The problem with vaccines
There are several people who are skeptical of vaccines, even if it is well proven that nearly all are overall safe and effective. However they have a point:
Vaccines work in two ways: they prevent the vaccinated subject from suffering a grave or fatal illness, and they stop the transmission of contagious diseases to other people.
In order to stop the transmission of contagious diseases they have to be given to everybody, which means also people with a low probability of infection.
In both cases, the essential property of a vaccine is that is not a cure for an illness, but a prevention of an illness, that is they are given to healthy people to keep them healthy.
Almost everything has a non-zero chance of resulting in damage or death.
Therefore giving vaccines to healthy people gives a non-zero chance of damage or death to healthy people.
There are many human genetic variations as to body biochemistry, as obvious for example there being a wide range of allergies that only a minority of people have; also if you read the information about medicines, there are lists of 1-in-100, 1-in-1000, 1-in-10000 side effects.
Thus presumably the non-zero chances of damage or death in giving a vaccine (or a medicine) to healty people may differ greatly between groups of people with different biochemistries, and vaccines (or medicines) that are tested to be quite safe for most people could be much worse for some small groups they could not be tested on.
But vaccines should be given to everybody, including small groups they were not tested on.
Therefore extremely cautious people might reason that since they are healthy, and they don't know whether they are part of a small group that has a particularly bad reaction to a vaccine (or medicine), they should not take the risk. There are two problems with that:
Vaccines cannot be taken after getting infected, so not being vaccinated makes sense only if the probability damage or death from infection is lower than that of damage or death from the vaccine, and that is practically never true.
In order to work to stop epidemics vaccines must be taken by everybody, so if there any significant minority of non-vaccinated people, they are putting many others are risk, not just themselves.
Consider the cases of measles: it is not a huge deal nowadays for most people, for most people it does little damage. The problem is that it often does very big damage to pregnant women and their babies, so the people who refuse measles vaccination because they think they don't need it and however small it is a risk they don't want to bear, are doing something that can have quite nasty consequences to other people.
