Vaccination a medical procedure whereby a vaccine against a particular pathogen is administered to a healthy subject for protection against possible future attack by the same pathogen. The essential component of all vaccines is the antigen- substance(s), which the body recognises as 'non-self' and hence foreign- and therefore reacts to it in a certain manner dictated by the defence system of the body, the result of which is the development of immunity. A vaccine may have the antigen administered as whole bacterial cells or virus particles or given as purified chemical components isolated from the pathogen.

There are six childhood diseases against which effective vaccines are available. These are: diphtheria, pertussis (whooping cough), tetanus, polio, measles and tuberculosis. The World Health Organisation has undertaken a programme for 'universal child immunisation' against these six diseases. In Bangladesh, as recently as 1985 only about 2% of children between the age of 12 and 23 months were immunised against these diseases. A programme called expanded programme on immunisation (EPI) was undertaken in the late 1980s with the result that by 1993 approximately 74% children were immunised and by 1998 the coverage reached nearly 85% for some of these vaccines. Bangladesh today stands as an outstanding success story for universal childhood immunisation. A contributing factor to this success is observation of national immunisation day (NID) throughout the country on which special countrywide arrangements are made to administer the vaccines through an elaborate network of community health care centres and with the participation of a large number of volunteers. A massive awareness campaign is set in motion a few weeks prior to the NID to make day widely known, urging particularly mothers to report to the nearest vaccination centre on the day.

Currently available vaccines for other vaccine-preventable diseases in Bangladesh include mainly the recombinant DNA-derived vaccine against hepatitis B virus. The vaccine is, however, costly and can be afforded by only a small section of the population. Anti-rabies vaccine is also used to a limited extent after bites by rabid dogs, the most common cause of rabies infection in Bangladesh. Vaccination against cholera through injection of killed cholera bacteria used to be an elaborate anti-cholera programme in the 1960s. However, more careful field studies carried out late in the 1960s and one extensive study carried out in 1974 by the Pakistan SEATO Cholera Research Laboratory (PSCRL, the progenitor of ICDDR,B) conclusively proved that the vaccine had no efficacy in providing protection against cholera. This prompted the WHO to recommend discontinuation of the use of the vaccine. As of the present time, there is, however, no effective vaccine against cholera. Similar is the case with vaccines for bacillary dysentery and rotavirus diarrhoea, but research in this area is in progress in which icddr,b is closely associated. Diarrhoeal diseases account for about 16% of deaths in Bangladesh, only next to acute respiratory infection or pneumonia which accounts for an estimated 25% of deaths. An effective vaccine against pneumonia is in field testing stage in Bangladesh and may soon be available for general use.

The cost of mass vaccination presents a painful economic scenario in Bangladesh. The cost of developing a vaccine is high and is a technically demanding exercise, much beyond the reach or ability of the developing countries. But there are diseases, which are strictly speaking developing country diseases due to poor hygiene and sanitation, unsafe drinking water, over-crowding, malnutrition and a host of other factors. Vaccines developed in the industrialised countries will be admittedly too costly to afford by average Bangladeshi given the fact that per capita health budget in Bangladesh is just about a meagre sum of $3 which is perhaps less than the cost of vaccination against just one disease. The six EPI vaccines currently being used throughout world were developed a long time back. Their development cost was recovered long before so that these vaccines can now be produced at a relatively low cost in many developing countries by using technologies that are readily available, many in the public domain, that is, not protected as intellectual property. Even then the estimated cost of full immunisation with the six EPI vaccines runs in the order of about $12 per child at the present time in Bangladesh. Mass vaccination programmes in Bangladesh using other vaccines now available in the market and those that are likely to be available in the near future is an issue that is closely linked with country's socio-economic development and may thus take a long time before reasonable vaccine coverage is achieved.

Vaccine Preventive agents against diseases caused by infection with pathogenic organisms such as virus, bacteria, and parasites, which upon administration into a healthy host stimulates body's defence system and prepares it to repel any future attack by the pathogen against which the vaccine has been administered. A vaccine uses the property of the human body to discriminate between self and non-self, so that when a bacterial pathogen for instance, invades the body it readily recognises it as a foreign agent by sensing the molecules and structures in the surface of the bacteria. Such foreign agents that can be recognised by the body as such and against whom the body can mount an immune attack are called antigens. Once an antigen is presented to the body such as in natural infections or as vaccines, the body mounts an attack by making corresponding antibodies. A vaccine thus contains antigens either as whole bacterial cells or virus particles or as purified active antigenic components of the pathogen. Successful vaccination often leaves a memory in the immune system so that when the individual is exposed to the agent containing the antigen at a later date, the immunological memory is activated and antibodies against the antigen are made. The antibodies are highly specific for antigens and this antigen-antibody reaction is the basis of immune protection. The antigen-antibody complex formation results in an incapacitation of the pathogen leading to its eventual destruction.

Bangladesh is a country with heavy load of infectious diseases many of which are vaccine-preventable. The WHO has taken steps for elimination of the six childhood diseases through an Extended Programme of Immunisation (EPI). Through the EPI programme Bangladesh has attained very high vaccine coverage. The success bears the promise that those who have completed fully the vaccination course may live the rest of their lives immune from these diseases. The EPI programme in Bangladesh has been widely alluded as a success story of considerable magnitude which other developing countries are trying to replicate.

Other vaccines that are currently in use in Bangladesh, although in a very limited scale, include vaccines against hepatitis B and anti-rabies vaccine. A vaccine against cholera had been in use for mass immunisation during the 1960s but it has been discontinued in the late 1970s because of the findings by the International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR,B) in a large field trial carried out in the matlab field station of the centre which suggested that the vaccine had no significant effect in providing immune protection against cholera. Other anti-diarrhoeal vaccines that are in various stages of development, are those against rotavirus diarrhoea and bacillary dysentery. Field trials of rotavirus vaccine are expected to be undertaken soon in Bangladesh under the auspices of the ICDDR,B and similar studies with vaccines against bacillary dysentery may soon follow although anti-dysentery vaccine development has been a difficult area of research because the pathogenic mechanism is complicated and still not fully understood. Vaccine against typhoid fever is not available at present but is urgently needed because of the high prevalence of the disease and frequent outbreaks of epidemics.

Although the large population base of Bangladesh offers a potential market for several vaccines, vaccine production in Bangladesh has drawn little interest. There is an estimated 100,000 rabies cases per year. The Institute of Public Health (IPH) produces a vaccine from infected sheep brain that can treat about 85,000 cases of rabies, but the vaccine has frequent neuroparalytic side effects which militates against its continued use. Tissue culture-based (using VERO cells) anti-rabies vaccine production technology is not available for large scale production of the vaccine in Bangladesh so that this vaccine is imported in limited quantities for use by those who are in a position to afford the vaccine. Hepatitis B vaccine produced through the recombinant DNA route is a promising and commercially viable product for which the technology is available for adaptation in a developing country setting but entrepreneur interest is lacking in this or indeed for any other vaccine in Bangladesh.

Oral vaccine A disease-specific agent designed to stimulate protective immunity against the particular disease when it is administered orally to a healthy individual. A vaccine contains certain components of the pathogenic organism, called antigens, which the organism uses to establish the disease. The antigens are recognised by the body as foreign or non-self substance. The defence system of the body therefore reacts against these foreign antigens by producing antibodies. The antibodies form complexes with the antigen that usually occur on the surface of the pathogen thereby disarming the pathogen. The antigen may be included in the vaccine either as whole cells or as purified chemical components isolated from the pathogen. The vaccine also triggers the development of a memory in the immune system so that when the offending pathogen invades the host at a later date, the antigen present on the invader is quickly recognised by the host's immune system resulting in the production of corresponding antibodies and rapid incapacitation of the invading pathogen.

Most vaccines are used as injections given in the muscle. From there, the vaccine is distributed through the blood circulation to different parts of the body in the course of which it interacts with the body's immune system and thus triggers the development of immunity. In recent years, interest has grown on the development of vaccines that are active when given orally in the form of oral drops or as pills. Recent studies have demonstrated that the human intestine is capable of taking up antigens presented through vaccine and send the antigen to the relevant parts of the body for development of protective immunity. This method of immunisation is called oral immunisation which is much easier to use and less costly and has the advantage of better compliance by vaccinees.

One of the six vaccines used in the Expanded Programme on Immunisation (EPI) in Bangladesh, the polio vaccine, is an oral vaccine. The remaining five vaccines-diphtheria, pertussis or hooping cough, tetanus, measles and tuberculosis- are injectible. Vaccines for enteric diseases such as diarrhoea (particularly cholera, bacillary dysentery and rotavirus diarrhoea) and for typhoid fever are being developed that are designed to be delivered orally with the idea that since in these diseases the site of infection is the gut, delivery of the vaccine at the site of infection may trigger a better immune response. Cholera and dysentery vaccines are in clinical trials in Bangladesh and soon field trials may be undertaken with some vaccines including an oral vaccine against rotavirus diarrhoea through the participation of the International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR, B).

DPT vaccine A combination vaccine given as one single injection against three diseases- Diphtheria, Pertussis (whooping cough) and Tetanus. The vaccine consists of inactivated toxin preparation (toxoid) of the diphtheria and tetanus causing bacteria (Corynebacterium diphtheriae and Clostridium tetani, respectively) and killed whole cells of pertussis causing bacteria Bordetella pertussis. Recommended immunisation schedule with DPT vaccine is at 2, 4, and 6 months of age, with two additional booster doses at 2 years and 5 years.

The immunisation coverage which was achieved with these three vaccines against the three most dreadful childhood diseases was disturbingly low prior to 1980s. Only about 1-2% of children had access to DPT immunisation which represented mainly the urban children. Beginning in the early 1980s the Extended Programme on Immunisation (EPI) was undertaken by the government of Bangladesh with assistance from the WHO with the result that by 1993 a coverage of over 80% was achieved. This is regarded as a commendable accomplishment on the part of a developing country like Bangladesh which is beset with many organisational problems and acute financial constraints. The establishment of community health care centres in the villages and the fact that Bangladesh represents the country with world's highest population density meant that with relatively small number of such centres a very large population could be covered. This in a large measure contributed to the success of the overall EPI programme in Bangladesh.

BCG vaccine An anti-tuberculosis vaccine widely used in the developing countries where the disease prevalence is very high because of poor living conditions, and the confounding effects of malnutrition. Tuberculosis is caused by a highly virulent bacillus Mycobacterium tuberculosis. The disease causes enormous suffering, incapacitation, and wasting both in adults and in children. A bacillus of bovine origin which is related to M. tuberculosis was found to be a virulent in human. The bacillus is called Bacille Calmette Guerin or BCG, named so after its discoverer, French scientists Albert Calmette and Camille Guerin. The BCG vaccine consists of attenuated live BCG bacteria. Because of antigenic similarities of BCG with Mycobacterium tuberculosis, the BCG vaccine provides immune protection against infection by the latter.

Bangladesh is a highly tuberculosis prone country because all the conditions that aid the spread of the disease such as poverty, malnutrition, unhygienic living conditions, etc are widespread in the country. Prevalence of the disease is on the increase in recent years as it is also the case in many other countries of the world, including many industrialised nations. The WHO considers the re-emergence of tuberculosis a global emergency at the present time and vaccination against tuberculosis is a high priority public health measure in many countries of the world.

In Bangladesh, the BCG vaccine has been a part of the national immunisation programmes for several decades now, but a major thrust has been put on it through the Expanded Programme on Immunisation (EPI) which was initiated in 1985. Prior to the launching of the EPI programme, the BCG vaccination coverage in Bangladesh represented a meagre 1% of the eligible population, but only a few years after the initiation of EPI, the coverage exceeded 95% which is considered to be a commendable accomplishment. [Zia Uddin Ahmed]

See also hepatitis; whooping cough.