A vaccine is a biologically active substance designed to protect children and adults from infections caused by bacteria and viruses. Vaccines are also called immunizations because they take advantage of our natural immune system’s ability to prevent infectious illness. To understand how vaccines work, we need to consider how our immune system protects us from infections.

Our bodies are armed with a variety of methods to protect against infectious microorganisms like bacteria and viruses. The most sophisticated of these methods involve activating specific immune system cells, some of which make proteins called antibodies. For the immune system to effectively respond to an infectious microorganism, the invader must carry some sort of identification that immune cells can recognize and respond to. These identifying markers are called antigens. Both bacteria and viruses carry their own antigens. In fact, different varieties, or strains, of the same microorganism possess their own unique antigen. Immune cells are able to recognize these highly specific antigens, appropriately identify their owners as threatening, and respond accordingly.

The immune system’s response actually consists of two parts. First, in the presence of a particular antigen, special immune cells, called lymphocytes, become active and take steps against the antigen and its owner, either by unleashing a direct assault on the invader or discharging antibodies to do the job. This usually works quite well, but the full response can take awhile, sometimes days to weeks, during which time we suffer the symptoms of the infectious illness. Symptoms can include discomforts like fever, sore throat,, or rash. It’s only after the immune system gains the upper hand that we begin to recover.

The second, and equally important, part of the immune system response involves creating memory. Not all of the immune cells and antibodies initially stimulated are destined to destroy the invader. A portion of them are held back so they can fight another day. The purpose of these memory cells and antibodies are to attack promptly and overwhelmingly, and to destroy the invader if it were to attack again. In most cases, this memory capability is so efficient that when the same antigen reappears again in the future, we are completely unaware that we’ve been exposed. The term immunity is used to describe the situation in which an effective memory response has attacked the antigen of a particular microorganism.

Consider the example of chickenpox (varicella), a common viral infection. If you were born prior to the early 1990s, when the varicella vaccine was first introduced, you probably remember staying home from school for about a week with a fever and rash. You probably have also noticed that the same illness never reoccurred. This is true even though you’ve almost certainly been exposed to the virus many times since. Your immune system successfully remembers the chickenpox antigen from its initial encounter with the virus and reliably responds each time it is confronted with the identical antigen.

Now consider the flu. Why is it possible, even likely, to suffer from the flu winter after winter despite a healthy immune response every time? Well, unlike the varicella virus, different strains of influenza infect humans each season. Being immune to last year’s flu strain may protect you for the duration of the season, but it will be of little use when next year’s strains come around. There is only one strain of varicella that infects humans.

Ways to Fool the System

So, where do vaccines fit in? The concept behind vaccinations is to stimulate a memory response without producing an actual illness. If successful, a vaccinated individual can enjoy the benefits of immunity without having to suffer through the original illness. To accomplish this, a vaccine must contain at least one antigen from the bacteria or virus of interest. The antigen may take many forms:

  • A part of the toxin responsible for the ill effects of the infection, as in tetanus and diphtheria
  • Tiny components of killed bacteria, as in pertussis
  • Viral protein produced by biotechnology, as in hepatitis B
  • Killed viruses or parts of viruses, as in inactivated polio
  • Live viruses that have been rendered harmless by a process called attenuation, as in measles, mumps, rubella, or chickenpox

Once the vaccine enters the body, its antigen(s) begins to stimulate the development of immune cells and antibodies, which build up over the course of several weeks. Since the immune response produced by vaccines is not as robust as the immune response produced by an actual infectious microorganism, a single vaccine dose usually only provides limited protection. This is why almost all vaccines require multiple doses to insure that the recipient is sufficiently immune. For example, until recently, the vaccine for measles was only given one time in early childhood. When outbreaks of measles began to appear in previously vaccinated adolescents, it became clear that a second, or booster, dose was necessary. Now all children are recommended to receive a booster dose.

It is important to point out that vaccine antigens are often combined with other components for a variety of reasons. To increase the magnitude of the immune response, particularly in young children whose immune systems have yet to mature, antigens are often chemically combined with so-called adjuvant substances such as aluminum salts. In addition, a vaccine may contain by-products from the medium in which it was produced, such as egg protein, as well as substances to preserve the effectiveness of the antigen and keep it sterile, such as antibiotics. An apparent allergy to a vaccine may actually result from these additives rather than the antigen itself.

Active Versus Passive Immunity

The discussion so far has focused on so-called active immunity, which occurs when a person is exposed to an actual infection or receives a vaccination instead. In either case, the immune system responds by activating its own supply of cells and creating its own antibodies. There is, however, an alternative way to become immune.

In passive immunity, a person can benefit from someone else’s immune response by receiving their pre-manufactured antibodies. This occurs naturally in the womb. Prior to birth, babies receive their mother’s antibodies, which cross the placenta and protect the newborn from the hostile, germ-laden, environment they encounter in the outside world. Were it not for these antibodies, infants would have a difficult time surviving the many months it would take for them to actively build up their own immunity.

Passive immunity can also be created artificially by administering antibodies retrieved from individuals who have already acquired active immunity to a particular infection. Passive vaccines contain immunoglobulins, which is another term for antibodies. Passive immunity is most commonly used in individuals who have recently been exposed to a serious infection, or who are at high risk for such an exposure, and may not be fully protected. This is because the protection afforded by passive immunization is immediate, whereas active immunization takes weeks or even months to become fully protective.

An example of this could be an infant who has not yet received the active measles vaccine. The infant may be given the passive measles immunoglobulin in the event of a household exposure, such as an older sibling with a measles infection. While passive immunizations are useful in selected cases, only active immunizations are used routinely. This is because passive immunity lasts a few months at best, whereas the protective effects of active immunity, with proper booster doses, should last a lifetime.

Vaccines to Prevent Other Diseases

All vaccines are designed to target infections. However, two commonly recommended vaccines have the added benefit of protecting against cancer. This is true because of the close association of certain viruses with the development of certain cancers. Hepatitis B is the first example of a vaccine (introduced in 1982) that also reduces the risk of cancer. Hepatitis B is a major cause of primary liver cancer, with others being alcoholiccirrhosis and hepatitis C. By essentially eliminating the risk of hepatitis B, the vaccine protects against its associated cancer, but has no affect on the risk of liver cancer associated with excessivealcohol consumption or hepatitis C.

A more recent example of an anticancer immunization is the human papillomavirus (HPV) vaccine, introduced in 2006. SinceHPV is the leading cause of cervical cancer, immunized women should experience a lower risk of Pap smear abnormalities including pre-cancers (cervical dysplasia) and cancer. Based on a number of studies documenting the vaccine’s effectiveness, the US Centers for Disease Control and Prevention (CDC) currently recommends that all girls aged 11-12 years receive the three-dose vaccination. Females aged 13-26 years should still get vaccinated if they missed getting vaccinated at age 11-12 years.

The CDC also recommends the HPV vaccination series in boys. In males, different types of HPV can cause genital warts. Others types can cause cancers in the penis, anus, and back of the mouth and throat.