Vaccination

Vaccination is the process of producing immunity against a disease by exposing individuals to weakened, dead, or closely related (but relatively harmless) versions of the pathogen that causes this disease. With the advent of widespread vaccination within populations, rates of vaccine-preventable diseases have dropped dramatically, leading to significant decreases in both morbidity and mortality. Through vaccination, for example, smallpox has been completely eradicated and other diseases, such as polio and measles, are in the process of becoming eliminated. In spite of these successes, however, vaccination is not without controversy. Concerns over possible adverse effects have caused individuals in many areas of the world to question the benefit of vaccines.

Credit for the development of vaccination is given to Edward Jenner, an 18th century English physician. In 1796, Jenner successfully vaccinated an 8-yearold boy against smallpox by exposing him to the related, but much less virulent, cowpox virus. Since Jenner's initial success, many more vaccines have been developed to combat a variety of diseases, including measles, polio, diphtheria, rabies, pertussis, and the flu. Vaccines continue to be developed today to combat diseases, such as HIV and malaria, as well as infections, such as human papillomavirus (HPV) that can cause certain types of cancer.

The central idea behind vaccination is that exposure to weakened or dead microbes, parts of these microbes, inactivated toxins, or, like Jenner's smallpox vaccine, closely related but relatively harmless pathogens, can cause an immune response within individuals that can prevent subsequent infection. More specifically, when a person is vaccinated, he or she is exposed to a version of a pathogen that has been altered so that it does not produce disease, but so that it still contains antigens, or the parts of the pathogen that stimulate the immune system to respond. The B lymphocytes in an individual's blood then detect these antigens in the vaccine and react as if the real infectious organism was invading the body. During this process, the B lymphocytes clone themselves producing two types of cells: plasma cells and memory B cells.

The plasma cells produce antibodies that attach to and inactivate the pathogen. This response is known as the primary immune response; it can take up to 14 days for this process to reach maximum efficiency. Over time, the antibodies gradually disappear, but the memory B cells remain. If an individual is exposed to the disease-causing pathogen again, these dormant memory cells are able to trigger a secondary immune response. This occurs as memory B cells multiply quickly and develop into plasma cells, producing antibodies that in turn attach to and inactivate the invading pathogen. Unlike the primary response, this secondary response usually takes only hours to reach maximum efficiency. It is through this process that vaccination is able to protect individuals from disease.

Vaccination is also beneficial at the population level. When a sufficient number of individuals in a population are immune to a disease, as would occur if a large proportion of a population was vaccinated, herd immunity is achieved. This means that if there is [Page 1070]random mixing of individuals within the population then the pathogen cannot be spread through the population. Herd immunity acts by breaking the transmission of infection or lessening the chances of susceptible individuals coming in contact with a person who is infectious. Herd immunity is important because it provides a measure of protection to individuals who are not personally immune to the disease—for instance, individuals who could not receive vaccines due to age or underlying medical conditions or individuals who received vaccines but remain susceptible. It is herd immunity that made the smallpox eradication campaign possible, and it is herd immunity that prevents the spread of diseases such as polio and measles today.

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