Vaccines contain the virus or bacteria (or parts of virus or bacteria) that cause disease. For example, measles vaccine contains measles virus. The virus in the measles vaccine is weakened to the point that it doesn’t cause disease. Some vaccines have a killed version of the germ, such as the vaccine against Hepatitis A. Others have only a piece of the germ, such as with the meningococcal vaccine. All of these vaccines are strong enough to make the immune system produce antibodies that lead to immunity. In other words, a vaccine is a safer substitute for a person’s first exposure to a disease. The person gets protection without having to get sick. Through vaccination, people can develop immunity without suffering from the actual diseases that vaccines prevent.

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Thanks to vaccines, some diseases are almost gone from the U.S. But experts still recommend that we immunize our children, ourselves as adults, and the elderly. It’s true, some diseases (like polio and diphtheria) are becoming very rare in the U.S. Of course, they are becoming rare largely because we have been vaccinating against them.

It’s much like bailing out a boat with a slow leak. When we started bailing, the boat was filled with water. But we have been bailing fast and hard, and now it is almost dry. We could say, “Good. The boat is dry now, so we can throw away the bucket and relax.” But the leak hasn’t stopped. Before long we’d notice a little water seeping in, and soon it might be back up to the same level as when we started.

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Unless we can “stop the leak” (eliminate the disease), it is important to keep immunizing. Even if there are only a few cases of disease today, if we take away the protection given by vaccination, more and more people will become infected and will spread disease to others. Soon we will undo the progress we have made over the years.

We don’t vaccinate just to protect our children. We also vaccinate to protect our grandchildren and their grandchildren. With one disease, smallpox, we “stopped the leak” in the boat by eradicating the disease. Our children don’t have to get smallpox shots anymore because the disease no longer exists. Smallpox is now only a memory, and if we keep vaccinating against other diseases, the same will someday be true for them too. Vaccinations are one of the best ways to put an end to the serious effects of certain diseases.

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Diseases that are almost unknown would stage a comeback. Before long we would see epidemics of diseases that are nearly under control today. More children would get sick and more would die.

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The United States has the safest, most effective vaccine supply in its history. The nation’s longstanding vaccine safety system ensures that vaccines are as safe as possible. As new information and science become available, the system is updated and improved.

As part of these vaccine safety efforts, scientists ensure the safety of vaccines by conducting different types of studies:

• Clinical trials are done before a vaccine is made available. Vaccine manufacturers conduct these studies as part of the development, testing, and approval process. The Food and Drug Administration (FDA) uses the information from clinical trials to decide if a vaccine is safe, effective, and ready to be licensed for use.
• Post-licensure safety studies are conducted after a vaccine is approved and in use. These studies are part of continuous vaccine safety monitoring and often look at populations that might be underrepresented in clinical trials. Importantly, these studies can look for rare adverse events.

The United States’ long-standing vaccine safety program closely and constantly monitors the safety of vaccines.

A critical part of the program, CDC’s Immunization Safety Office identifies possible vaccine side effects and conducts studies to determine whether health problems are caused by vaccines.

Data show that the current U.S. vaccine supply is the safest in history.

Immunity to a disease is achieved through the presence of antibodies to that disease in a person’s system. Antibodies are proteins produced by the body to neutralize or destroy toxins or disease-causing organisms. Antibodies are disease-specific. For example, measles antibody will protect a person who is exposed to measles but will have no effect if he or she is exposed to mumps.

Active immunity results when exposure to a disease organism triggers the immune system to produce antibodies to that disease. Exposure to the disease organism can occur through infection with the actual disease (resulting in natural immunity), or introduction of a killed or weakened form of the disease organism through vaccination (vaccine-induced immunity). Either way, if an immune person comes into contact with that disease in the future, their immune system will recognize it and immediately produce the antibodies needed to fight it.

Active immunity is long-lasting, and sometimes lifelong.

Passive immunity is provided when a person is given antibodies to a disease rather than producing them through his or her own immune system.

A newborn baby acquires passive immunity from its mother with antibodies that cross the placenta. A person can also get passive immunity through antibody-containing blood products such as immune globulin, which may be given when immediate protection from a specific disease is needed. This is the major advantage to passive immunity; protection is immediate, whereas active immunity takes time (usually several weeks) to develop.

However, passive immunity lasts only for a few weeks or months. Only active immunity is long-lasting.

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