Herd immunity is based on naturally acquired immunity, which is a life-long immunity developed from natural exposure to a pathogen.
Vaccine-induced immunity is temporary—hence why so many boosters are required, why more and more boosters have been added over time. It’s also why we see outbreaks in highly “vaccinated” populations.
Primary Vaccine Failure
Vaccines are not always capable of eliciting the antibody response in a person—this is called “Primary Vaccine Failure”. You do not know if you are a non-responder unless you get your titers checked.
Just being injected with a vaccine does not guarantee antibody production. Unresponsiveness to vaccinations, specifically Hep B, is associated with gene polymorphisms related to different types of immune-mediate inflammatory conditions such as Type 1 Diabetes (T1D), Celiac disease (CD), rheumatoid arthritis (RA), obesity and systemic lupus erythematosus (SLE). Up to 15% of people will not respond to Hep B vaccine at all.
Secondary Vaccine Failure
All vaccines wane over time, and this is known as “Secondary Vaccine Failure”. Some vaccines, like the flu shot wane in a matter of weeks. Others like the pertussis component wanes in a year (bordatella pertussis has mutated to be pertactin deficient, and is mismatched for the vaccine, which targets the surface protein pertactin). The mumps component of the MMR is about 88% effective (range: 31-95%) so it could be as little as a few years.
Lifelong immunity from natural infection affords a few things: it creates a natural cocoon around the most vulnerable groups. For example, a nursing baby is protected by her mother’s breastmilk which contains antibodies providing “passive immunity”. Historically, measles was rare in the first year of life because of this.
When people have natural infection, they develop lifelong immunity that does not wane, and this creates “Herd Immunity”. It is the lasting immunity that is protective against infection, that protects the vulnerable around you.
It is not possible to reach herd immunity through vaccination, given that peoples vaccines will wane in a staggered way, leaving exposed pockets in an unpredictable manner. It is not possible to determine ‘immune status’ by evidence of receipt of vaccine.