Canada: Delta variant 'growing rapidly' in Ontario, warns Yaffe

CTV News talks about how many people in Ontario need to be vaccinated to achieve herd immunity. Dr. Barbara Yaffe isn't sure because of Delta variant.

"Herd Immunity" (also called herd effect, community immunity, population immunity, or mass immunity) is certainly a form of indirect protection from infectious disease 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, thereby reducing the likelihood of infection for individuals who lack immunity. Immune individuals are unlikely to contribute to disease transmission, disrupting chains of infection, which stops or slows the spread of disease. The greater the proportion of immune individuals in a community, the smaller the probability that non-immune individuals will come into contact with an infectious individual.

Individuals can really become immune by recovering from an earlier infection or through vaccination. Some individuals cannot become immune because of medical conditions, such as an immunodeficiency or immunosuppression, and for this group herd immunity is a crucial method of protection. Once the herd immunity threshold has been reached, disease gradually disappears from a population. This elimination, if achieved worldwide, may result in the permanent reduction in 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 contact exposed you to the infectious agent, but something infectious isn't always contagious.

Infectious diseases are caused by microscopic germs (such as bacteria or viruses) that get into 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 bacteria that makes you sick, but you can't give your food poisoning to someone else just by shaking their hand or even giving them a kiss.