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.

The Immune System

Our bodies are armed with a variety of methods to protect against infectious microorganisms (eg, 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 (days to weeks), during which time we suffer the symptoms of the infectious illness (eg, fever, ]]>sore throat,]]> , 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 is 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 ]]>chickenpox 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 (chickenpox) 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 (poison) responsible for the ill effects of the infection (eg, ]]>tetanus]]> and ]]>diphtheria]]> )
  • Tiny components of killed bacteria (eg, ]]>pertussis]]> and pneumococcus)
  • Viral protein produced by biotechnology (eg, ]]>hepatitis B]]> )
  • Killed viruses or parts of viruses (eg, inactivated ]]>polio]]> )
  • Live viruses that have been rendered harmless by a process called attenuation (eg, ]]>measles]]> , ]]>mumps]]> , ]]>rubella]]> , 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 (eg, aluminum salts). In addition, a vaccine may contain by-products from the medium in which it was produced (eg, egg protein), as well as substances to preserve the effectiveness of the antigen and keep it sterile (eg, 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 alcoholic ]]>cirrhosis]]> 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 excessive ]]>alcohol consumption]]> or hepatitis C.

A more recent example of an anticancer immunization is the ]]>human papillomavirus (HPV) vaccine]]> , introduced in 2006. Since ]]>HPV]]> 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 currently recommends that all girls aged 11 or 12 receive the three-dose vaccination. There's a "catch-up" vaccine for females aged 13-26.

Why Should Everyone Be Vaccinated?

Imagine for a moment a disease contracted every year by more than 50 million people worldwide. Thirty percent die of this disease and the majority of those that survive are left with disfiguring skin lesions (most prominently on the face), rendered totally blind or both. Now imagine this same disease, which has been in existence for at least 3,000 years, being completely eliminated from the face of the earth in as few as 25 years. From over 50 million to not a single case anywhere! What medical advance could possibly be that powerful? The ]]>smallpox vaccine]]> .

No medical intervention in the history of humanity has been more successful than immunizations, as measured by the sheer number of people who have benefited. While smallpox is the shining example of a totally successful vaccine, the success rates of other vaccines are equally impressive. Where vaccines are readily available and routinely administered, the rates of measles, mumps, rubella, polio, hepatitis B, and many other infections have plummeted. In 1952, for example, there were 21,000 cases of polio resulting in paralysis in the US. By 1980, there were zero, and health officials are on the brink of worldwide polio eradication.

Based on this tremendously successful track record, great efforts are made in many countries to ensure that every child is immunized against a constantly growing number of infections. Despite these efforts only 83% of children received all their required vaccines by three years of age in 2004. While this level of compliance is apparently sufficient to keep extremely rare infections from returning, it has probably contributed to recent localized epidemics of measles and pertussis infections (whooping cough).

To completely eradicate an infectious illness in a population, high levels of immunization rates are necessary. This is due to the concept of herd immunity. Say an inadequately immunized person with measles enters a new community. That case of measles will have no impact on that community if all of its members (the entire “herd”) are already immune to measles, ideally through appropriate and timely vaccination. However, if some members of community are insufficiently immunized against measles, it will likely spread rapidly with the introduction of this new case. It can also spread outside the community if an infected member travels elsewhere. The only way to prevent such a scenario is to make sure the entire community is immunized against measles.

Ironically, some reluctant parents argue against vaccinating their children precisely because the universal vaccination program has been so successful. They reason that these infections have become so rare, that they are no longer even worth the small risk of harmful effects. In essence, they are saying that their children need not contribute to herd immunity since so many other children already have. After all, the smallpox vaccine was discontinued in the US in 1972, five years before the infection was officially declared eradicated worldwide. Health professionals counter that as long as these infections continue to exist in the world, they can be easily reintroduced into a population that is not adequately protected and cause considerable devastation.

All states require students to have updated immunizations before enrolling in public schools. However, many states also permit parents to send their children to public schools without being immunized by officially registering their objection.

Vaccines recommended universally for all children, adolescents, and/or adults in the US include:

]]>Haemophilus influenzae type b]]>]]>Pertussis (whooping cough)]]>
]]>Hepatitis A]]>]]>Pneumococcus]]>
]]>Hepatitis B]]>]]>Polio (poliomyelitis)]]>
]]>Human papillomavirus]]> (females) ]]>Rotavirus]]>
]]>Influenza]]>]]>Rubella (German measles)]]>
]]>Meningococcus (meningitis)]]>]]>Varicella (chickenpox)]]>

In addition to the routinely recommended immunizations there are a number of other vaccines indicated for only select people at increased risk. These include:

]]>BCG (tuberculosis)]]>]]>Smallpox (vaccinia)]]>
]]>Herpes zoster (shingles)]]>]]>Typhoid]]>
]]>Japanese encephalitis]]>]]>Yellow fever]]>

How and When Are Vaccines Given?

Most vaccines are injected with a needle deep inside a muscle. In infants, the preferred site for injections is in the thigh a little off the side because it is the largest muscle suitable for this purpose. In older children and adults the preferred site is the deltoid muscle (upper arm) because any lingering pain will not interfere with walking. The buttocks area is not ideal because it contains a lot of fat, and it risks damaging the sciatic nerve, which runs nearby.

Vaccines are also administered just beneath the skin (eg, measles, mumps, rubella, varicella); orally (eg, rotavirus); and inhaled through the nose (one type of an influenza vaccine).

The optimal timing of vaccines is based on two, sometimes competing, factors: when a recipient is first capable of responding to the antigen and when he first needs the protection. If a vaccine is given too early, a child will not be able to mount an adequate immune response and will not become adequately protected. This is true, for example, with the live viral vaccines (measles, mumps, and rubella), which should not generally be given prior to a child’s first birthday. If a vaccine is given too late, however, a child may contract the infection.

Another consideration involves the need for booster doses. Most vaccines do not stimulate an adequate immune response after the first dose and must be repeated with booster doses at optimal intervals to insure adequate protection for a lifetime. This increases considerably the number of doses a child must receive before he is fully immunized. Fortunately, multiple vaccines can be administered simultaneously without compromising safety and effectiveness. This is particularly important now that children routinely receive 14 different immunizations by the time they reach age seven.

With the rapid growth in the number of recommended vaccines, the schedule has become quite complicated. Here is a timetable of all routine vaccines in children up through age six.

Recommended Vaccine Schedule: Aged 0-6 Years (2010 )
Indicates age range to administer dose(s)
Indicates vaccines only recommended for certain high risk groups
VaccineBirth1 Month2 Months4 Months6 Months12 Months15 Months18 Months19-23 Months2-3 Years4-6 Years
Hepatitis BHepatitis BHepBHepBHepB
Diptheria, tetanus, pertussisDTaPDTaPDTaPDTaPDTaPDTaP
Haemophilus influenza type bHibHibHib Hib aHib
Inactivated poliovirusIPVIPVIPVIPVIPV
Measles, mumps, rubellaMMRMMRMMR
Hepatitis AHepatitis AHepA (2 doses)HepA Series
MeningococcalMCV MCV

Source: Centers for Disease Control and Prevention. Available at http://www.cdc.gov/vaccines.

Recommended Vaccine Schedule: Aged 7-18 Years (2010 )
Indicates age range to administer dose(s)
Indicates vaccines only recommended for certain high risk groups
Catch-up immunizations
Vaccine7-10 years11-12 years13-18 years
Diptheria, tetanus, pertussisTdapTdap
Human papillomavirusHPV (3 doses)HPV series
Pneumococcal PPSV b
InfluenzaInfluenza (yearly)
Hepatitis AHep A series
Hepatitis BHep B series
Inactivated poliovirusIPV series
Measles, mumps, rubellaMMR series
VaricellaVaricella series

Source: Centers for Disease Control and Prevention. Available at http://www.cdc.gov/vaccines.

a Not required if PedvaxHIB or ComVax were given at 2 and 4 months

b With underlying medical conditions

Children who fall off the recommended schedule and those in certain high-risk groups may receive these and other vaccines on a modified schedule. Parents and caregivers of children who have missed doses on the recommended schedule should discuss with their child's pediatrician an alternate vaccination schedule. There are catch-up schedules for children who miss doses: http://www.cdc.gov/vaccines/recs/schedules .

How Safe Are Vaccines?

Vaccines are one of the safest and most effective medical interventions every devised. Consider measles. The risk of a life-threatening reaction to the measles-mumps-rubella (MMR) vaccine is approximately 1-2 per 1,000,000 doses given. Conversely, the risk of death from measles itself is 1-3 per 1,000 cases, which means that a child who contracts measles is approximately 500 times more likely to die from the infection than a child who is immunized against it (children receive two MMR vaccines). All routinely administered vaccines have a similar mortality rate measured in cases per million.

The fact that vaccines are rarely life threatening does not, of course, mean they are completely safe. Vaccines are associated with a variety of adverse effects ranging from mild discomfort at the injection site to serious neurologic complications. The vast majority of unwanted reactions are mild and transient. These include localized pain, redness and swelling, as well as low-grade fever and rash. The benefits of the routine vaccinations far outweigh the risks of these mild, self-limited and unavoidable reactions.

However, since routine vaccines are given to perfectly healthy individuals who only have a small risk of future infection, any potential for significant, long-term harm must be taken extremely seriously. As the potential for adverse reactions increase, at some point the risk of harming a healthy child in the present no longer outweigh the benefits of protecting him or her in the future. Despite repeated assurances from numerous government agencies and independent research scientists that routine vaccinations meet the highest standards of safety and effectiveness, a small yet vocal group of parents and consumer advocacy organizations remain unconvinced. They have raised concerns about the connection between various vaccines and a variety of serious health conditions including ]]>autism]]> , ]]>multiple sclerosis]]> , ]]>sudden infant death syndrome (SIDS),]]> and ]]>cancer]]> . One reason this controversy persists is that it can be difficult to prove a causal connection between vaccines, which are given often, and most of these conditions, which occur rarely. Even if a child develops, say, autistic behavior soon after receiving a vaccine, it does not mean the vaccine was responsible. To date, there has been no convincing study published that supports a cause and effect relationship between vaccines and a substantially increased risk of these chronic conditions.

Nevertheless, serious adverse reactions do rarely take place, and it is essential to monitor these events carefully. The National Childhood Vaccine Injury Act of 1986 requires healthcare providers who administer vaccines to maintain permanent immunization records and to report serious adverse reactions to the Vaccine Adverse Event Reporting System (VAERS). Consistent reporting by physicians nationwide provides clues to unanticipated adverse reactions from all vaccines, but particularly the newer ones for which there is relatively limited experience. In addition, VAERS can monitor the frequency of known reactions, identify possible risk factors for these reactions, and help locate a bad vaccine batch. If you are concerned your child may have developed a serious reaction to a recently administered vaccine, contact your child’s pediatrician. In addition, you may report your concerns directly to VAERS, where patient-identifying information is kept confidential.

In the event of serious injury or death thought to be caused by a vaccination, patients and their families can be compensated through the National Vaccine Injury Compensation Program, a no fault system covering most of the vaccines given routinely to children and adults.

Precautions and Contraindications

Contraindications refer to situations in which a vaccine or vaccines should not be administered to an individual under any circumstances because the risks clearly outweigh the benefits. Precautions refer to a situation where the vaccine may be safely given, but the benefits and risks must be carefully weighed before proceeding. Contraindications and precautions may be temporary or permanent, and they may apply to all vaccines or only certain ones. A severe, life-threatening allergic (anaphylactic) reaction to a particular vaccine is a contraindication to its further use, although, the same individual may receive other vaccines safely.

As a rule of thumb, precautions should be observed if a recipient has ever had a serious, but non-life-threatening reaction from which he completely recovered (eg, high fever, ]]>seizure]]> , or persistent inconsolable crying). Future precautions are, of course, unnecessary in situations where the recipient experienced minor, transient reactions that are not unexpected. Furthermore, if a recipient arrives for a vaccine with acute moderate or severe illness, it is usually best to wait until the illness resolves before administering the vaccine. However, precautions are generally unnecessary for minor illnesses; vaccines can generally be administered safely even in the presence of a low-grade fever.

Special considerations are necessary for patients with compromised immune systems (ie, immunodeficiency). The safety and effectiveness of immunizations will depend on the severity and cause of the immunodeficiency (eg, ]]>AIDS]]> or congenital condition) and the type of vaccine being considered. Vaccines containing live viruses, for example, may actually cause the infectious illness in such patients, and even perfectly safe vaccines may not produce a sufficiently protective immune response. In some cases, the household contacts of immunocompromised patients (eg, siblings) should not receive live vaccines (eg, oral polio) since the virus can be spread on rare occasions. In addition to immunodeficiencies, other illness, or medications to treat them, can impair vaccine safety and effectiveness. For example, children receiving corticosteroids, which at high doses can have profound effects on the immune system, may require special precautions. The same may be true for children with a history of seizures.

Many children have their immunization unnecessarily delayed or even forgone due to common misconceptions among both physicians and parents regarding when vaccines are contraindicated. Routinely administered vaccines are generally not contraindicated under the following common circumstances:

  • Mild acute illness with or without low-grade fever in an otherwise well child
  • A child in the recovery phase of illness
  • Current use of antibiotics
  • Recent exposure to an infectious illness
  • Reaction to a previous vaccine dose involving only localized soreness, redness, or swelling
  • Infants born prematurely
  • Breastfeeding mother
  • History of nonspecific allergies or relatives with allergies
  • History of allergies to penicillin or any other antimicrobial agent, except anaphylactic reactions to neomycin or streptomycin
  • Family history of seizures, sudden infant death syndrome, or adverse events after immunization