- (written as Herd Immunity)The protection offered by vaccines is rarely 100 per cent. Any vaccine will be more effective at the population level if more people have been vaccinated because some diseases may be able to jump from a vaccinated person to a person who has not been vaccinated but is unlikely to jump from one vaccinated person to another who has been vaccinated. Empirically, when a particular percentage of a population is vaccinated, the spread of the disease is effectively stopped. This critical percentage varies according to the disease, the interactions between members of the population and the vaccine, but 90 per cent is not uncommon. This is herd immunity - the fact that others in the herd or population are vaccinated provides protection to all, whether or not vaccinated themselves. An obvious implication is that 100 per cent vaccination is not normally a technically necessary target that is necessary to obtain effective 100 per cent population protection. Of course, a cost-effective rate of vaccination will normally be even lower, depending on the social value of the marginal reduction in risk and the cost of increasing vaccination from a lower to a higher percentage of the population at risk. (It will be lower than the herd immunity level.) The marginal costs of increasing vaccination rates may rise quite sharply as one seeks to immunize groups who are reluctant (for a variety of reasons, including religious objections, fear of the needle, imaginary risks, lack of contact with health care services, ignorance, misinformation).