Hi Spledan, it is very important that all children have their vaccinations if they are able to. One reason for this is to protect the child, but another is to maintain 'herd immunity'.
When we vaccinate enough children we finally reach a point where it becomes much more difficult for a particular disease to spread, and sometimes we can eradicate a disease completely - in regards to the latter that doesn't mean that the disease won't come back if we stop vaccinating. Bacteria and viruses can still exist in water, soil, faeces e.t.c. When we reach the point where it becomes much more difficult for a disease to spread, this is known as 'herd immunity'.
'Herd immunity' is very important because it helps to protect those who can't be vaccinated. For example, they may be too young, have a health condition, or on rare occasions, are allergic to something added to the vaccine (not the vaccine itself). For individuals who can't be vaccinated, 'herd immunity' is the only protection that they have.
'Herd immunity' also helps to protect those who are immunocompromised, i.e. have a weakened immune system. For these individuals, particularly those with a very weakened immune system as a result of a health condition or medication, an infection, even one that may be seen as a relatively minor infection can make them seriously ill, leave them with permanent disabilities, or kill them. In order to try and protect these individuals we need to keep the spread of infection under control and infection rates as low as possible. One of the ways in which we do this is by maintaining 'herd immunity'.
If we don't vaccinate enough children, 'herd immunity' is lost and those who can't be vaccinated or have weakened immune systems will no longer be protected.
Unfortunately, there is a lot of misinformation and scaremongering about vaccines on the internet, and this information doesn't tend to come from reputable or trustworthy sources.
I can understand why you are so concerned that your grandchild was unwell after the vaccination, but a fever after the Men B vaccine is quite common. Also, it is much better than the alternative - serious illness, permanent disability or worse.
If you haven't read it already, NHS Choices has some excellent information on the Men B vaccine that will hopefully help to put your mind at rest: www.nhs.uk/conditions/vaccinations/pages/meningitis-b-vaccine.aspx