Answer:
Social Benefit / Positive Externality
Explanation:
Each economic transaction has benefits & costs to society.
Eg: Vaccinations purchase by patients- benefits patients by preventing them from a disease for which they pays monetary cost to the doctor, which is latter's income benefit.
However, these both are patient's & doctor's private benefit & costs.
Externalities imply extra harm or benefit to other un-indulged parties, without any monetary exchange for that harm or benefit. Socially Beneficial are positive externalities, Socially harming are negative externalities.
Eg - In this case, vaccination is the positive externality : It has extra benefit for other people who are less probable to transmitting illness, without having paid for that prevention in any way.
But, Individual consumers (here patients) & producers (here doctors) decisions are based on their private benefit & cost. So, consumers' (here patients') willingness to pay will depend only on their private benefit of disease prevention & ignore the extra social benefit from the positive externality vaccination - as society less illness probability.
