Below is an excerpt from the above link to the website for the Association for Professionals in Infection Control and Epidemiology (APIC) —
What is herd immunity?
Herd immunity (or community immunity) occurs when a high percentage of the community is immune to a disease (through vaccination and/or prior illness), making the spread of this disease from person to person unlikely. Even individuals not vaccinated (such as newborns and the immunocompromised) are offered some protection because the disease has little opportunity to spread within the community.
Vaccines prevent many dangerous and deadly diseases. In the United States, smallpox and polio have both been stamped out because of vaccination. However, there are certain groups of people who cannot get vaccinated and are vulnerable to disease: babies, pregnant women, and immunocompromised people, such as those receiving chemotherapy or organ transplants. For example, the earliest a baby can receive their first pertussis or whooping cough vaccine is at two months, and the earliest a child can receive their first measles vaccine is at one year, making them vulnerable to these diseases.
Herd immunity depends on the contagiousness of the disease. Diseases that spread easily, such as measles, require a higher number of immune individuals in a community to reach herd immunity. Herd immunity protects the most vulnerable members of our population. If enough people are vaccinated against dangerous diseases, those who are susceptible and cannot get vaccinated are protected because the germ will not be able to “find” those susceptible individuals.
When can we expect herd immunity for COVID-19?
COVID-19 is a very contagious disease. A large percentage of the population will need to be immune against the disease (through infection or vaccination) before herd immunity will be achieved. It is not known when that will happen, but it will depend on how many people develop immunity after COVID-19 infection, how soon a COVID-19 vaccine is widely available, how many vaccine doses will be available for distribution, and how many people get vaccinated. Throughout this time period until herd immunity is achieved, it is very important to continue to wear masks in public and social distance to slow the spread of COVID-19.
Several drug makers have developed promising vaccines for COVID-19 and have submitted applications for emergency use authorization to the U.S. Food and Drug Administration (FDA). If the FDA approves the vaccines for use, then the focus will shift to manufacturing the vaccine, educating members of the public about the vaccine, and distribution of the vaccine. Although these efforts will take some time to achieve, vaccines have proven to be the safest, most cost-effective way to protect people from disease. APIC continues to monitor and advocate on policies affecting each one of these steps and will continue to share consumer information via our social media channels."