When a large portion of a population develops immunity to a disease through past infection or a vaccine, the spread of the disease becomes very unlikely. The community including those that are not immune become protected. 

The herd immunity threshold is the percentage of a population that needs to become immune to an infection, so that those that are not immune are protected from infection. In order to determine the herd immunity threshold, the number of nonimmune people an infectious person can infect must be determined. This number is currently still unknown for COVID-19 due to uncertainties such as speculation around super-spreaders, the role of children in viral spreading, and inaccuracy of some diagnostic testing. 

Another key consideration for herd immunity is the uncertainty around the decay of antibodies. It is currently unknown at this time if and for how long antibodies may offer immunity to COVID-19. Two studies3-4 in Science Immunology have however demonstrated IgG antibodies persisting for at least 3 months.

1. https://www.nature.com/articles/s41577-020-00451-5

2. https://jamanetwork.com/journals/jama/fullarticle/2769704

3. https://immunology.sciencemag.org/content/5/52/eabe0367/tab-pdf

4. https://immunology.sciencemag.org/content/5/52/eabe5511/tab-pdf