"Herd" immunity is a theory which purports to explain how an entire group can be protected from disease.  It is thought to result in protection of everyone once a magic percentage of vaccinated or otherwise immune individuals is reached, by preventing transmission of the virus to those who remain "unprotected", either because a vaccine didn't happen to work for them, or was too risky in their case.

It relies on getting as high a percentage vaccinated as possible, which means requiring that everyone who doesn't have an obvious contraindication to getting a vaccine, be vaccinated.

However, it is ironic and worth noting that it is the failure of vaccination, not its success, which is used as justification for requiring its use.  Thus, children attending school are required to get vaccinations, whether their parents want them to or not, whether it is personally good for them or not, simply because vaccination doesn't always work.   This, all in the name of "herd immunity".

Notwithstanding the questionable validity of using the weakness of a product as justification for forcing it on people (more on that another time), and whether or not there is any justice in forcing someone to potentially sacrifice their own health in order to "protect" someone else's, there seem to be problems with the theory itself,  i.e., the notion that if you reach a high enough percentage that is vaccinated, a "herd immunity" will result. 

One of those potential problems is that outbreaks have occurred in highly vaccinated populations, including those documented to be 100% vaccinated. 

If the "herd" cannot be protected with 100% vaccination rates, maybe there is something wrong with the theory.

Another possible problem is that vaccines often do not prevent transmission, instead merely preventing full-blown cases of disease (i.e., causing mild or subclinical cases).  In fact, circulating virus is known to boost the immunity of the vaccinated (and perhaps even those naturally immune), thereby prolonging the apparent effectiveness of the vaccine (and maybe even natural immunity).  In the absence of circulating disease, vaccine-induced immunity is more likely to wane, and boosters will be required.  In any event, it is hard to understand how something which does not necessarily prevent transmission can result in immunity for the "herd". 

Is the notion of "herd immunity" scientifically valid?

If circulation of disease cannot always be prevented by vaccination, and instead merely drives it underground in the guise of mild or subclinical cases, can "herd immunity" be counted on to work?

Is one of the reasons outbreaks have occurred in highly vaccinated populations because vaccines are not as effective as we have been led to believe? Are the methods currently used to measure/determine immunity inadequate or flawed? 

How valid is the justification for mandating vaccination, given what appears to be some evidence against the theory used to support that policy?

Sandy Mintz