For accurate information on the efficiency of vaccines, consult the WHO website.
In 2008, a nine-year-old girl in Georgia received a vaccine that aggravated her pre-existing but undiagnosed cellular disease. After receiving immunisations, the disease caused a brain disorder that led to brain damage. In children, it is not always possible to detect underlying conditions. Any child could be carrying a disease that could be exacerbated by vaccines without any doctor knowing. This makes vaccines exceptionally risky and unsafe.
Side-effects are extremely rare - far less than other medicines. These are utterly dwarfed by the effects of the infectious diseases they prevent. In the rare cases where the child may have a pre-existing condition that could be aggravated by immunisations, doctors are able to provide medical exemptions for diseases. In the US, approximately 0.05% of children enrolled in schools are exempt from immunisations on medical grounds. If parents ruled out immunisations and medical procedures that had the possibility of aggravating an undiagnosed illness, they would deny their child almost all medical treatment. Almost all treatments carry the risk of aggravating an undiagnosed illness in some form. We must devise a standard for "safety" that means that the risk of aggravating an underlying illness or disease is sufficiently small to justify the widespread use of the treatment. Vaccines would meet this standard for safety, even if it cannot be called 100% safe.
[P1] Vaccines can aggravate underlying existing conditions. [P2] Therefore, they are not safe.
Rejecting the premises
[Rejecting P1] This only occurs in very exceptional cases. [Rejecting P2] Therefore, we can accept that vaccines, relatively speaking, are a safe form of medical treatment.