Contact immunity

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Contact immunity is the property of some vaccines wherein contact of unimmunized individuals with a vaccinated individual can confer immunity. In other words, if Mark has been vaccinated for virus X and John has not, John can receive immunity to virus X just by coming into contact with Mark.

Contact immunity is primarily a potential property of vaccines that consist of live virus of attenuated (weakened) virulence. Vaccination with a live but attenuated virus is the intentional infection of a person with the virus to produce immunity to the more dangerous wild form of the virus. These attenuated viruses produce little or no illness in most people, though the live virus multiplies briefly, may be shed in body fluids or excrement, and can be contracted by another person. If this contact produces immunity and carries no risk, it benefits an additional person and further increased the immunity of the group.

The most prominent example of contact immunity was that of the OPV, the oral polio vaccine. This live, attenuated polio vaccine was widely used in the US between 1960 and 1990 and continues to be used in polio eradication programs in developing countries because of its simple and cheap administration. It is popular in part because it is capable of contact immunity. Recently immunized children "shed" live virus in their feces for a few days after immunization. An unimmunized family member who is exposed to this shed virus usually develops immunity as well.

The principal drawback of live virus use is that a few people who are vaccinated or exposed to those who have been vaccinated may develop severe disease. Those with defective immune function were most vulnerable. In the case of OPV, there were rare instances of adults contracting paralytic polio from contact with a recently immunized child. As the risk of wild polio diminished, the small risk of contact infection with the attenuated polio virus outweighed the small advantages of oral administration and contact immunity, and OPV was replaced by the injectable, inactivated virus form of the polio vaccine.

Contact immunity should be distinguished from herd immunity, a different type of group protection, in which risk for unimmunized individuals is reduced if they are surrounded by immunized individuals who are unlikely to contract, harbor, or transmit the disease.