Difference Between Polysaccharide and Conjugate Vaccines

The key difference between polysaccharide and conjugate vaccines is that polysaccharide vaccines contain only free polysaccharides as antigens while conjugated vaccines contain polysaccharides combined with a protein molecule.

Polysaccharide and conjugate vaccines are two types of vaccines. Polysaccharide vaccines contain only the polysaccharide capsule as the antigen to stimulate an immune response. Therefore, they cause a negligible immune response compared to conjugate vaccines. Conjugate vaccines contain polysaccharides conjugated to an immunogenic protein. Hence, they provoke a stronger immune response. Moreover, conjugated vaccines establish B-cell memory and long-term immunization. At present, conjugated vaccines have vastly replaced the polysaccharide vaccines.

CONTENTS

1. Overview and Key Difference
2. What are Polysaccharide Vaccines
3. What are Conjugate Vaccines
4. Similarities Between Polysaccharide and Conjugate Vaccines
5. Side by Side Comparison – Polysaccharide vs Conjugate Vaccines in Tabular Form
6. Summary

What are Polysaccharide Vaccines?

Polysaccharide vaccines contain the polysaccharide capsule of the bacterium as the antigen to stimulate an immune response. They are unconjugated vaccines. These free polysaccharide vaccines cause a weak immune response. In very young children, polysaccharide vaccines are too simple to stimulate antibody production. In fact, they cause a negligible immune response in children younger than two years (limited ability to protect children less than two years of age) and do not induce anamnestic reaction at any age.

The response produced by polysaccharide vaccines is not a T cell-dependent response. Moreover, it does not establish B cell memory. Furthermore, polysaccharide vaccines show a decreased immune response after repeated doses. Polysaccharide vaccines are available for three diseases: pneumococcal disease, meningococcal disease, and Salmonella typhi.

What are Conjugate Vaccines?

Conjugated vaccines are the vaccines that contain polysaccharides conjugated to the carrier protein. In addition to polysaccharides, they have an immunogenic protein. Hence, they provoke a stronger immune response. Conjugated vaccines produce a T cell-dependent response. Moreover, they establish B cell memory and longer-lasting immunity. Most importantly, conjugated vaccines are able to offer a protective immune response in infants, unlike polysaccharide vaccines.

Conjugated vaccines are less likely to induce a diminished immune response. Due to these advantages, conjugated vaccines have now replaced polysaccharide vaccines. However, there are several disadvantages of conjugated vaccines. They are dependency on a T cell response and the smaller coverage of pneumococcal serotypes.

What are the Similarities Between Polysaccharide and Conjugate Vaccines?

• Both polysaccharide vaccine and conjugate vaccine protect against bacteria with a polysaccharide capsule.
• These vaccines are considered safe.
• Both vaccines contain polysaccharides of the bacterial capsule as antigens.

What is the Difference Between Polysaccharide and Conjugate Vaccines?

Polysaccharide vaccines contain only free polysaccharides as antigens while conjugated vaccines contain polysaccharides combined with a protein molecule. So, this is the key difference between polysaccharide and conjugated vaccines. Moreover, another significant difference between polysaccharide and conjugated vaccines is that polysaccharide vaccines produce an immune response that is T-cell independent while conjugated vaccine produces a T cell-dependent immune response.

The below infographic tabulates more differences between polysaccharide and conjugated vaccines.

Summary – Polysaccharide vs Conjugate Vaccines

There are two types of vaccines that are developed to combat bacteria with a polysaccharide capsule. They are polysaccharide vaccines and conjugated vaccines. Polysaccharide vaccines contain only the free or plain polysaccharides while conjugated vaccines contain polysaccharides conjugated with an immunogenic protein. So this is the key difference between polysaccharide and conjugated vaccines. Moreover, conjugated vaccines produce a T cell-dependent response with the establishment of B cell memory and long term immunization, unlike polysaccharide vaccines. Hence, conjugated vaccines have now replaced the polysaccharide vaccines.

Reference:

1. “Principles of Vaccination.” CDC. Available here.
2. “Conjugate Vaccine”. En.Wikipedia.Org, 2020, Available here.

Image Courtesy:

1. “NmVac1” By Peter – JNI Medical Corp R&D (CC BY-SA 3.0) via Commons Wikimedia
2. “Haemophilus influenzae 01” Public Domain) via Commons Wikimedia