Vaccine

DEFENITION OF VACCINE

A vaccine is a substance that is introduced into the body to prevent infection or to control disease due to a certain pathogen (any disease-causing organism, such as a virus, bacteria or parasite); the vaccine ‘teaches’ the body how to defend itself against a pathogen by creating an immune response. Vaccines can be introduced in different ways, such as injection into the muscle (intramuscular) or into or under the skin (intradermal or subcutaneous); by application to the skin (transdermal); by application to the inside of the nose (nasal); or by being swallowed (oral).Right now, there is no vaccine to protect against HIV/AIDS.

A vaccine’s efficacy refers to how well it protects against disease or infection when it is tested in a large trial in humans; a vaccine’s effectiveness refers to how well it reduces the amount of disease once it is used in the overall population.

How Preventive Vaccines Work The following steps outline how a preventive vaccine protects an individual from infection or disease: The vaccine introduces a small piece or a non-harmful form of the pathogen into the body. This is called the foreign antigen (‘foreign’ indicates that it is not from the person’s own body). The immune system in the body produces an immune response to the pathogen by making antibodies, killer cells or both. The immune system has memory B cells (producing antibodies) and memory T cells (helping the production of antibodies or killer T cells). The next time the real pathogen is encountered, the immune system remembers it and mounts a much larger and quicker response than it would have if the person had never received the vaccine. This is called ‘immune memory’. This larger and quicker immune response can act in several ways to fight infection and/or disease: By stopping replication of the pathogen, so it cannot infect more cells By producing antibodies that attach to the pathogen, rendering it harmless (antibody response) By producing immune cells that attack and kill other cells that have been infected with the pathogen (killer cell response) Preventive vaccines are the traditional type of vaccine, defined above. They are intended for people who have not yet been infected. They prepare the immune system to respond in case of future exposure to the pathogen. Common examples include polio, measles, hepatitis B and tetanus vaccines. All vaccines now marketed throughout the world are preventive vaccines, although a few can work if given immediately after exposure (such as a rabies vaccine given right after a dog bite or a tetanus ‘booster’ vaccine given after a wound, provided that the patient has been vaccinated before and has immune memory). Most of the AIDS vaccine candidates now being tested are preventive vaccines. Another way that a vaccine might work would be to start an immune response after a person has been infected with HIV; this would be called a therapeutic, or ‘treatment’, vaccine. Right now, there is no HIV vaccine that works this way, although some scientists are trying to develop one.

NOTES A vaccine ‘teaches’ the immune system how to defend itself against a disease-causing agent, known as a pathogen. A vaccine is designed to prevent one specific disease or pathogen; therefore, a vaccine ‘matches’ with a certain disease. A preventive vaccine is meant for people who have not been infected with the pathogen that the vaccine is designed to protect against. A preventive vaccine is not a treatment or cure for someone who is already infected with the specific pathogen.