[Answer]
It is common knowledge that preventive vaccination produces immunity, which then prevents the disease, but this question also reflects the doubts that many parents have.

This question is actually two-fold. First, it asks about the two types of preventive vaccinations: routine vaccinations and voluntary vaccinations. Routine vaccinations are given for measles and polio while vaccinations for the mumps, and the flu are given on a voluntary basis. Routine vaccinations are also called recommended vaccinations. Routine vaccinations are recommended under the Preventive Vaccination Act in Japan, and the cost is free or nearly free because it is subsidized by the national government. On the other hand, there are voluntary vaccinations, that is, vaccinations that are "desirable," but not stipulated by law, and while they are partly subsidized by the local government, the cost is basically paid by the individual.

Why do these differences exist? Some may wonder why national government doesn't bear the cost of inoculating everyone in the country on a regular basis if preventive vaccinations create immunity?

The reasons are somewhat complicated. The diseases for which preventive vaccinations are given under the Preventive Vaccinations Act are highly contagious with a high mortality rate and a high rate of after-effects. Measles, for example, is one such infectious disease. Statistics show that before a preventive vaccination for measles was available, several thousands of people died every year of the disease. (Approximately 8,000 deaths were recorded in 1951.) Since the development of a measles vaccination in 1976, the mortality rate has dramatically decreased and currently stands at around 20 per year.

The infectious diseases for which voluntary vaccination is given do not have the high mortality rate of measles. For example, the annual number of deaths due to encephalitis, a rare complication of the mumps, is estimated to be fewer than 10 per year. However, one serious complication of mumps is hearing loss, and without preventive vaccination, it is projected that approximately 500 children would suffer hearing loss every year. Given these statistics, vaccination is recommended although the cost of vaccination is borne by the individual.

The reason why mumps vaccination is voluntary is that it might cause encephalitis. This vaccination-related encephalitis is much milder than that caused by a wild-type mumps virus since mumps vaccine is prepared from live-attenuated (weakend) virus. In the case of natural infection, no one bears responsibility, but when encephalitis results from vaccination, the doctor who administered the vaccine or the national government is held responsible. Given that this is principle at work in society, countries do not actively support routine vaccinations. Even so, however, in the United States, routine vaccination is stipulated for mumps. Some pediatricians also assert that it is better to be naturally infected by mumps, but this is clearly wrong from a medical point of view.

As for the second question, immunity resulting from vaccination for mumps is adequate to prevent infection by a wild-type strain, but this immunity declines little by little with age. In this case, despite having been vaccinated, infection is possible, but because this is rare, it is not necessary to be revaccinated.

I am sure that many questions remain regarding vaccinations, and I will address them at my next opportunity.