It is not uncommon to see online discussions with arguments of the form if your child is vaccinated, what risk is there from my unvaccinated child?.
The fact is that while vaccines are generally very good, they are not 100% effective in terms of seroconversion rates. Some vaccinated individuals can still catch and transmit the diseases they have been vaccinated against; although often the symptoms are usually less severe than in the unvaccinated case.
Ideally vaccines should be effective enough to reach the requirements needed for herd immunity. Herd immunity is a wonderful phenomenon whereby some individuals can escape both the risk of infection and the risk of vaccination (vaccination does carry small risks). It can be shown for example that assuming an ideal 100% effective vaccine, we need to vaccinate about 94% of children to eliminate measles. This 6% ‘breathing space’ is fortuitous because there are a number of people who cannot be vaccinated because they are too young or their immune system is compromised in some way.
When people deliberately don’t vaccinate their children for no other reason than personal choice, they are using up some of this precious 6%. Their children will generally be safe because they are being protected by the herd (freeloading), but at some point the space will be used up and herd immunity will break down. By implication, vaccination is not simply be a matter of personal choice because your choices affect your neighbours; it’s far more of a social collaboration.
Imagine a measles vaccine that is 99% effective after the scheduled number of doses. By implication we need to vaccinate more than 94% of children to maintain herd immunity and it follows that if the measles vaccine is less than 94% effective, herd immunity cannot be achieved because we have to theoretically vaccinate at a rate of over 100%. This is why we try to vaccinate all children, to create as much space as possible using herd immunity for those that really need it and to counter the less than perfect effectiveness of vaccines.
Even if a vaccine is not effective enough to achieve herd immunity it can still be highly desirable. Most diseases go though epidemic cycles. By vaccinating, both the magnitude and frequency of the epidemics are reduced. A side effect is that the average age of infection increases and for some vaccines this may outweigh the benefits because some diseases are worse when contracted later in life. Pertussis is an example of a disease that cannot presently be eliminated by childhood vaccination alone because of the efficacy of the current vaccines, but is still desirable because it massively reduces epidemic magnitude and frequency, and is less severe when caught later in life.