Private benefits + external benefits = Social benefits
The private benefits are those that accrue solely to the individual making the action.
When a person has a vaccination against tuberculosis, they receive the private benefit of being immune to the disease.
External benefits are the positive side-effects of production or consumption incurred by third parties.
When a person has a vaccination against tuberculosis, other people are also protected from this highly contagious disease.
The social benefits of a decision are all of the benefits that accrue from that decision.
The true benefit of the vaccination is called the social benefit.
This is an example of market failure because there are external benefits to society of vaccination programmes. If vaccinations were left to the choice of individuals, they would be under-consumed, mainly due to the price that would be charged for them.