A conjugate vaccine is a type of subunit vaccine which combines a weak antigen with a strong antigen as a carrier so that the immune system has a stronger response to the weak antigen.
Vaccines are used to prevent diseases by invoking an immune response to an antigen, part of a bacterium or virus that the immune system recognizes. This is usually accomplished with an attenuated or dead version of a pathogenic bacterium or virus in the vaccine, so that the immune system can recognize the antigen later in life.
History
thumb|For bacteria with a polysaccharide coating, like [[Haemophilus influenzae type b, the best way to prevent infection is to use a conjugate vaccine.]]
The idea of a conjugate vaccine first appeared in experiments involving rabbits in 1927, when the immune response to the Streptococcus pneumoniae type 3 polysaccharide antigen was increased by combining the polysaccharide antigen with a protein carrier. The first conjugate vaccine used in humans became available in 1987. Soon after the vaccine was made available the rates of Hib infection dropped, with a decrease of 90.7% between 1987 and 1991.
Even in adults, bacterial polysaccharides only produce a B cell response independent of T cell stimulation. The T cells cannot be activated by polysacchrides because polysaccharides by themselves cannot be loaded onto the major histocompatibility complex (MHC) of antigen presenting cells (APC) because MHC can only bind peptides. In contrast, proteins are much more effective at provoking the immune system, activating both types of cells. When the polysacchride is conjugated (chemically attached) to a protein, carrier peptide linked to the polysaccharide target antigen is able to be presented on the MHC molecule and the T cell can be activated. This improves the vaccine as T cells stimulate a more vigorous immune response and also promote a more rapid and long-lasting immunologic memory. Conjugation can also point the immune system towards "self" targets, such as in immunocontraception where an animal is made infertile by causing the immune system to intercept GnRH, a signal that activates the gonads, as an alternative to surgical neutering. The same idea is being used in cancer vaccines as a way to cause the immune system to target tumor antigens.
Approved conjugate vaccines
thumb|A vial of [[Soberana 02 vaccine in Iran for use in the phase III clinical trials]]
The most commonly used conjugate vaccine is the Hib conjugate vaccine. Other pathogens that are combined in a conjugate vaccine to increase an immune response are Streptococcus pneumoniae (see pneumococcal conjugate vaccine) and Neisseria meningitidis (see meningococcal vaccine), both of which are conjugated to protein carriers like those used in the Hib conjugate vaccine. which may be more effective and prevents typhoid fever in many children under the age of five years.
In 2021, Soberana 02, a conjugate COVID-19 vaccine developed in Cuba, was given emergency use authorisation in Cuba and Iran.
Select list of other conjugate vaccines
- Various immunocontraception vaccines for animal use, including GonaCon (GnRH linked to keyhole limpet hemocyanin)
- NicVAX, which aims to vaccinate against nicotine using a chemically modified hapten version linked to exotoxin A
- TA-CD, cocaine linked to inactivated cholera toxin
- TA-NIC, nicotine linked to inactivated cholera toxin
See also
- Toxoid
- Vaccine
- T cell
- B cell
- Immunogenicity
- Immune system
- Immune response