Usually when you are exposed to viruses or bacteria they cause infections that make you sick. To fight this infection, your immune system produces antibodies. These are special disease-fighting cells that attack the virus, destroy it, and make you better. In many cases, once you have made antibodies against a virus, you are then immune to the infection that it causes. This means that you cannot get sick from the same infection. For instance, if you had chickenpox as a child, you are immune to it later in life because your body has produced antibodies against the varicella virus (the virus that causes chickenpox). If you are exposed to the virus again, your antibodies recognize it and destroy it before it makes you sick.
Vaccines work with your body’s natural defenses to help you safely develop immunity to certain diseases. A vaccine uses a small piece of the virus or bacteria that causes the infection. Usually this virus is greatly weakened or it is killed. But it looks enough like the live virus to make your body react and make antibodies to attack the virus in the vaccine. This allows you to become immune to the disease without having to get sick first. For example, after you get the chickenpox vaccine, you will develop antibodies against the varicella virus, but you will not get chickenpox first. This factsheet from the CDC explains the body’s immune response to disease and how vaccines work in much more detail.
There are two main types of vaccines: weakened, live virus or inactivated, killed virus.
Vaccines that use weakened, live viruses include measles, mumps, rubella, rotavirus, flu mist, and chickenpox (varicella). Natural viruses reproduce thousands of times when they infect an individual. But weakened viruses can only reproduce about 20 times. This is not enough to make you sick, so they can’t cause disease. But even a few copies of the virus will cause your immune system to react and to make antibodies against the disease. The advantage of live, weakened vaccines is that typically you only need one or two doses (or shots) to provide immunity. However, live, weakened vaccines cannot be given to people with immune systems that don’t work as well as they should, because even such a small amount of virus could make them sick.
Vaccines that use inactivated or killed viruses include polio, hepatitis A, and the flu shot. The inactivated virus cannot reproduce and therefore cannot cause disease. But the immune system still makes antibodies to protect you against disease. The advantages of inactivated viruses are that the vaccine cannot cause the disease at all, and the vaccine can be given to people with weakened immune systems. The limitation of this method is that several doses of the vaccine are required before you are immune to the disease.
August is National Immunization Awareness month. It is important for people of all ages to protect their health with vaccines. In the upcoming weeks, we will be posting more information about vaccines for women who are thinking about getting pregnant, pregnant women, and babies.
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