Although vaccines have been an incredibly useful and important advancement in the practice of medicine, there are some valid reasons certain members of the population cannot be vaccinated. 

In many cases, these contraindications are caused by the adjuvants, or added ingredients which generally make a vaccine more effective or better received, helping to activate the immune system and build disease immunity. However, these additives can cause allergic reactions in some patients, and if an allergic reaction has been shown to a certain type of vaccine or adjuvant, subsequent vaccination is not recommended. Examples include allergy to gelatin begin contraindicated against MMR vaccination, as well as severe egg allergies for yellow fever vaccinations.

Other causes for concern are pregnancy, during which certain vaccines such as measles and hepatitis B are not recommended, due to possible adverse effects to the developing child, as well as severe immunosupression, during which the small amounts of active antigen given in certain vaccines may not be able to be overcome by the damaged immune system. 

It is a common misconception that current illness or fever is a true contraindication against vaccination, but according to the CDC, this does not automatically mean a patient cannot be vaccinated, merely that precautions should be taken and the medical care provider should evaluate the patient on a case-by-case basis. 

Situations in which an individual may not be able to receive a vaccine leave them vulnerable to infection. These cases highlight why herd immunity is so important to maintain in a population, as it is one of the only ways to protect these individuals. 

Sources:

"Contraindications." Types of Vaccine and Adverse Reactions. World Health Organization, n.d. Web. 28 Sept. 2016.

"Chart of Contraindications and Precautions to Commonly Used Vaccines."Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 06 Mar. 2014. Web. 28 Sept. 2016.

"Vaccine Contraindications and Precautions for ADULTS Only." Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 06 Mar. 2014. Web. 28 Sept. 2016.

"Conditions Commonly Misperceived as Contraindications to Vaccination."Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 17 July 2012. Web. 28 Sept. 2016.