Herd immunity: protecting people who cannot be vaccinated
Herd immunity is the indirect protection a community gets when a high enough proportion of people are immune (protected from disease), so infectious people rarely meet anyone who is not.
Remember
Herd immunity protects people who cannot respond to vaccines: when enough people are vaccinated, the pathogen is passed on less often, so they are exposed less often, provided the vaccine reduces spread.
Vaccines protect the person who receives them, but they also protect the community. When a high enough percentage of a community is vaccinated and immune, an infectious person rarely meets anyone who is not immune. Pathogens are then passed on less often, so transmission, the spread of the infection from one person to another, falls. This is herd immunity, and it works only for infections where vaccination really does reduce spread.
Some people cannot be protected by a vaccine themselves. This is commonly due to immune defects, which are weaknesses of the immune system. If a patient cannot produce antibodies, a vaccine will not give them immunity. Such defects can be inherited, like a congenital antibody deficiency (an inherited problem present from birth), or acquired through treatment such as chemotherapy, which weakens the immune system. These people depend on the people around them being immune.
Each person makes their own antibodies and keeps them in their own blood, so contacts do not hand their antibodies over. Fewer infectious people are circulating, so the vulnerable person has fewer chances of being exposed.
The proportion of a community that needs to be immune depends on how transmissible the disease is, which is how infectious it is. Measles needs about 95% of the population to be vaccinated. Herd immunity should come from vaccination, not from letting disease spread, because that causes unnecessary cases and deaths.
Key facts
- Herd immunity
- the indirect protection a community gets when a high enough proportion of people are immune, so an infectious person rarely meets someone who is not immune.
- Who relies on it
- people unable to be vaccinated, or whose immune systems cannot respond to a vaccine, for example those with immune defects.
- Mechanism
- people who cannot respond to a vaccine are protected indirectly, because fewer infectious people are circulating and so they are exposed less often.
- Threshold
- the proportion needed varies with the disease, and it is about 95% for measles.
- Antibodies
- vaccinated people keep the antibodies they make in their own blood, and these are not shared between people through everyday contact.
- Principle
- herd immunity should be reached through vaccination, not by letting a disease spread, since that would cause unnecessary cases and deaths.
Exam traps
- Antibodies made by vaccinated people are not passed to other people through contact, because each person keeps their own antibodies in their own blood. Protection of people who cannot be vaccinated comes from the pathogen circulating less often.
- A vaccine acts in the person who receives it and does not weaken the pathogens circulating in a community.
- A threshold of about 50% is far too low for measles, because the figure needed is about 95%, and it depends on how transmissible the disease is.
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