What Is Herd Immunity and Why Does It Matter?
Herd immunity can play an important role in controlling the spread of contagious diseases in a community. Diseases can spread when a large portion of the community is at risk of getting the disease due to lack of immunity (resistance) to the infection. When many people become ill at the same time, it may strain healthcare resources (doctor’s offices, hospitals, and clinics) and cause serious health issues or death to individuals.
Why is herd immunity important in communities?
Having a significant number of people immune to disease will reduce the risk of it spreading from person to person. Herd immunity may not eliminate the spread of disease but will decrease the likelihood of people coming in contact with an infected individual. Immunity within a local community is important to control the spread of disease. If immunity to a disease is low in a specific area or community, localized outbreaks of disease may occur.
Herd immunity is also important to reduce the chance of infection to those in the community who are unable to be vaccinated and people who are immunocompromised or with reduced immunity.
How does immunity occur?
People can get immunity to an infectious disease in one of 3 ways:
- Vaccines – an injection that causes the body to produce antibodies against the disease. The antibodies remain in the body to fight disease in the future.
- A previous infection – exposure to the virus or bacteria and recovery will cause the body to make antibodies to protect against future infections.
- Antibodies from an outside source – antibodies from a pharmaceutical product may be injected. Also, newborn infants may receive antibodies from their mother before birth or via breastmilk.
Generally, vaccination provides the best immunity from disease as prior infection and antibodies from another source may only provide protection for a limited amount of time.
How much immunity within a group is needed for herd immunity protection?
Herd immunity protection will vary depending on the virus or bacteria’s ability to spread. Most infectious diseases require about 70 to 80% of a group of people to be immune to an infection to prevent widespread disease. However, very contagious diseases, such as measles, will require most everyone to have immunity to prevent the spread of infection.
Is herd immunity long lasting?
Diseases may reappear in communities with previous herd immunity if new members entering a group do not have immunity from the disease. In many cases this is usually due to reduced vaccination of children.
One example of this change is measles, a highly contagious disease. In 2000, measles had nearly stopped spreading in the US. However, even a small loss of vaccination status in the community can cause measles to spread. Non-immune Individuals can be infected with measles when traveling outside the area and then spread disease when the individual returns if enough people in the community are not immune. This often occurs in unvaccinated children, allowing the disease to spread widely throughout the community, state, or region.
Rarely, elimination of a disease can occur with global herd immunity. This occurred with smallpox.
Summary
Immunity to disease is important within a community to reduce disease burden to healthcare systems and individuals in the community. It is important that new members of a community be immunized to maintain herd immunity.