Your Immune System is your Body’s Defense Against Infection (Part 2 of 2)

In our August blog, we provided an overview of how vaccines work, and a look at some of the vaccines in development worldwide. This month, we discuss the different types of vaccines, and why some may require more than one dose.

There are many things to consider in the development of a vaccine. Scientific considerations include knowledge about the infections the vaccines are intended to prevent (i.e. viral or bacterial), how germs infect cells, and how the immune system responds. Practical and logistical considerations include the regions of the world the vaccine is intended for, so that prevalent virus strain, environmental conditions, supply chain, and administration can be considered.

Types of Vaccines

There are currently four main types of vaccines that infants and young children commonly receive in the U.S.:

Live, attenuated vaccines

fight viruses and bacteria. They contain a weakened version of the living virus or bacteria that does not cause serious disease in people with healthy immune systems. These vaccines most closely resemble a natural infection, and the immune system responds well to them. Examples of live, attenuated vaccines include measles, mumps, and rubella vaccine (MMR) and varicella (chickenpox) vaccine. However, not everyone is able to receive this type of vaccine. A weakened immune system—such as in people undergoing chemotherapy—makes a person ineligible for live, attenuated vaccines.

Inactivated vaccines

use the inactivated version of the disease-causing germ to trigger the immune response. Inactivated vaccines usually don’t provide the same level of immunity as live vaccines, so several doses over time (booster shots) are recommended.

Toxoid vaccines

prevent diseases caused by bacteria that produce toxins (poisons) in the body. With a toxoid vaccine, a person develops immunity to only the parts of the bacteria that cause disease, instead of to the bacteria itself. In other words, the immune response is targeted to the toxin instead of the whole germ. Booster shots to ensure ongoing protection against disease may be needed.

Subunit, recombinant, polysaccharide, and conjugate vaccines

use specific pieces of the germ — like its protein, sugar, or capsid (a casing around the germ), and they give a very strong targeted, immune response. Because of the specific targeting in their action, they can be given to almost anyone who may need them, including those with weakened immune systems and long-term health problems. One limitation of these vaccines is that booster shots may be needed to ensure continued effectiveness.

Why Some Vaccines Require More Than One Dose

There are four reasons that people may need more than one dose of a vaccine:

  1. For some vaccines (especially inactivated vaccines), the first dose does not provide maximum immunity, so an additional dose is needed. The vaccine that protects against the bacteria Hib, which causes meningitis, is a good example.
  2. For some vaccines, immunity begins to wear off after some time has passed, and a “booster” dose can bring immunity levels back up.
  3. For some vaccines (primarily live vaccines), studies have shown that more than one dose is needed for maximum immune response to develop. For example, one dose of the MMR vaccine may not help the body to produce sufficient antibodies to fight off infection. The second dose helps make sure that fuller immunity results.
  4. Finally, in the case of flu vaccines, adults and children (6 months and older) are advised to get a dose every year because the flu viruses that are most active may be different from season to season. Each year, research is conducted regarding which viruses are expected to be most common, and flu vaccines are developed for those particular viruses.

Vaccines are an important part of the healthcare landscape. Many serious diseases like polio have been virtually wiped out in the developed world, thanks to vaccines. When getting vaccinated, it is important to be informed through discussion with a healthcare provider, and credible sources like and, to avoid the misinformation that can sometimes be found online. Well-researched, informed vaccination decisions support a healthier life, and a healthier community.