In
the year 1798, Edward Jenner first demonstrated that vaccination
offered protection against smallpox. He used cowpox (poxvirus bovis) for
the immunization of man against the smallpox virus (poxvirus varialae).
For the last 200 years, the use of vaccines has continued to reduce the
burden of many bacterial and viral diseases.
Smallpox itself has been eradicated, and poliomyelitis in the verge of eradication. In Sri Lanka, the last case of virologically confirmed poliomyelitis patient was reported in 1993.
In Sri Lanka, the introduction of routine immunization has generally reduced the incidence of several vaccine preventable diseases. Similar success in disease reduction has been demonstrated by immunization programmes in many other countries. The World Health Organization’s (WHO) Expanded Programme on Immunization (EPI), with assistance from the United Nation’s Children’s Fund (UNICEF) and other donors, has made great strides in extending these benefits to developing countries. Immunizations permitted the global eradication of smallpox, and may do the same for poliomyelitis and some other diseases.
Immunizing
a child not only protects that child but also other children by
increasing the general level of immunity and minimising the spread of
infection.
History of Immunization in Sri Lanka
The history of immunization in Sri Lanka goes back to the 19th century. The law relating to compulsory vaccination (against smallpox) is referred to in the Vaccination Ordinance of 1886.
The
Expanded Programme on Immunization (EPI) established in 1978, has
continued to make excellent progress over the past two decades, most
notably in terms of achieving high immunization coverage and disease
control. The milestones of immunization in Sri Lanka are given below.
1886 - Vaccination against smallpox introduced under the Vaccination Ordinance
1949 - BCG Vaccination introduced against tuberculosis
1961 - “Triple” vaccination introduced against diphtheria, whooping cough and tetanus
1962 - Oral polio vaccine introduced
1963 - BCG vaccination of newborn introduced
1969 - Tetanus Toxoid administration to pregnant mothers introduced
1978 - Launching of the Expanded Programme on Immunization
1984 - Measles vaccination introduced
1991 - Revision of Tetanus Toxoid schedule
1995 - First National Immunization Days conducted.
1996 - Introduction of Rubella vaccine
2001 - Introduction of revised National Immunization Schedule with MR and ATd
2003 - Introduction of Hepatitis B Vaccine on phase basis
2008 - Introduction of Hib containing Pentavalent vaccine
1995 - First National Immunization Days conducted.
1996 - Introduction of Rubella vaccine
2001 - Introduction of revised National Immunization Schedule with MR and ATd
2003 - Introduction of Hepatitis B Vaccine on phase basis
2008 - Introduction of Hib containing Pentavalent vaccine
Immunization Schedule
With the commencement of the EPI Programme in 1978 focus was to control childhood T.B., tetanus, whooping cough, diphtheria, polio and neo-natal tetanus. In 1988, the focus shifted to disease elimination. In 1991, a fifth dose of OPV was introduced at school entry to facilitate the polio eradication process. Rubella, Hepatitis B and Hib containing Pentavalent vaccines introduced to the programme gradually over the years. The present EPI vaccination schedule is given in annex below. JE vaccine was introduced to high-risk areas in 1987. Primary Immunization against JE consists of 3 doses at an interval of 2 – 4 weeks between the first and second doses and one year between the second and third. A booster dose is given every 4 years after the primary immunization. JE immunization is offered children below the age of ten years living in identified high-risk areas.
Principles for Determination of Immunization Schedule
- Age specific risk of disease
- Age specific immunological response to vaccines
- Potential interference with the immune response by passively transferred material antibody
- Age specific risk of vaccine associated complications
- Programmatic feasibility
- In general vaccines should be administered to the youngest age group at riks for developing the disease
Objectives of the National Immunization Programme
- Eradication of poliomyelitis Elimination of neonatal tetanus, diphtheria, measles and rubella
- Reduction of morbidity and mortality due to whooping cough prevention of outbreaks
- Reduction of morbidity and mortality due to hepatitis B
- Reduction of morbidity and mortality due to Japanese Encephalitis
- Reduction of morbidity and mortality due to Haemophilias influence B disease
To achieve the above objectives, immunity has to be create in population against the specific organisms causing above diseases by administering potent vaccines in correct dosage using correct technique and according to the national immunization schedule.
‘Immunity’ is a term that originally implied exemption from military service or taxes; it was introduced into medicine to refer to those people who did not get further attacks of smallpox or plague once they had had the disease. In a wide sense the term refers to the resistance of a host organism to invasive pathogens or their toxic products. Immunity is divided into two main types
Non-specific immunity (sometimes called ‘innate immunity’): which includes the general protective reaction of the organism against invasion.
Specific immunity: Immunity is the ability of the body to tolerate material that is indigenous to it and eliminate material that is foreign. The immune system is comprised of organs and specialized cells that protect the body by identifying harmful substances, known as antigens, and by destroying them by using antibodies and other specialized substances and cells. There are two basic ways to acquire this protection – active immunity and passive immunity.
Passive immunity: involves either the transfer of antibodies or in some diseases, of sensitised white blood cells, from an immune to a non-immune person. Natural passive immunity is transferred from mother to child across the placenta (and in the colostrum in subhuman primate species). Artificial transfer is the therapeutic use of various antitoxins or gammaglobulins, as in the treatment of tetanus, diphtheria, gas gangrene, snakebite, and immunodeficiency states. The passive immunity is short-lived, depending on the life-span of the antibody or the transferred cells in the recipient. Once they disappear, the host is again susceptible to the disease.
Active immunity: is provided by a person’s own immune system. This type of immunity can come from exposure to a disease or from vaccination. Active immunity usually lasts for many years and often is permanent. Live microorganisms or antigens bring about the most effective immune responses, but an antigen does not need to be alive for the body to respond.
Types of Vaccine
Live attenuated vaccines are derived from disease-causing viruses or bacteria that have been weakened under laboratory conditions. They will grow in a vaccinated individual, but because they are weak, they will cause either no disease or only a mild form. Usually, only one dose of this type of vaccine provides life-long immunity, with the exception of oral polio vaccine, which requires multiple doses.
Inactivated vaccines are produced by growing viruses or bacteria and then inactivating them with heat or chemicals. Because they are not alive, they cannot grow in a vaccinated individual and therefore cannot cause the disease. They are not as effective as live vaccines, and multiple doses are required for full protection. Booster doses are needed to maintain immunity because protection by these vaccines diminishes over time.
Inactivated vaccines may be whole-cell or fractional. Whole-cell vaccines are made of an entire bacterial or viral cell. Fractional vaccines, composed of only part of a cell, are either protein- or polysaccharide-based. Polysaccharide-based vaccines are composed of long chains of sugar molecules taken from the surface capsule of the bacteria. Unless coupled with a protein, pure polysaccharide vaccines are generally not effective in children under the age of two years. This coupling process is known as “conjugation.” Recombinant vaccines are produced by inserting genetic material from a disease-causing organism into a harmless cell, which replicates the proteins of the disease-causing organism. The proteins are then purified and used as vaccine.
• Live attenuated
– Virus, e.g., oral polio vaccine (OPV), measles, yellow fever
– Bacteria, e.g., BCG
• Inactivated
Whole
– Virus, e.g., inactivated polio vaccine (IPV)
– Bacteria, e.g., whole-cell pertussis
Fractional
– Virus, e.g., oral polio vaccine (OPV), measles, yellow fever
– Bacteria, e.g., BCG
• Inactivated
Whole
– Virus, e.g., inactivated polio vaccine (IPV)
– Bacteria, e.g., whole-cell pertussis
Fractional
– Protein-based
– Subunit, e.g., acellular pertussis
– Toxoid, e.g., diphtheria and tetanus
– Polysaccharide-based
– Pure, e.g. meningococcal
– Conjugate, e.g., Haemophilus influenzae type b (Hib)
• Recombinant,
– Subunit, e.g., acellular pertussis
– Toxoid, e.g., diphtheria and tetanus
– Polysaccharide-based
– Pure, e.g. meningococcal
– Conjugate, e.g., Haemophilus influenzae type b (Hib)
• Recombinant,
-Hepatitis B
Impact of Immunization on Disease Transmission
An
infectious disease is an illness that occurs when an infectious agent
is transmitted from an infected person, animal, or reservoir to a
susceptible host. Some of the factors that influence transmission
include:
A basic concept of public health is that every individual who is protected from a disease as a result of an immunization is one less individual capable of transmitting the disease to others. Individuals who have been immunized serve as a protective barrier for other individuals who have not been immunized, provided that the number immunized has reached a certain level. Reaching and maintaining that level, which varies by communicable disease, provides “herd immunity” to unimmunized individuals. The figure bellow illustrates the concept of herd immunity. It shows two hypothetical populations in which each individual comes in contact with four other members of the population. Both populations have been exposed to a hypothetical disease that is 100% contagious. The first group has no immunity to the disease and, therefore, the disease spreads to everyone. The second population is partially immune due to vaccination services that have protected 75% of the population. Even though only 75% are immune because of vaccination, the disease does not spread to all of the remaining 25% of susceptible individuals. This is because some of the remaining susceptible are protected by the fact that they do not come in contact with an infected individual. This is how herd immunity can protect more people than those who actually receive vaccinations and thus inhibit the spread of disease.
Note that if the susceptible individuals are unevenly distributed, e.g., if they cluster in urban slums, the level of protection in that population will need to be higher to prevent transmission. However, immunization coverage is not near 100 % these unimmune persons (susceptible) can accumulate over the years and can cause outbreaks as experienced in 1999 measles outbreak. Therefore, it is important to plan and organize our immunization programme in such a way that minimum susceptible accumulate over the years.
Source - http://www.epid.gov.lk
