PIC Education Committee- Online Educational Modules

Vaccine-Preventable Diseases and Vaccines


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General Information:
There are vaccines against several viruses and bacteria. Many of these vaccines are recommended for subgroups of the population based on age or increased risk. There are some vaccines that are recommended only for travel.


Shingles and chickenpox are both caused by the varicella virus. There is a vaccine for children that prevents chickenpox and a shingles vaccine for people over 60 that protects against shingles. Although the vaccines are both made using the same strain of varicella virus, they can not be used interchangeably because they contain different quantities of the virus.

Measles, mumps and rubella are all caused by viruses and can be prevented by the measles-mumps-rubella (MMR) vaccine. The MMR vaccine, while highly effective, has been protested by some because of disproven claims that it leads to the development of autism in some children.

Human papillomavirus (HPV) causes cervical cancer and genital warts. It can be prevented with the HPV vaccine.

Hepatitis A and hepatitis B both are vaccine-preventable diseases that affect the liver. Whereas hepatitis A is transmitted through contaminated food and water, hepatitis B is transmitted through blood. Hepatitis B can lead to liver cancer.

Influenza is a virus that commonly infects the respiratory tract and can cause severe infection leading to hospitalization and death. There are 2 types of vaccines available to protect against influenza—a live intranasal vaccine and an inactivated shot. The influenza vaccine is unique because each year it must be reformulated to offer the best immunologic protection. The influenza virus changes so rapidly that a vaccine from the previous year may not allow the immune system to recognize the current form of the virus.

Polio and rotavirus are also viruses that are vaccine-preventable. Both viruses infect the gastrointestinal tract. Rotavirus infection can quickly lead to severe dehydration and death primarily in infants. Polio can also infect the central nervous system leading to paralysis and death. Both vaccines are given to infants. The polio vaccine is given as a shot, but a different version used to be given orally. The oral version provided excellent protection from polio, but sometimes the vaccine virus was known to revert to its more virulent form and cause paralysis. The rotavirus vaccine is given orally.


Diphtheria, tetanus and pertussis are all bacteria that produce harmful toxins. These toxins cause illness in humans, but a vaccine containing chemically inactivated forms of these toxins (called toxoids) is available. There are specific formulations for infants, adolescents and adults.

Haemophilus influenzae type b (Hib), pneumococcus and meningococcus are all types of bacteria that can cause meningitis, an infection of the brain and spinal cord. Bacterial meningitis can be fatal. All three of these bacteria are vaccine-preventable. They tend to occur at different ages and, therefore, the vaccines against them are also recommended for different age groups.  Hib and pneumococcus are given to infants. There is also a version of the pneumococcal vaccine that is given to adults over 65 years old.  Meningococcus is available as a vaccine for young children and adolescents.

Travel vaccines

Some vaccines are only required if a person is traveling to a part of the world where the infection being prevented is common. These include vaccines for typhoid fever, Yellow fever, Japanese encephalitis virus, and rabies.

Vaccine Science

Vaccines work by exposing a person’s immune system to the part of the virus or bacteria that causes illness. However, the vaccine is created so that it will not make the person ill. This can be done by:

  1. using just a piece of the bacteria,
  2. inactivating the virus, bacteria or its harmful products (toxins)
  3. weakening the virus by making it grow in cell types other than those which it prefers, or
  4. attaching a protein to the piece of the bacteria so that the body will make an immune response against both the protein and the part of the bacteria that causes illness


The branch of science that studies the distribution of disease, illness and health in a population.

Clinical Trials

Clinical trials are the methods by which vaccine safety and efficacy are determined. First, there are pre-clinical studies that occur in animals. Once a vaccine is ready for trial in people, there are four phases of studies that must be completed—three prior to licensure and a fourth after the vaccine has been licensed for use in people. Studies during this fourth phase are often referred to as post-marketing studies. The post-marketing studies are important because they monitor the vaccine’s safety in a larger population than can be tested during the earlier trials. Some side effects may be so rare that they are only noticed when the vaccine reaches the much larger general population.

Specific Information:

Vaccine Preventable Diseases
Diphtheria: Diphtheria is caused by a bacterium called Corynebacterium diphtheriae that produces a harmful toxin.  This toxin can invade the heart, kidneys, and nervous system.  Diphtheria causes the formation of a thick white coating in the nose, mouth, or airway, making it difficult to swallow or breathe and in some cases leading to suffocation.  Infection with diphtheria can lead to heart failure, paralysis, or death.  Diphtheria is spread by coughing.  Because of immunization, there are very few cases of diphtheria in the United States.  About 2-3 cases per year occur in the U.S. Diphtheria is still common in other countries.

Haemophilus influenzae type b (Hib): Haemophilus influenzae are a group of bacteria, and there are six types.  Hib is the most common, accounting for 95% of disease caused by H. influenzae.

At one time Hib was the leading cause of bacterial meningitis in children under 5 years of age.  Serious consequences of the disease include permanent brain damage, blindness, deafness, or death. The bacteria spread by droplets in the air from coughing, sneezing, or breathing.  If the bacteria stay in the nose or throat, they are harmless; however, if the bacteria travel into the lungs or bloodstream they can cause meningitis, pneumonia, epiglottitis, or bloodstream infections.  Symptoms of Hib vary depending upon the system the bacteria have affected.  Symptoms of Hib meningitis are fever, decreased mental status, and stiff neck.  Epiglottitis, an infection and swelling of the epiglottis, can lead to airway obstruction and death. Infants and young children, especially those in day-care settings, are particularly at risk for developing the disease.   In 2005 there were between 9 and 200 cases of Hib infections in the U.S.   Hib remains a major cause of lower respiratory tract infections in infants and children in other countries around the world.

Hepatitis A: Hepatitis A is a virus that infects the liver.  The virus is shed in feces, and can be spread from person to person or in contaminated food and water.  Hepatitis A infection may be asymptomatic, or its symptoms may range in severity from a mild illness lasting 1-2 weeks to a severely disabling disease lasting several months. Some symptoms can include fever, malaise, anorexia, nausea, abdominal discomfort, or jaundice.  Unlike other forms of hepatitis, hepatitis A does not cause cirrhosis or liver cancer; however, it is still serious and can cause a severe infection of the liver, and death. In the United States, approximately 30,000 people are diagnosed with hepatitis A annually, and about 100 people die from the disease.  The disease is very common in the developing world due to the use of sewage-contaminated water for drinking or in growth or preparation of food. 

Hepatitis B: Hepatitis B is also a virus that infects the liver; however, unlike hepatitis A, hepatitis B is spread through the blood of an infected person.  Those who have sex without condoms or share needles have a higher risk of contracting hepatitis B.  Tattooing and body piercing have also been reported to result in the spread of hepatitis B when good infection control practices have not been used. Unsafe injections in medical settings are a major source of hepatitis B disease spread in many developing countries.  Signs and symptoms of hepatitis B can include nausea; lack of appetite; tiredness; muscle, joint, or stomach pain; fever; diarrhea or vomiting; headache; dark urine; light-colored stools; or jaundice; however, children who develop the infection rarely show any symptoms.  Even though people might eventually recover from an acute infection, they may become chronically infected.  These people are often known as carriers as they may infect others with the virus. Those with chronic hepatitis B are at higher risk for developing cirrhosis, liver cancer, or liver failure.  Every year, approximately 200-300 Americans die of an acute infection of hepatitis B.  Approximately 3,000-4,000 people die each year of hepatitis B-related cirrhosis, and another 1,000-1,500 die each year of hepatitis B-related liver cancer.  Approximately 350 million people around the world are chronically infected with hepatitis B and approximately 1 million of these people die each year from cirrhosis leading to liver failure or liver cancer. Hepatitis B is the 10th leading cause of death worldwide.  If not vaccinated, 5 out of every 100 people will develop the disease during their lifetime.  The hepatitis B vaccine was the first vaccine to prevent cancer.

Human Papillomavirus (HPV):  HPV is the name of a group of viruses that infect the epithelium.  There are over 100 different types of HPV.  Some strains of the virus cause warts on the hands or feet; however, approximately 40 of the types infect the mucosal epithelium.  Some types of genital HPV cause genital warts. Other types cause abnormal cell changes that can lead to cervical cancer and to a lesser extent cancers of the vulva, vagina, penis, and anus.  Genital HPV is spread through direct contact, usually sexual.  Condom use does not completely protect against infection with HPV.  In most cases, HPV infections are asymptomatic and go away without treatment, but other times cell changes progress abnormally leading to cervical cancer.  Routine Pap smears have helped to decrease disease and death caused by HPV.  However, each year cervical cancer kills almost 4,000 women in the U.S. and 200,000 women worldwide.  Nearly three out of four sexually active women in the U.S. will have been infected with genital HPV by the age of 50.  Nearly 20 million women in the United States are currently infected, with another 6 million becoming infected every year.  The vaccine is currently licensed in the U.S. for girls and women ages 9 –26 years.

Influenza:  Influenza is a virus that causes infection of the respiratory tract, including the nose, throat, and lungs.  Types A and B are responsible for the influenza infections that occur every winter (typically from October to April).  Influenza is spread from person to person through droplets produced while coughing or sneezing.  It can also be transmitted by touching a contaminated object (doorknob, table, telephone, etc.) and the eyes, nose or mouth.  Symptoms of the infection include fever, chills, cough, sore throat, runny nose, headache, muscle aches, and fatigue.  Most people recover from the flu completely; however, in the elderly or those with chronic health problems, influenza can cause severe or fatal pneumonia, or exacerbation of chronic conditions (heart failure, asthma, diabetes, etc.).  Most of the individuals who die from influenza are over the age of 65 years, but many of those who are hospitalized are under age 4 years of age.  In the United States, about 200,000 people are hospitalized and 36,000 die from infection with influenza each year.  Between three and five million cases of severe illness and up to 500,000 deaths occur from influenza worldwide.  Two types of influenza vaccines are available; one is given as a shot and the other is given intranasally. Neither vaccine can be given to people who have egg allergies.

Measles: Measles is caused by a virus that infects the epithelium of the nasopharynx. It is spread from respiratory droplets.  Symptoms include fever, runny nose, cough, loss of appetite, conjunctivitis (pink eye) and a rash. The rash usually lasts 5-6 days and begins at the hairline, moves to the face and upper neck, and then continues down the body. The first symptom is usually a fever, with the rash following approximately 2-3 days later.  Prior to widespread immunization, measles was common in childhood, with over 90% of infants and children infected by age 12.  Only 37 cases were reported in 2004; however, measles is still common in many other countries in the world and can easily be brought into the United States, so continued vaccination against the disease is still important.

Meningococcus:  Meningococcus is a bacterium that is found on the lining of the nose and throat.  Meningococcal infection is transmitted through contact with droplets of nasal or throat secretions from infected individuals. Children under the age of five years, and particularly those under age one year, are at highest risk of acquiring the disease, followed by teenagers between 15 and 19 years of age.  Close and prolonged contact (kissing, sneezing and coughing on someone, living in close quarters or dormitories, etc.) facilitates the spread of infection. For example, college freshman living in dorms are more likely to get the disease than those the same age who do not attend college. If the bacterium enters the bloodstream and the lining of the brain and spinal cord (meningitis), it can cause serious disease.  Symptoms of meningococcal infection can include fever, headache, stiff neck, nausea, vomiting, confusion, or sleepiness.  Seizures may develop as the disease progresses from the high fever associated with meningitis.  Infection with meningitis can also cause sepsis, pneumonia, arthritis, otitis media, and epiglottitis.  Meningococcal infection can progress so rapidly that someone can appear healthy and be dead hours later.    In the United States, 2,500 people are infected and 100-300 die from meningococcal disease every year.   

Mumps: Mumps is a viral infection.  It is spread by infected secretions sneezed or coughed out of the nose or throat.  Initial symptoms usually include swelling and tenderness of one or both of the salivary glands.  Headache, loss of appetite, and low-grade fever may also be present.  Many people show no symptoms at all, or just have general respiratory symptoms.  Both children and adults can get the disease, however it is usually more serious in adults.  Complications can include meningitis, encephalitis or in males, orchitis (testicular inflammation), which can lead to sterility.  Before vaccination was available, mumps was relatively common in the United States, with approximately 200,000 cases and 20-30 deaths reported each year.  In 2004, only 258 cases were reported, with no fatalities.  In 2006, a mumps outbreak in the U.S. resulted in about 6,000 reported cases.

Pneumococcus: Pneumococci are a group of bacteria that naturally occur in the nose and throat of people of all ages.  The bacteria spread by respiratory droplets.  Pneumococci can infect the middle ear or sinuses, the lungs (causing pneumonia), the central nervous system (causing meningitis), or the bloodstream (causing bacteremia).  Symptoms vary depending on the site of infection.  Pneumococcal pneumonia is the most common presentation in adults; symptoms may include abrupt onset of fever, chills, chest pain, cough, shortness of breath, rapid breathing and heart rate, and weakness.  Symptoms of pneumococcal meningitis include headache, tiredness, vomiting, irritability, fever, seizures, and coma.  Serious pneumococcal infections are most common in infants, toddlers, and the elderly. Each year in the United States among children younger than five, pneumococcal disease accounts for at least 1,400 cases of meningitis, 17,000 cases of bacteremia, 71,000 cases of pneumonia, and 5 to 7 million middle ear infections. These numbers rise to an estimated 150,000 to 570,000 cases of pneumococcal pneumonia, 16,000 to 55,000 cases of pneumococcal bacteremia, and 3,000 to 6,000 cases of pneumococcal meningitis when looking at Americans of all ages. Before a vaccine was available in the United States, pneumococcal disease caused about 200 deaths per year.

Pertussis (Whooping Cough): Pertussis is caused by a bacterium that produces a toxin that causes inflammation of the respiratory tract.  Whooping cough usually starts with cold or flu-like symptoms, such as runny nose, sneezing, fever, and a mild cough. These symptoms can last up to 2 weeks and are followed by increasingly severe, painful coughing spells.  The coughing can be so severe that sufferers can crack their ribs.  The “whooping” sound can be heard while trying to breathe between coughs.  A person’s lips and mouth can also turn blue due to lack of oxygen.  Pertussis is spread by respiratory droplets and is highly contagious.  Many times pertussis goes undiagnosed or is confused for bronchitis or a cold, particularly among adults in whom the disease may not be as severe.  Infants and young children are at highest risk of life-threatening consequences from the disease because of their small windpipes.  Adults are the most common source of infection, often transmitting this disease to children. In the United States in 2005, over 25,000 cases were reported; however, since the disease is often undiagnosed, it is underreported. You can see pictures of and hear the sound of pertussis at www.pertussis.com.

Polio: Polio is a disease caused by a virus that infects the intestine and the central nervous system.  Polio is transmitted through contaminated food, water, or saliva.  In some people it does not cause serious illness. However, in others, it can cause paralysis and even result in death from paralysis of the breathing muscles.  Initial symptoms can include fever, fatigue, headache, vomiting, stiffness in the neck, or pain in the limbs.  At one time, polio was extremely common in the United States, paralyzing up to 21,000 per year.  Today, due to vaccination, polio has been eliminated from the U.S. and the entire Western Hemisphere, although it remains a threat in some countries.  Polio is nearing eradication throughout the world.  In 2005, there were fewer than 2,000 cases globally.

Rotavirus:  Rotavirus is a virus that infects the lining of the intestines.  The virus is shed in the stool and can be spread by contact with contaminated hands or objects.  The most common cause of gastroenteritis (severe diarrhea), rotavirus is particularly dangerous for infants and young children due to the potential for dehydration.  Symptoms include severe diarrhea, vomiting, and fever.  Those who are between 3 and 35 months of age are at the highest risk for acquiring the infection, and during the first 3 to 5 years of life most children will become infected.  In the United States, an estimated 2.7 million children become infected with the virus, and approximately 20-60 children per year die from it.  Worldwide, rotavirus causes up to 500,000 deaths per year. The rotavirus vaccine is given orally.

Rubella (German Measles): Rubella is caused by a virus and is spread from person to person through the air.  In children, the first symptom of rubella is usually a rash which starts on the face and eventually moves to the rest of the body.  Older children and adults usually first have a low-grade fever, swollen glands in the neck or behind the ears, and upper respiratory infection before they develop a rash, and many people with the virus show no symptoms at all.  In pregnant women, rubella can cause congenital rubella syndrome (CRS).  Up to 85% of infants infected during the first trimester of pregnancy will be born with some type of birth defect, including deafness, eye defects, heart defects, mental retardation, and more.  Two rubella outbreaks in 1990-1991 resulted in the birth of 58 infants with CRS; however, in 2006, rubella was declared eradicated from the U.S.  Rubella is still common in other parts of the world.  The primary reason for giving the rubella vaccine is to protect a female’s unborn children.  Males get the vaccine to decrease transmission in the community.

Tetanus (Lockjaw): Tetanus is caused by a bacterium that produces a toxin causing painful muscle spasms.  The bacterium is found in the soil, so unlike other diseases, it is not passed from person to person and can never be eradicated through immunization programs.  Tetanus is transmitted mostly through wounds, but can also develop after surgery, burns, ear infections, dental infections, and animal bites. The bacterium enters the body through an opening in the skin and first affects the nerves that control the muscles near the wound.  As it circulates more widely, the toxin interferes with the normal activity of nerves throughout the body, leading to generalized muscle spasms.  Without treatment, tetanus can be fatal.  In the United States because of vaccination, only approximately 40 people are diagnosed with tetanus annually; however, in places like Africa and Southeast Asia, approximately 84,000 and 82,000 deaths from tetanus, respectively, were reported annually in 2002. 

Varicella Zoster Virus: This virus causes two diseases. When people are initially exposed to varicella zoster virus, they may develop chickenpox. Some virus particles then remain dormant in the nerve ganglia, and sometimes reactivate later in life to cause shingles.

Varicella (Chickenpox):  Infection causes a blister-like rash, itching, and fever.  The rash first appears on the trunk and head, then moves to the extremities. Lesions can also occur on mucous membranes, such as in the nose, mouth, eyes and vagina.  The disease is highly infectious and is spread by direct contact with fluid from lesions, respiratory droplets containing virus, or inhalation of aerosols from lesions. In some cases serious complications result from infected skin lesions or pneumonia.  Those with weakened immune systems, older teens and adults, children less than one year, and newborns of infected women are at greater risk for serious complications.  Before the vaccine became available, there were approximately 11,000 hospitalizations and 100 deaths from chickenpox annually in the United States.  Worldwide, it is estimated that 60 million cases occur annually.

Herpes Zoster (Shingles): Shingles begins with pain, itching, or tingling followed by a rash in the same area.  The rash starts as blisters that scab and clear with 2-4 weeks.  Other symptoms can include fever, headache, chills, or upset stomach.  Shingles most commonly occurs in people over 50, although people of any age who have already had chickenpox can develop shingles, even children.  People who are immuno-compromised and infants exposed to varicella while in the womb are at greater risk for getting shingles.  Shingles cannot be spread from one person to another, however the virus that causes them can.  If a person who has never had chickenpox comes in direct contact with the shingles rash, he or she will develop chickenpox, not shingles.  Once the rash becomes crusted or scabbed, the virus cannot be spread.  In the United States there are about one million cases of shingles per year.  The shingles vaccine is made using the same virus that is used for the chickenpox vaccine, but at a dose that is 14 times greater. Because of the larger dose, the two vaccines may not be used interchangeably.

Travel Vaccines:
The following diseases have vaccines that are recommended for travel. Some of these vaccines are also recommended for certain laboratory workers or others with potential for occupational exposure.

Rabies: Rabies is a virus that is most commonly transmitted from the bite of a mammal with rabies.  Animals that most often transmit rabies include raccoons, skunks, bats, and foxes. Domestic animals once accounted for the majority of rabies transmission; however, today these animals (such as cats, cattle, or dogs) are responsible for only about 10% of rabies infection in the U.S. due to the routine use of veterinary rabies vaccines.  Rabies attacks the central nervous
system, resulting in encephalopathy (disease of the brain that alters brain function or structure).   Because encephalitis can result from various infections, it is important to determine the cause. Rabies encephalitis may be distinguished by the presence of the following symptoms, particularly after a known exposure: pain near the site of exposure, hydrophobia, hypersalivation, agitated behavior, and asymmetric or ascending paralysis.  Death is certain within days of the onset of symptoms if no treatment is received.  In the early 1900’s rabies was responsible for more than 100 deaths annually in the United States; however, due to vaccination, this number has dropped to 1-2 cases per year.  Rabies still occurs in countries with inadequate public health funding or poor animal vaccination programs, thus the need for this vaccine when traveling to some areas.  Worldwide, there are over 50,000 people who are infected with rabies each year.  Those travelers who will have increased exposure to outdoor activity (such as biking, hiking, camping) are at higher risk, especially in rural areas. 

Typhoid Fever: Typhoid is caused by the bacteria Salmonella enterica Typhi.  Typhoid is carried in the bloodstream and intestinal tract of an infected person and is shed in the stool.  Typhoid is spread from ingesting food or drink that contains the bacteria, either from improper food handling or from sewage-contaminated water supplies.  Therefore, typhoid is common in the developing world such as some countries in Asia, Africa, the Caribbean, and Central and South America where sewage disposal may be inadequate, necessitating vaccination for travelers.  Signs and symptoms include a high fever that persists, weakness, stomach pains, headache, loss of appetite, and in some cases a red-spotted rash.  Rarely, those who recover from typhoid fever still carry the bacteria, and are called “carriers.”  In the United States approximately 400 cases occur annually; however, 75% of these were acquired during travel outside of the U.S.  In developing countries, approximately 21.5 million people are infected and 200,000 die each year from typhoid fever. 

Yellow Fever: Yellow fever is a virus whose effects range from flu-like disease to hepatitis or hemorrhaging.  The virus is transmitted to humans by mosquitoes and occurs mainly in Africa and South America.  Symptoms may include fever, chills, muscle pain, headaches, black vomit, hemorrhage, or jaundice.  Worldwide, yellow fever occurs in approximately 200,000 people and causes about 40,000 deaths annually.  People traveling to countries in Africa or South America should be vaccinated and also take precautions against getting mosquito bites including wearing insect repellent and protective clothing and sleeping under mosquito netting.  Yellow fever is one of three quarantinable diseases subject to International Health Regulations.  The other two are plague and cholera.

Japanese Encephalitis Virus: Japanese Encephalitis is a virus that is transmitted to humans from the bite of an infected, rice paddy-breeding mosquito with the virus.  Infections range from mild (most common) to severe. Symptoms range from minor headache or fever to headache with quick onset, high fever, neck stiffness, stupor, disorientation, coma, tremors, or convulsions.  The disease is fatal in 5-30% of the cases and causes permanent neurologic disability in 30% of those who survive.  Cases in the United States are very rare despite 2-3 million Americans traveling to Asia per year. Most travelers do not go to high-risk, rural areas or engage in high-risk activities, such as outdoor, nighttime exposure.  Thirty thousand to 50,000 cases are reported annually in Asia.    

Science of Vaccines:
How Do Vaccines Work?
Vaccines work by inducing an immune response against the part of the virus or bacteria that makes a person ill. Because the viruses or bacteria are weakened, killed or only introduced in part, the recipient will not become ill. However, the immune responses generated will protect the person from the whole, live virus or bacteria during subsequent exposures.

How are Vaccines Made?
 There are several ways that vaccines are made. They include live, attenuated; inactivated; subunit; and conjugate vaccines. A live, attenuated vaccine is a vaccine that contains a live virus or bacteria that has been weakened. Live vaccines generally provide long-lasting protection with one or two doses. The limitation of this approach is that these vaccines usually cannot be given to people with weakened immune systems (such as people with cancer or AIDS). Live vaccines are not routinely given to pregnant women. The following are live, attenuated vaccines: measles, mumps, rubella, varicella and Yellow Fever.
Inactivated or killed vaccines are vaccines that contain a virus or bacteria that has been killed with a chemical. By killing the virus, it cannot reproduce itself or cause disease. The inactivated polio, hepatitis A, influenza, and rabies vaccines are made this way. The advantages of this approach are that the vaccine is not capable of causing the disease that it prevents and it can be given to people with weakened immune systems. The limitation of this approach is that several doses are often necessary to achieve life-long immunity.
Subunit vaccines contain pieces of the virus or bacteria that the immune system recognizes and makes antibodies against.  Subunit vaccines are similar to killed vaccines in that they are not infectious and therefore also present a low risk for adverse reactions. However, as with killed vaccines, subunit vaccines cannot replicate and, therefore, do not provide protective immunity for long periods of time. Because of these factors as well as the higher cost for production, subunit vaccines are used less frequently than modified live and killed vaccines. Examples include the subunit vaccine against hepatitis B vaccine that is composed of only the surface proteins of the virus, and the virus-like particle vaccine for human papillomavirus (HPV).  In some cases the vaccine is not made using a piece of bacteria, but a harmful chemical that it makes, called a toxin. In these cases, the toxin is chemically treated to inactivate it. After such treatment the toxin is called a “toxoid.”  Examples of this type of vaccine include the diphtheria, tetanus, and pertussis vaccines.
Some bacteria have polysaccharide outer coats that do not provide protection unless they are linked to proteins.  The immune system then recognizes the polysaccharide as if it were a protein antigen.  Vaccines in which the polysaccharide is chemically joined to a harmless protein carrier are called conjugate vaccines. Conjugate vaccines were developed primarily for very young children because their immune systems are not able to make a sufficient response to the bacteria alone. Examples of conjugate vaccines include Hib and pneumococcus.


What is Epidemiology?: 
Epidemiology is concerned with the distribution and determinants of health and diseases, morbidity, injuries, disability and mortality in populations. (Friis, R.H. & Sellers, T.A. Epidemiology for public health practice. Boston, MA: Jones & Bartlett, p. 5.)

Types of Epidemiologic Studies:
A variety of study designs are employed in epidemiology. In experimental studies, the investigator manipulates the study factor (or intervention), maintains control over the research setting, and randomly assigns subjects to the control or intervention groups. Experimental studies, also called randomized controlled trials, are the gold standard of research, and are the definitive test of an intervention's effectiveness. Quasi-experimental studies involve an intervention, but not randomization of subjects. Quasi-experimental studies are often used to evaluate situations when random assignment is not possible or ethical. Observational studies involve the measurement of patterns of exposure and disease in populations. They are used to generate hypotheses, or "educated guesses," about the causes of disease. There are 2 types of observational studies: descriptive and analytic. Descriptive studies include case reports, case series, and cross-sectional surveys. Analytic studies include ecologic, case-control and cohort studies.

Vaccine Clinical Trials:

Before a vaccine can be offered to the public, it must undergo extensive testing in controlled scientific studies. These are called pre-licensure studies and consist of three different phases. Once the vaccine has been licensed for use, there are additional post-marketing or phase IV studies. The requirements for all of these studies are established in the Investigational New Drug (IND) regulations.

Pre-clinical trials: Studies completed prior to testing in humans. Pre-clinical studies must show promise in the lab and be safe in animals prior to advancing to clinical trials.

Phase I studies: These studies are conducted in a small number of healthy adults, ranging from less than 20 to 100 people. There are two main goals in a phase I study. First, these studies rule out serious negative outcomes, or adverse events. Second, they establish immunogenicity; that is, they answer the question, “Does the immune system respond to this vaccine?”

Phase II studies: These studies enroll a few hundred people and can last for several months to a few years. In addition to continuing to monitor safety, these studies seek to determine the best dosage and timing for administering the vaccine. These studies are usually designed as randomized, placebo-controlled trials.

Phase III studies: The largest pre-licensure studies, these studies typically include thousands of people and last several years. These are usually well-designed placebo-controlled studies that monitor safety of the expected final formulation. They also determine the vaccine’s effectiveness, or efficacy, in the desired population.

Vaccine Licensing: In the United States, vaccines are licensed by the Food and Drug Administration’s Center for Biologics Evaluation and Research (CBER). Data from the pre-clinical studies must be submitted to CBER in the Biologics License Application (BLA). In addition to trial data, CBER will also review buildings and facilities, equipment, organization and personnel, production and process controls, packaging and labeling, record-keeping
practices, lot consistency and all other aspects surrounding preparation of the product. Products must be made using current Good Manufacturing Practices (cGMP) in order to be licensed.

Phase IV studies: After the vaccine is licensed, studies continue to evaluate its safety; these are often referred to as phase IV or post-marketing studies. These studies are observational, rather than experimental (see “Types of epidemiologic studies” section). If these studies suggest a potential safety issue, use of the product may be suspended until additional data can be gathered to determine whether the concern is causal or coincidental. Phase IV studies are also used to determine long-term effectiveness.

Vaccination Schedules
Immunization Schedules and Recommendations Module

Vaccine Safety

 Social and Economic Aspects of Immunizations

To learn more about the topics covered in this section, please visit the following:

All Diseases:


www.pertussis.com (National Association of Pediatric Nurse Practitioners)


Travel Vaccines:

Good Manufacturing Practices (GMP):

Science of Vaccines:


Friis, R.H. & Sellers, T.A. Epidemiology for public health practice. Boston, MA: Jones & Bartlett, p. 5.)


UPDATED: July 2009





©2006 Philadelphia Immunization Coalition