CTV News talks about how many people in Ontario need to be vaccinated to gain herd immunity. Dr. Barbara Yaffe is unsure due to the Delta variant.
“Herd immunity” (also called herd effect, community immunity, population immunity, or mass immunity) is certainly a form of indirect protection from infectious diseases that can occur with some diseases when a sufficient percentage of a population has become immune to an infection, whether through vaccination or previous infections, thus reducing the likelihood of infection for people without immunity. Immune individuals are unlikely to contribute to disease transmission by disrupting the chains of infection, which halt or slow the spread of the disease. The greater the proportion of immune individuals in a community, the less likely it is that non-immune individuals will come into contact with an infected individual.
Individuals can truly become immune by recovering from a previous infection or through vaccination. Some individuals cannot become immune due to medical conditions, such as immunodeficiency or immunosuppression, and herd immunity is a crucial method of protection for this group. Once the herd immunity threshold is reached, the disease gradually disappears from a population. This elimination, if achieved worldwide, can result in the permanent reduction of the number of infections to zero, called eradication.
Infectious vs contagious:
Contagious diseases are spread by contact, while infectious diseases are spread by infectious agents. Something “contagious” is by default “infectious” because the contact has exposed you to the infectious agent, but something infectious is not always contagious.
Infectious diseases are caused by microscopic germs (such as bacteria or viruses) that enter the body and cause problems. Some (but not all) infectious diseases spread directly from one person to another. Infectious diseases that spread from person to person are said to be contagious.
“Food poisoning” is a good example of something infectious but not contagious: food can be contaminated with a bacterium that makes you sick, but you can’t give your food poisoning to someone else just by shaking their hand or even kissing them.