Contact immunity

An overview of contact immunity in the context of vaccination

Contact Immunity

Oral polio vaccine being administered, which can lead to contact immunity.

Contact immunity refers to the phenomenon where individuals who have not been directly vaccinated gain immunity to a disease through exposure to others who have been vaccinated. This occurs primarily with live attenuated vaccines, which contain a weakened form of the virus that can still replicate to a limited extent.

Mechanism

The mechanism of contact immunity involves the shedding of the vaccine virus by the vaccinated individual. This shed virus can then be transmitted to close contacts, who may develop immunity without having received the vaccine themselves. This is particularly relevant in the case of the oral polio vaccine (OPV), which is known to shed the attenuated virus in the feces of vaccinated individuals.

Examples

One of the most well-known examples of contact immunity is with the oral polio vaccine. When a person receives the OPV, the weakened virus replicates in their gut and can be excreted in their stool. In areas with poor sanitation, this can lead to the spread of the vaccine virus to others, who may then develop immunity to poliovirus.

Benefits

The benefits of contact immunity include:

• Increased herd immunity: By indirectly immunizing individuals who have not been vaccinated, contact immunity can contribute to the overall level of immunity in a population, helping to prevent outbreaks.
• Protection of vulnerable populations: Individuals who cannot be vaccinated due to medical reasons, such as immunocompromised individuals, may gain some level of protection through contact immunity.

Risks

While contact immunity can be beneficial, there are also potential risks:

• Vaccine-derived virus: In rare cases, the attenuated virus from a live vaccine can mutate and regain virulence, leading to vaccine-derived infections. This is a concern with the oral polio vaccine, where vaccine-derived poliovirus (VDPV) can occur.
• Unintended spread: In areas with inadequate sanitation, the spread of the vaccine virus can be uncontrolled, leading to potential health risks.

Related pages

• Vaccination
• Herd immunity
• Live attenuated vaccine
• Poliovirus