Two health workers give a child polio vaccine during a vaccination campaign in the Democratic Republic of the Congo.
Credit: WHO, Christopher Black
Vaccines
A vaccine is a biological preparation that stimulates immunity to a particular pathogen. A vaccine typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the microbe or its toxins. The agent stimulates the body’s immune system to recognize it as foreign, destroy it, and ”remember” it, so that the immune system can more easily identify and destroy any of these microorganisms that it encounters later. The body’s immune system responds to vaccines as if they contain an actual pathogen, even though the vaccine itself is not capable of causing disease. Because vaccines are widely used in the United States, many once common diseases—polio, measles, diphtheria, whooping cough, mumps, tetanus, and certain forms of meningitis—have become rare or well controlled.
The body’s immune system responds to vaccines as if they contain an actual pathogen, even though the vaccine itself is not capable of causing disease.
Unfortunately, however, there have been some steps backward in recent years. In the United States, more cases of measles and whooping cough have been reported due to the decision of some parents to forgo vaccination for their children. According to the Centers for Disease Control and Prevention (CDC), 189 cases of measles in 24 states and 5 outbreaks were reported in 2015 compared to less than 100 annual reported cases in the early 2000s. Similarly, in 2014, 32,971 cases of whooping cough were reported, a 15 percent increase from the 28,639 cases reported in 2013.
Several analyses of the safety of vaccines, especially childhood immunization schedules, have been completed. Although no vaccine is without the risk of adverse events, the currently recommended childhood immunization schedule has been determined to be safe.
Vaccinated people produce antibodies that neutralize a disease-causing virus or bacterium. They are much less likely to become infected and transmit those germs to others. Even people who have not been vaccinated may be protected by the immunity of the “herd” because the vaccinated people around them are not getting sick or transmitting the infection. The higher the proportion of vaccinated people in a community, the lower the likelihood that a susceptible person will come into contact with an infectious individual—leading to greater herd immunity.
In the past, thimerosal, a preservative that contains mercury, was used in some vaccines and other products. Use of this product became the subject of controversy, with some arguing that the substance caused autism in children. Extensive independent research has presented no convincing evidence of harm associated with the low levels of thimerosal present in vaccines. Nevertheless, since 2001, thimerosal has not been routinely used as a preservative in recommended childhood vaccines.
