How does immunization work?

© UNICEF/HQ02-0289/Pirozzi
Information session with community women in Begene, a remote village near the town of Bla, Mali

Immunization works by tricking the body into believing it is experiencing a full-scale invasion by an infectious agent so that the immune system can fortify its defenses. During vaccination, a harmless version of a germ is introduced to the body and the immune system responds by producing antibodies to attack the intruder. Thereafter, a memory of this “invasion” remains so that the immune system can quickly recognize and neutralize disease-causing agents when they appear.

The Chinese performed a version of vaccination called variolation in the 16th century when they discovered they could prevent smallpox by exposing a healthy person to matter from the lesions of an infected person. In 1796, Edward Jenner, an English doctor, performed the first vaccination in Europe when he used a cowpox virus to vaccinate a young boy against the more deadly smallpox virus. (Dr. Jenner called this process vaccination after the Latin word for cow, vacca.)

Today there are several types of vaccines. Some, such as the oral polio vaccine (OPV), are live, “attenuated” vaccines which means the virus has been weakened so that it stimulates antibody production, but does not cause the disease. Others such as the “whole-cell” pertussis vaccine use an inactivated, or killed, virus that still triggers an immune response. Tetanus toxoid (TT), the vaccine that protects mothers and newborns from tetanus, is a detoxified version of the toxin (poison) that causes the disease. A fourth variety of vaccine, such as that for Haemophilus influenzae type b (Hib), uses only the components of the virus or bacteria that provoke an immune response.

Mothers can pass on immunity to their babies across the placenta during the final months of pregnancy. The amount of inherited immunity varies by disease and is an important factor in deciding when a child should be immunized. A mother's antibodies may protect a child from measles for 6 to 12 months. But, in the case of diseases such as pertussis, immunity may last only for a few weeks. Tetanus is one example where inherited immunity is critical and the mother must be immunized to offer protection to her newborn.

For many diseases, immunity is built up over several doses of vaccine. The World Health Organization (WHO) recommends that the first polio vaccine be given at birth, along with the vaccine for childhood tuberculosis (BCG). In countries where transmission of hepatitis B from mother to child is common, these infants should be immunized against the disease at birth.

The remaining doses of polio vaccine and the combination diphtheria, pertussis, tetanus vaccine (DPT) should be given three times before the age of one: at six weeks, 10 weeks and 14 weeks. Due to inherited immunity, measles vaccines are typically given at nine months. Yellow fever is also given at this time for children in high-risk regions.

The more children in a community that are vaccinated, the less likely it is that any children, even those who have not been immunized, will get sick because there are fewer hosts for the infectious agents. This is referred to as “herd” immunity and it is particularly vital with extremely contagious diseases such as measles, where immunization of 90 to 95 per cent of infants is needed to protect a community from measles. However, this is not true for all diseases, such as tetanus, therefore an individual’s vaccination status is important, not just group immunity.



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