Irrelevant. The social contract in today's society is for children to be vaccinated, not adults. Anti vaxers complaining that the social contract isn't quite as rigorous as it could be (via lack of boosters) is an evasion.
One of the cornerstones in herd immunity is* life long* immunity. Each disease has a different threashhold for the WHOLE population from newborn to elderly. Waning immunity is creating a drop in the % of immune people in the population. The social contract is equally applicable for adults as it is for children and babies, if you want herd immunity.
Not all vaccines can contribute to herd immunity as they do not prevent transmission, but rather are supposed to make the illness less severe in the vaccinated person.
Herd Immunity theory doesn't work IMO when discussing vaccination. I understand epidimic theory as going with the approach of preventing disease from ever happeing, and not looking at what the conditions are that result in disease becoming dangerous. I am not seeing disease prevention as the risk free way forward. I am more confident trying to understand how to best ensure that my family is able to manage disease without serious complications. Factors like individual health status and sanitation were not factored in from what I have understood. Contracting the disease is not a death sentence or even a hospital sentence. While it is a burden on the health system, with good nutrition and sanitation and optimal living conditions these diseases do not need to be scary unless you are immune compromised. Also that people were arrogant enough to think they could make diseases extinct through vaccination will probably become laughable down the line.
The question does have to be asked if Herd Immunity from vaccination doesn't work, why we are not seeing huge increases in adults being ill with VPD.