregnant women. As a first priority, TT will be replaced by Td in all countries that have had DTP-3 coverage of 70% or more for at least 5 years. Where school-based boosters of DT are given, evaluation of the use of Td to replace the first dose of DT should be considered 2. It is also appropriate to use Td following trauma in adolescents whose immunization history for tetanus is inadequate or uncertain. Additional evidence from Nigeria points to the risk to young women from unsafe abortion practices. Vaccinating adolescent girls before first conception can be expected to reduce tetanus related maternal deaths where abortion rates are high. The 'high risk approach' to control neonatal tetanus is increasingly being used in developing countries where neonatal tetanus remains a serious public health problem. The high-risk approach consists of supplementary immunization rounds delivering two doses of TT to all women of childbearing age in areas identified as being at 'high-risk'.
Summary
WHO recommends giving a booster dose of Td during adolescence, assuming that several doses of tetanus and diphtheria toxoids as DTP or DT have been given in the face of an epidemic of diphtheria. WHO also recommends Td or TT should be given to those female adolescents targeted in neonatal tetanus elimination campaigns in areas at high risk for the disease. As part of routine immunization, women (including adolescents) at risk for tetanus or neonatal tetanus should receive at least two doses of tetanus toxoid (as Td or TT) vaccine at least four weeks apart, with the last dose at least 2 weeks before delivery. A booster dose of TD should be offered to all adolescents who have not already received five doses of vaccine containing tetanus toxoid as resources and opportunities permit. A booster dose of Td should be used instead of TT following trauma.
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Shastri D D D.. Available From : http://www.pediatriconcall.com/fordoctor/ Conference_abstracts/report.aspx?reportid=354
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