Why cant a vaccine ever be completely safe?

Answer 1

For the most part, they are completely safe (as much or more safe than any medicine is), and much more safe than getting the infectious disease they are designed to prevent; most of which can be deadly in some people. This is the position of most health care professionals, the Centers for Disease Control and Prevention (CDC) in the US, and the World Health Organization (WHO), to name only a few of the public health and medical entities who recommend vaccinations.

The most severe problem with vaccinations are in people who are allergic and have a reaction. In some people these can be life-threatening if not treated immediately to prevent anaphylactic shock. Having said that, serious allergic responses are not common from most vaccines in use today. (See the related question below about vaccinations in those with allergy to eggs or chicken protein.) The US and most nations have monitoring programs in place for reporting by doctors or patients when there is a suspected problem caused by the vaccines so they can be investigated.

There are ingredients in some vaccines that have been inappropriately linked to various diseases and disorders by some people (usually those who oppose all vaccine programs) and by a few who believe there is a conspiracy to introduce foreign substances/chemicals in our bodies by government authorities. Most of these claims are not proven and many have been studied and disproved scientifically. See more related questions below for additional information about the safety of vaccinations.

Answer 2

Vaccines can never be completely safe because the vaccine contains a small amount of the virus corresponding to the virus you are actually getting vaccinated for. For example flu viruses are in the flu vaccination. They do this so your anti-bodies can prepare for when they have to fend off larger attacks of the virus; like getting sneezed on. The problem with this is your anti-bodies sometimes are not ready for the vaccination and it is possible for you to become infected with the virus (i.e. the flu) from the shot.