Immunizing individuals against diseases such as measles, rubella, polio, and cholera to significantly enhance overall social welfare by boosting population health and productivity is an example of what?

Explanation

Vaccinations that improve public health and productivity generate benefits beyond the vaccinated individuals, which are known as positive externalities. This differs from economies of scale, which relate to cost advantages from increased production, and from negative externalities, which are harmful side effects. Net present value is a financial metric unrelated to external benefits.