Immunoprophylaxis Against Infectious Diseases

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APPLIED MICROBIOLOGY Immunoprophylaxis against Infectious Diseases Prevention of infections depends upon three concepts: • • •

Elimination of source of infection Prevention of transmission of infective agents Protection of susceptible persons by active or passive immunization

Vaccine is an immunobiological substance designed to confer specific protection against a disease. It stimulates the immune system (either humoral or cell mediated or both) to generate specific protection against an infectious agent. Vaccines may be prepared from live modified organisms, inactivated or killed organisms, toxoids or combination of these. Live Versus Killed Vaccines Live attenuated as well as inactivated vaccines for different diseases are available. The former are also called as replicating and the latter as non-replicating vaccines. The live vaccines are prepared from live and generally attenuated organisms which have lost their ability to induce disease but retain their immunogenicity. In general, live vaccines are more potent immunizing agents because of following reasons: a. Can multiply in the host thus increasing the antigen dose manifold. b. Possess all major and minor antigenic components. c. Occupy natural niches for the pathogen in the body thus blocking colonization by the pathogen. d. May persist for longer time in the body in latent stages. The killed vaccines are prepared by subjecting the organism to the action of physical or chemical agents. These are usually safe but generally less efficacious than the live vaccines. Differences between Attenuated and Inactivated vaccines Feature Preparation Administration Route Dose Adjuvant Safety Cold chain requirement Cost Duration of immunity Immune response Humoral CMI

Attenuated Attenuation

Inactivated Inactivation

Usually natural route May be single Not required May revert to virulence ++++ Low Usually long IgG, IgA +

Parenteral Usually multiple Usually required Safe ++ High May long or short Mainly IgG Little or no

Adverse reactions to vaccines Normal toxicity Faulty production

Faulty administration

Abnormal inherent toxicity Presence of foreign toxin Bacterial contamination Wrong culture used Viral contamination Use of nonsterile apparatus

Contamination from operator Allergy

Local Serum sickness Neurological illness General anaphylaxis

Other causes Indirect effects

Abnormal sensitivity of vaccine Damage to fetus Provocation of disease

Contraindications to Vaccinations WHO has recommended a limited number of contraindications to vaccinations as summarized below: a. Immunization should be delayed in case of severe illness with fever, so that any sign of illness will not be attributed to the vaccination. Malnutrition, moderate fever, respiratory infections, common diarrhoea and any other benign ailment do not constitute contraindication for vaccination. Hospitalized children may receive necessary vaccinations before their discharge, and, in some cases, immediately following admission, particularly in the presence of nosocomial measles risk. b. Discontinuation of DPT immunization is recommended in case of occurrence of a severe postvaccinal reaction as collapse, shock, fever above 40.5°C, convulsions and other neurological symptoms. Diarrhoea is not considered a contraindication for oral poliomyelitis vaccination. Extra doses corresponding to those administered during the bout of diarrhoea should be given. c. No live vaccine is to be given to a person with an immunodeficiency or undergoing immunosuppressive treatment, corticosteroids therapy, radiotherapy, antimetabolite therapy, etc. d. Measles, mumps or rubella immunization should be delayed for at least six weeks when a recent injection of polyvalent immunoglobulin has been given. IMMUNOGLOBULINS AND ANTISERA Immunoglobulins (Ig) Two types of immunoglobulins are available: normal human Ig and specific human Ig. Normal human Ig is an antibody rich fraction obtained from a pool of at least 1000 people. The preparation is rich in IgG, almost whole of which is in free form (and not in aggregates). It contains very little of IgA. Normal human Ig is administered to prevent measles in highly susceptible individuals and to provide temporary protection (up to 12 weeks) against hepatitis A infection for travelers to endemic area and to contacts of case of hepatitis A in an outbreak. There should be a gap of 3 months between the administration of normal Ig and any live vaccine. Specific human immunoglobulins are prepared from the plasma of patients who have recently recovered from infection or who have been immunized against a specific infection. The plasma of donor should contain at least five times the amount of specific antibody as is present in standard reference serum. The immunoglobulins are usually given intramuscularly. Peak blood levels are reached in two days after intramuscular injection. The average half-life is 20-35 days. Generally immunoglobulins should not be given just before or after active immunization. National Immunization Schedule Every country has devised a schedule for immunizing children against common infectious diseases (which have been included in EPI of WHO) to obtain optimal results with the available resources. As per WHO recommendations one dose of BCG, three doses of combined OPT vaccine, three (if possible additional zero dose at birth) doses of oral poliovaccine and one dose of measles vaccine is to be given to the child in his first year of life to protect him against these six diseases. Of these BCG is given as soon after the birth as is possible along with a dose of OPV. OPT is injected from the age of 6 weeks onwards with a gap of four weeks each between three doses. Each injection of OPT is accompanied by a dose of OPV to reduce the contact of child with health functionary. Vaccine against measles is given on the completion of 9 months of age. Upto the age of 9 months, a child is protected against measles by the antibodies passively transferred to child from the mother.

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