1. When most of a population is immune to an infectious disease, this provides indirect protection—or herd immunity (also called herd protection)—to those who are not immune to the disease.
2. When a large percentage of the population becomes immune to a disease, the spread of that disease slows down or stops.
3. Many viral and bacterial infections spread from person to person. This chain is broken when most people don’t get or transmit the infection.
· This helps protect people who aren’t vaccinated or who have low functioning immune systems and may develop an infection more easily, such as: older adults, babies, young children, pregnant women, people with weakened immune systems, people with certain health conditions
· For some diseases, herd immunity can go into effect when 40 percent of the people in a population become immune to the disease, such as through vaccination. But in most cases, 80 to 95 percent of the population must be immune to the disease to stop its spread.
· For example, 19 out of every 20 people must have the measles vaccination for herd immunity to go into effect and stop the disease. This means that if a child gets measles, everyone else in this population around them will most likely have been vaccinated, already have formed antibodies, and be immune to the disease to prevent it from spreading further.
· The goal of herd immunity is to prevent others from catching or spreading an infectious disease like measles.
· However, if there are more unvaccinated people around the child with measles, the disease could spread more easily because there is no herd immunity.
· To visualize this, picture someone without immunity as a red dot surrounded by yellow immune dots. If the red dot can’t connect to any other red dots, there is herd immunity.
· The percentage of people that must have immunity to safely slow or stop an infectious disease is called the “herd immunity threshold.”
