21.3A: Vaccines and Immunity

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Vaccinations prevent viruses from spreading by building immunity to the virus.

Learning Objectives

Explain how vaccination protects vaccinated individuals and the community

Key Points

Vaccinations are prepared with live viruses, killed viruses, or molecular subunits of the virus.
A live vaccine consists of a small dose of the active virus.
A killed vaccine contains the inactivated virus.
It is possible, though rare, for live vaccines to cause the disease they’re used to prevent.
Live vaccines are made by growing the virus in a lab, which causes mutations that allow them to grow better in the lab than in the host, thereby inhibiting their ability to cause disease.
Even though live vaccines are designed to cause few symptoms, back mutations can occur and cause the virus to readapt to the host and the disease to spread.

Key Terms

vaccination: inoculation in order to protect against a particular disease or strain of disease; causes a primary immune response without illness, allowing the secondary response to destroy subsequent infection
live vaccine: consists of an active microbe (virus or bacteria)
killed vaccine: (inactivated vaccine) consists of virus particles which are grown in culture and then killed using a method such as with heat or formaldehyde

Vaccines for Prevention

While we do have limited numbers of effective antiviral drugs, such as those used to treat HIV and influenza, the primary method of controlling viral disease is by vaccination, which is intended to prevent outbreaks by building immunity to a virus or virus family. Vaccines may be prepared using live viruses, killed viruses, or molecular subunits of the virus. The killed viral vaccines and subunit viruses are both incapable of causing disease.

Figure \(\PageIndex{1}\): Vaccinations: Vaccinations are designed to boost immunity to a virus to prevent infection.

Live viral vaccines are designed in the laboratory to cause few symptoms in recipients while giving them protective immunity against future infections. Polio was one disease that represented a milestone in the use of vaccines. Mass immunization campaigns in the 1950s and 1960s significantly reduced the incidence of the disease, which caused muscle paralysis in children and generated great fear in the general population when regional epidemics occurred. The success of the polio vaccine paved the way for the routine dispensation of childhood vaccines against measles, mumps, rubella, chickenpox, and other diseases.

The danger of using live vaccines, which are usually more effective than killed vaccines, is low, but significant since the possibility that these viruses will revert to their disease-causing form by back mutations is still present. Live vaccines are usually made by attenuating (weakening) the “wild-type” (disease-causing) virus by growing it in the laboratory in tissues or at temperatures different from what the virus is accustomed to in the host. Adaptations to these new cells or temperatures induce mutations in the genomes of the virus, allowing it to grow better in the laboratory while inhibiting its ability to cause disease when reintroduced into conditions found in the host. These attenuated viruses still cause infection, but since they do not grow very well, they allow the immune response to develop in time to prevent major disease. Back mutations occur when the vaccine undergoes mutations in the host such that it readapts to the host and can again cause disease, which can then be spread to other humans in an epidemic. This type of scenario happened as recently as 2007 in Nigeria where mutations in a polio vaccine led to an epidemic of polio in that country.

Some vaccines are in continuous development because certain viruses, such as influenza and HIV, have a high mutation rate compared to other viruses and normal host cells. With influenza, mutations in the surface molecules of the virus help the organism evade the protective immunity that may have been obtained in a previous influenza season, making it necessary for individuals to get vaccinated every year. Other viruses, such as those that cause the childhood diseases measles, mumps, and rubella, mutate so infrequently that the same vaccine is used year after year.