A vaccine primer
Vaccines are designed to “teach” the body’s immune system to recognize and fight off invading pathogens. They do this by mimicking a natural infection, because they look like disease-causing agents, either in whole or in part. Types of vaccines include:
Live, attenuated vaccines
Viruses aren’t “alive” in the sense that they can reproduce by themselves; they must hijack the machinery of the cells they infect in order to make copies of themselves. By “live,” scientists mean that the viruses used in these vaccines are still capable of infecting cells, but the viruses have been “attenuated,” or weakened, so they cannot cause disease.
Viruses can be weakened by growing them in cells in which they don’t reproduce well. As they adapt to their new homes, changing their genetic material in the process, they become less able to cause disease in their natural host. Live, attenuated vaccines can also be created using recombinant DNA technology to alter genes in the viral genome so the viruses can’t replicate as well.
Live, attenuated vaccines are good teachers of the immune system because they closely mimic a true infection. But the possibility exists that the living microbes that make up such a vaccine might cause illness, particularly in immunocompromised individuals, or the attenuated virus might revert to a virulent form and cause disease. Some children may experience a very mild form of measles (generally a rash and fever) a week to 10 days after receiving the vaccine for measles, mumps and rubella. And because the attenuated virus in the oral polio vaccine can revert in rare cases to a more virulent form that can cause paralysis, only inactivated polio vaccines are now used in the United States.
Inactivated (killed) vaccines
Inactivated vaccines are produced by growing large batches of disease-causing microbes and killing them with chemicals, heat, or radiation. Inactivated vaccines are more stable and safer than live vaccines, but they usually stimulate a weaker immune response than live vaccines. Examples include vaccines for influenza, hepatitis A, and rabies.
Subunit vaccines use only important parts of a microbe—the parts that will best stimulate the immune system. Because subunit vaccines do not include the entire microbe, they usually provoke fewer adverse reactions, but also a less vigorous immune response than live vaccines. Subunit vaccines can be produced by purifying proteins from whole microbes or by using recombinant DNA technology to produce the desired proteins in another cellular system. Hepatitis B and pertussis are subunit vaccines.
For bacteria that secrete a harmful toxin, the purified, but inactivated toxin—called a toxoid—can be used to stimulate a protective immune response. Examples include vaccines for diphtheria and tetanus.