Photo courtesy Children's Healthcare of Atlanta
Vaccines have prevented millions of deaths around the world. modern medicine pictures.
Medicine has come a long way over the years. The development of the vaccine kicked off an era of illness prevention unlike anything the world had ever seen. In fact, vaccinations are largely viewed as the most successful medical advancement in the history of public health. Before vaccines were introduced, smallpox killed millions, nearly 20,000 were paralyzed by polio, and rubella (German measles) caused serious birth defects in about 20,000 newborns.
In this article, we'll learn about the inspiration for vaccines, the basic science behind how they prevent illness and the diseases they keep at bay. We'll also go head-to-head with some of the common myths circulated about vaccines.
The Inspiration for Vaccines
Who knew that cows would save the lives of countless humans? In 1796, a physician named Edward Jenner decided to prove a theory that had been circulating for some time. Smallpox once killed millions of people worldwide. Cowpox was a less serious disease related to smallpox that milkmaids often caught through exposure to infected cows. Jenner noticed that milkmaids who had contracted cowpox were later immune to smallpox. Jenner tested this theory when he took some infected cowpox matter and exposed an otherwise healthy boy through a cut in his arm. After the boy caught and recovered from cowpox, Jenner exposed him to smallpox via an injection. The boy remained healthy, and the world's first vaccine was born. The cows, for their part, were honored when the term "vaccine" was coined -- "vacca" is Latin for cow. According to the national Centers for Disease Control and Prevention (CDC) the world's last case of naturally occurring smallpox was in 1977. The disease has since been eliminated from natural occurrences in the world, so the vaccine is no longer given.
On the next page, we'll learn about how vaccines work in your body to destroy diseases.
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Children in Indonesia line up for vaccinations.
Jenner was operating on the now widely accepted principle that once a person catches a certain disease, he or she is immune to it for the rest of their life. For example, once you've had the chickenpox, it's extremely unlikely that you'll ever catch it again. This is because your body, when exposed again, will recognize the disease and fight it off. The beauty of vaccines is that they help the body develop disease-fighting abilities without making you sick. Vaccines accomplish this amazing feat by tricking the body into believing it already has the full-blown disease. Here are the steps in this process, known as the "immune response":
Why aren't vaccines available for all diseases?
The procedure for developing a vaccine takes many years and even more money, often hundreds of millions of dollars. According to John Bradley, M.D., member of the committee on infectious diseases of the AAP, it's because of this that vaccines are prioritized in this order:
Additionally, vaccines are studied and produced by companies, so the return on investment must be significant in order to justify the large expense. Vaccines are currently in development to prevent malaria. The malaria vaccine has been slighted in the past because the financial return was not worth the investment the industry had to make, according to Dr. Bradley. However, the Malaria Vaccine Initiative recently announced the beginning of Phase 1 trials of a new vaccine. The trial is currently underway in Brisbane, Australia.
Another reason that vaccines can be tricky to produce is that some viruses mutate so quickly that traditional vaccines are ineffective. A prime example is the HIV/AIDS virus. Despite these hurdles, there is currently a tremendous movement to develop a vaccine to fight HIV/AIDS.
The vaccine is administered. It contains weakened or dead forms of the disease.
- The immune system identifies these foreign substances (viruses and bacteria), also known as antigens.
- Once antigens are identified, the immune system develops proteins that circulate in the blood. These proteins are called antibodies. They fight the infection by killing the antigens. Antibodies are made by white blood cells called lymphocytes, also known as B cells. The main purpose of B cells is to create antibodies to fight infection.
- The body stockpiles these antibodies so they are available to fight off the disease if exposed later on. Unfortunately, antibodies are disease-specific, so previously acquired chickenpox antibodies will be useless if faced with other diseases.
It's very important to note that when the actual disease infects a person, the antigens multiply thousands and thousands of times until a raging infection is under way. The vaccine provides just enough of these antigens for the body to recognize them and complete the immune response process, therefore protecting them from exposure to the disease in the future.
Next, we'll discuss the two types of vaccines.
Photo courtesy CDC
A re-creation of the 1918 influenza virus
Vaccines are usually given via a hypodermic injection, but some are given through the mouth or nose. There are two main groups of vaccines: live-attenuated vaccines and inactivated vaccines.
Live-attenuated vaccines: Live-attenuated basically means alive, but very weak. These vaccines are made when the virus is weakened to such a level that they reproduce only about 20 times in the body. By comparison, natural viruses reproduce thousands of times. When the vaccine is made, the virus or bacteria is weakened in a laboratory to the point where it's still alive and able to reproduce, but can't cause serious illness. Its presence is enough to cause the immune system to produce antibodies to fight off the particular disease in the future.
"Live-attenuated vaccines can cause very mild illness in a small proportion of people," says John Bradley, M.D., member of the committee on infectious diseases of the American Academy of Pediatrics (AAP). "However, these side effects are usually very mild and limited to a low-grade fever or runny nose." Dr. Bradley also notes that about 5 to 10 percent of children who receive the varicella (chickenpox) vaccine develop a few pox spots, but it's nothing compared to the full-blown illness.
To weaken the virus, scientists must isolate it through a specimen from an infected person. They then grow the virus in a test tube. They "pass" the virus into a second test tube, then a third, a fourth and so on. Scientists perform this "passage" many times -- the measles virus was passed 77 times! The virus is periodically taken out of the test tube to see if it has mutated. Eventually, the virus gets so used to living in the comfortable test-tube environment that it loses its capacity to produce illness in humans. These passages are performed in a very controlled environment in exactly the same way each time. This discovery was considered the "hallelujah" of vaccine development, according to William Schaffner, M.D., professor and chair of the Department of Preventive Medicine at Vanderbilt University School of Medicine.
Vaccine Side Effects
As with any drug, there's always the potential for side effects. Vaccine side effects are generally pretty mild and limited to soreness at the injection site, headache or a low-grade fever. Serious reactions aren't completely impossible, though they are rare. If you suspect a severe reaction, call the doctor immediately to have it evaluated. Information about possible reactions to specific vaccines is available here.
Inactivated vaccines: When inactivated vaccines are made, the bacteria is completely killed using a chemical, usually formaldehyde. Dead pieces of disease-causing microorganisms (usually bacteria) are put into the vaccine. Because the antigens are dead, the strength of these vaccines tend to wear off over time, resulting in less long-lasting immunity. So, multiple doses of inactivated vaccines are usually necessary to provide the best protection. The benefit of inactivated vaccines is that there is zero chance of developing any disease-related symptoms -- allergic reactions are possible but extremely rare.
Why are some vaccines live and some dead?
"The bottom line is that the decision is entirely driven by the science," says Dr. Schaffner. "If scientists can make a killed vaccine that is effective, that is what they will do. It's all about trial and error." Most viral diseases, he says, require live-attenuated vaccines, but the vast majority of bacterial illnesses are prevented with inactivated vaccines. There are some exceptions to this rule, though. For example:
- Some travelers to less-developed countries get the vaccine to prevent typhoid fever. There are live and killed forms of this vaccine.
- Rabies is a viral infection that is 100 percent fatal once it has progressed. The disease is simply too dangerous to give, even in a weakened state. Fortunately, science allowed the development of an inactivated rabies vaccine.
So what, exactly, are the ingredients of a vaccine? Read on to find out.
Other than the antigen, a couple of things have to be included in the vaccine in order for it to be effective. The requirements vary depending on the specific vaccine, but the gist is the same.
The vaccine has to be stable because it leaves the manufacturing plant, gets bounced around on trucks and so forth. Sometimes small chemicals are added to act as stabilizers so that the vaccine material remains potent. These chemicals are thoroughly regulated by the Food and Drug Administration (FDA) to ensure their safety and are usually present only in trace amounts.
In multidose vials, a disinfectant is required. This is so that each time a dose is removed, any foreign matter that intrudes is killed instantly. Traditionally, Thimerosol has been the most popular among scientists. However, the industry has largely abandoned its use because of concerns that the chemical causes adverse reactions (see the page about vaccine myths). In fact, multidose vials are being phased out in favor of single-dose vials, even though these are more expensive. Thimerosol is currently present only in trace amounts in the influenza vaccine, but that will be a thing of the past in a few years. "We would like to put the controversy behind us," says Dr. Schaffner. "None of us associated with vaccines believe there is merit to that belief, but we are going the extra hundred miles to reassure parents."
On the next page we'll find out how vaccines are made.
Vaccine Production Process
Photo courtesy CDC
A CDC scientist fills a syringe with vaccine.
The process of creating, testing and producing a vaccine in mass quantities can take many years from start to finish because the industry is highly regulated. Before scientists even begin to formulate a vaccine, researchers have to study the particular virus or bacteria. Basically, they have to isolate it in a laboratory setting and figure out how it causes the disease. Then they develop the vaccine as either a live-attenuated or inactivated vaccine, depending on the type of virus or bacteria.
Once they have a good grip on that, researchers study the best ways to protect people from the disease using the vaccine they have developed. They figure out the best dosage amounts, whether or not one shot is good enough, or if more is necessary. They also estimate how long protection from the vaccine lasts to determine if booster shots will be necessary. Most of this early research is conducted in laboratories in an academic setting and is paid for by foundation or government grants.
Once the vaccine has been developed, the testing process is conducted over four phases and a period of many years. This testing is funded by pharmaceutical companies and can rack up hundreds of millions of dollars in bills.
- The first studies usually test the vaccine on about a hundred healthy adults who are at low risk for complications. The researchers want to find out if the vaccine is safe and if it induces the immune response necessary to fight off disease. If this phase isn't successful, the vaccine is taken back to the drawing board or abandoned.
- If Phase 1 is successful, the vaccine moves to the second phase of study, involving a few hundred people from the vaccine's intended group. For example, the chickenpox vaccine was probably tested on children at this phase because they are the target audience. The vaccine is tested on several hundred people to again ensure its safety. Researchers also want to verify that the vaccine causes the desired immune response consistently.
- The third phase can take many years and studies anywhere from a thousand to tens of thousands of people in various lifestyles and geographic locations. Researchers want to make sure that the vaccine works on people of all types and in all environments. The FDA reviews all of the data and the study methodology. If everything is satisfactory, the FDA deems the vaccine safe, but then it must be approved by a board of vaccine experts who advise the CDC. The experts then distribute their recommendations about dosage, who should receive the vaccine, when they should receive it, etc.
- Even after the vaccine has been distributed, it still requires many more years of study to make sure that no unforeseen side effects occur. The CDC monitors the vaccine and possible side effects very closely in four types of study:
- Areas especially prone to the disease are closely monitored. All people who receive the vaccine are monitored closely. Results are reported to the CDC.
- The CDC keeps an eye on any occurrence of the disease in the United States. If any strange pop-ups of the disease occur after the vaccine is introduced, it must find out if the vaccine and the pop-ups are related.
- Health care professionals or consumers who believe that a serious side effect has occurred in a vaccine recipient can file a report via the Vaccine Adverse Events Reporting System. This data is continuously monitored to identify trends.
- The Vaccine Safety Datalink Project provides data on millions of people regarding their vaccination histories and medical outcomes (whether or not they had side effects). This is carefully monitored by the National Center for Health Statistics' Research Data Center to make sure that no major side effects or health risks are occurring.
Who should get vaccinated -- and when? For what? Read on for a thorough explanation.
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Mary Mead, 83, receives a flu shot in Chicago.
Babies and young children have always been a major focus of the vaccine movement. However, public health experts want to make sure that adolescents, adults and the elderly stay up-to-date on their immunizations.
"Vaccines are not just for babies anymore," says Angie Matthiessen, MSW, of Immunize Georgia and Children's Healthcare of Atlanta. "Many adolescent and adult vaccines are now in place, often to protect the very young and elderly."
Visit your doctor regularly to stay up-to-date on vaccinations. Many insurance plans cover the majority of vaccine-related charges because they're considered necessary preventative measures. Often, walk-in clinics provide vaccinations for a nominal fee.
Some physicians are able to give vaccines to children as part of the Vaccines for Children Program free or at very little cost. Children under 18 are eligible if they're at least one of the following:
- Medicaid eligible
- Uninsured (no health insurance)
- Underinsured (health plan does not pay for vaccines)
- American Indian or Alaska Native (Indian Health Services Act)
Vaccines Around the World
Some countries, such as the United States, have highly developed vaccine programs. As mentioned earlier, though, many diseases eradicated in the United States are still alive and well elsewhere. Many initiatives exist to reduce illness and death from vaccine-preventable diseases, such as the Measles Initiative Immunization program. This program is made up of partners like the World Health Organization (WHO), the CDC, the United Nations Foundation and UNICEF. A major success story is the decline of measles, largely in Africa. Worldwide deaths from measles fell 60 percent from 1999 to 2005. Most of these were in Africa, were measles deaths fell 75 percent. The decline is attributed to childhood immunization programs. Similar global programs exist for polio, pneumococcal disease and meningitis.
School requirements for vaccinations vary from state to state. The requirements for each state can be accessed here.
Travel requirements are pretty limited. International Health Regulations mandates that the yellow fever vaccination is necessary for travel to certain countries in tropical South America and sub-Saharan Africa. The Saudi Arabian government requires that visitors receive the meningococcal vaccination if visiting during the Hajj. Other than that, the CDC recommends that all international travelers be up-to-date on regular vaccinations. Whether or not travelers need extra vaccines depends on factors such as the destination country, season and whether rural areas will be visited. The CDC provides a comprehensive destination list for travelers to review here.
On the next page, we'll give you a full list of infant vaccinations.
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An infant in India gets the polio vaccine.
Vaccines are most effective at preventing disease when the recommended schedule is followed. Over the next three sections, we'll go over the schedules for infants, children and adults.
Infants receive the lion's share of vaccines. First, babies have very immature immune systems and are at greater risk of complications and death if exposed to disease. Second, experts believe that a strict vaccine schedule from birth on helps to ensure that these diseases don't pop back up and cause large-scale epidemics. It is recommended that babies from birth to 15 months receive the following vaccines (see the vaccination schedule for exact ages):
- HebB - Hepatitis B is transmitted through blood transfusions, direct contact with infected bodily fluids, sexual contact, body piercing and tattoos. According to the CDC, of the children who acquire lifelong hepatitis B, 25 percent die of liver disease in adulthood.
- DTaP - Combination vaccine that prevents three diseases in one shot.
- Diphtheria is spread through person-to-person contact. Diphtheria once took the lives of more than 15,000 children a year.
- Tetanus (lockjaw) is transmitted via contact with infected soil to cuts and punctures in the skin. It causes muscle spasms, often of the jaw (hence the name lockjaw) and can cause major complications, including lung infections, heart damage and death. It's recommended every 10 years because it wears off over time and poses risk to people of all ages, causing death in 10 to 20 percent of cases.
- Pertussis (whooping cough) is transmitted through coughing and sneezing fits. Pertussis is especially dangerous to the very young, the elderly and those with compromised immune systems, such as cancer patients. See sidebar for more information on the rising number of pertussis cases in the United States.
- Haemophilus influenzae type b (Hib) - Hib meningitis once caused terrible complications in survivors, including mental retardation, deafness and seizures. Roughly 600 children died annually from the disease.
- IPV (inactivated poliovirus) - Spread through direct contact with fecal matter, this disease once caused 13,000 to 20,000 cases annually in the United States alone. Paralytic polio left many victims relegated to wheelchairs, crutches and leg braces.
- PCV (Pneumococcus) - Causes complications such as meningitis, pneumonia, blood infection (sepsis) and brain damage, is spread through coughing and sneezing, and is especially dangerous to the elderly.
- MMR (Measles, mumps and rubella) - Another combination vaccine. These three diseases are spread through coughing and sneezing.
- Measles is highly infectious. The CDC estimates that 2.7 million people worldwide would die if vaccinations stopped. The trademark rash begins at the hairline and progresses downward.
- Mumps causes swollen glands and cheeks and complications like deafness and brain damage.
- Rubella (German measles) is often transmitted from pregnant women to their unborn fetuses. It caused many premature births, miscarriages and birth defects.
- Varicella (chickenpox) - Chickenpox used to cause approximately 4 million cases, 11,000 hospitalizations and 100 deaths per year. Complications are worse for teens and adults, and include lung and brain damage and death. The trademark itchy rash usually covers the entire body.
- Hepatitis A - Usually spread via contaminated food and water, often in situations with bad food preparation habits and in developing countries. It can cause very low energy for as long as a year and much worse in people with liver problems.
- Rota (rotavirus) - A common disease that infects almost all children in the United States before school age. A very new oral vaccine is now available. The disease usually causes fever, nausea and diarrhea. In less developed countries it causes more than 500,000 deaths annually.
- Influenza (the flu) - The disease, which is spread through coughing, sneezing and other direct contact, can cause complications like brain swelling, pneumonia and death in severe cases. The Spanish flu pandemic of 1918 caused 550,000 deaths in the United States and 21 million deaths across the globe, according to the CDC (see sidebar for more information on the flu vaccine).
After all these vaccines, babies get a few years to rest before their next round of immunizations. Next we'll talk about childhood vaccinations.
The Problem with Pertussis
Pertussis, better known to many as whooping cough, is currently the only vaccine-preventable disease on the increase in the United States. Most infants receive the vaccine, but it wears off by adolescence. Many teens and parents aren't aware that middle-school-age children should receive a booster vaccine to protect themselves and others.
While teens and adults can catch and suffer from the disease, it's often fatal to the very young and very old. Babies receive the vaccine in three doses, so they aren't protected completely from the disease for many months. According to the CDC, 91 percent of infant deaths from pertussis (from 2001 to 2003) occurred in babies under 6 months old. The elderly and people with poor immune systems are also at elevated risk because their immune systems are much weaker.
"The point is that if you are around an elderly person or a baby you really need to have this shot," says Joyce Allers, R.N., of Children's Healthcare of Atlanta. "We as a society have to protect our more fragile members. They [people whose children are not up-to-date on vaccines] are taking their child out in public where they may be exposing a grandfather recovering from cancer to a potentially deadly disease."
Pertussis is often misdiagnosed because it shares symptoms with other common respiratory ailments. Symptoms include a severe cough that can last eight weeks or more, momentary loss of consciousness, low-grade fever and a runny nose.
Early childhood - From age 18 months to 6 years, children receive subsequent vaccinations of HepB, DTaP, IPV, MMR and varicella. This is to ensure maximum protection against the diseases.
Later childhood/adolescence - After age 6, children don't have too many shots to worry about until adolescence. Around age 11 they should get the Tdap vaccine to protect against tetanus, diphtheria and pertussis because the vaccine they received as an infant has worn off (see sidebar below for more info). The following are also recommended at age 11 or 12:
- HPV (human papillomavirus) - This three-dose shot is one of the latest and greatest in the vaccine industry. HPV is a sexually transmitted disease that causes cervical and genital warts. HPV is usually symptom-free, so women become aware of the infection only when they have a positive Pap smear. Since the vaccine is very new it's also recommended for women up to age 26 who have not already received it.
- MCV4 (meningococal) - The age for this vaccine has recently been moved up to 11. The disease occurs when the spinal fluid and fluid around the brain become infected. The symptoms of meningitis are high fever, headache and a very stiff neck. It can be viral or bacterial (bacterial is the most severe) and is sometimes contagious, depending on the bacteria. This is why middle- and high-school students, as well as college students living in dormitories, are recommended to get the shot.
Adults have to get shots, too. Find out about them on the next page.
Vaccines aren't just for humans. Pets are lining up for shots to prevent illness and death as part of their annual check-ups. Experts agree that vaccines have protected millions of animals over the years. Pet owners should talk to a veterinarian about which vaccines are necessary (core) and which are optional (non-core), because there is some variation depending on where you live and whether or not your pet goes outdoors.
Some debate is heating up between people who think that animals need yearly revaccinations and those that think it's unnecessary. Unfortunately, there's not enough evidence to prove beyond a reasonable doubt that immunity lasts more than a year.
Adult (18-65) - Adults should be revaccinated for any of the already-mentioned diseases if there's evidence of waning immunity. For example, when a woman becomes pregnant, her doctor usually runs tests to make sure that she's still immune to German measles. Also, not all adults have received every vaccine because they weren't available when they were children, or they were never immunized in the first place.
Elderly (over 65) - Adults over age 65 are at an increased risk of complications from disease, simply because their immune systems aren't what they used to be. Pertussis, pneumococcus and the flu pose extra threats to this age group. The elderly should make sure that their vaccinations are up-to-date because some may have worn off over time. The following is also recommended:
- Zoster (shingles) - Adults over age 60 should get the vaccine for herpes zoster (shingles). This painful disease causes a blistery rash.
High-risk groups - These are children and adults who don't have the best immune systems. These are usually people with compromised immune systems from diseases such as cancer. The schedule for these patients is largely controlled by their particular specialist and should be maintained because the disease can be harsher on them than others [source: CDC].
There can be confusion and misunderstanding about vaccines. On the next page we'll debunk some of the myths you might have heard.
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A French lab is working on the vaccine for the H5N1 virus.
MYTH: Vaccines cause autism.
TRUTH: "Study after study has shown that there is no link between vaccines and autism," says Angie Matthiessen with Children's Healthcare of Atlanta. This myth gained popularity when children were diagnosed with autism around the time that they received the MMR vaccine (around 18 months of age), according to Joyce Allers, RN, also with Children's Healthcare of Atlanta. At the time, the mercury-containing preservative Thimerosol was being put in some vaccines -- although not the MMR vaccine -- to keep bacteria from growing, so many people connected that to autism. However, there still appears to be no link between previous use of Thimerosol and autism. Thimerosol was completely removed from all infant vaccines in the United States in 2001, yet autism rates continue to increase. Thimerosol is currently present only in minute amounts in the influenza vaccine.
MYTH: Vaccines aren't necessary anymore because all the diseases are gone anyway.
TRUTH: According to the CDC, if we stopped vaccinating, many diseases that are largely unknown would come back with a vengeance. The only vaccine-preventable disease that's been completely wiped out is smallpox. "These diseases are coming into our country, into our schools and into our workplace," says Matthiessen. "Don't believe that your child is 100 percent protected because the disease isn't currently in the United States. International travel and adoption are two of the ways that dangerous diseases make their way into the country."
The Future of Vaccines
Vaccines are currently being developed for a myriad of diseases, including HIV/AIDS, cancer, the plague and even the common cold. Although it may be years before any of these vaccines make it to pharmacy shelves, their development is critical because as surely as we are able to destroy a particular disease another one is probably just around the corner. A list of potential new vaccines is available here.
MYTH: Babies are too fragile to get so many shots.
TRUTH: "A lot of young parents are worried about giving their babies so many shots, thinking that they will overwhelm their immune system," says Allers. "I tell them that as human beings we are exposed to disease all the time, like at the mall and church, so we can't overwhelm their systems." According to the Children's Hospital of Philadelphia's Vaccine Education Center, diseases often occur in very young infants, so the best way to prevent them is to give vaccines as soon as possible after birth. "We have a wonderful history of eliminating disease, but what worries us is all the good progress we've made is going to be diminished by families choosing their own vaccination schedule or choosing not to vaccinate at all," Allers says.
MYTH: A live vaccine can give me the disease it's supposed to prevent.
TRUTH: As we've discussed, live vaccines can cause extremely mild symptoms. However, experts agree that they are very minor and much better than coming down with the full-blown disease.
For more information on vaccines and related topics, check out the links on the next page.
Related HowStuffWorks Articles
- How Cancer Vaccines Will Work
- How Childhood Vaccinations Work
- How Your Immune System Works
- Are we on the brink of an AIDS vaccine?
- How could the flu vaccine prevent heart attacks?
More Great Links
- Centers for Disease Control and Prevention
- Immunization Action Coalition
- American Academy of Pediatrics
- National Network for Immunization Information
- Children's Hospital of Philadelphia, Vaccine Education Center
- Allers, Joyce, R.N. Personal interview. 22 Aug. 2007.
- American Academy of Pediatrics.
- American History.
- American Veterinary Medical Association.
- "A Parent's Guide to Vaccine-Preventable Diseases for Children." Toronto, Ontario, CA: Sanofi Pasteur, 2005.
- Bradley, John, M.D. Personal interview. 30 Aug. 2007.
- Centers for Disease Control and Prevention.
- Children's Hospital of Philadelphia's Vaccine Education Center.
- Foster, DVM Race; Marty Smith, DVM and Holly Nash, DVM, MS. "Vaccination Recommendations for Dogs." Pet Education. 4 Sept. 2007.
- Foster, DVM Race; Marty Smith, DVM and Holly Nash, DVM, MS. "Vaccination Recommendations for Cats and Kittens." Pet Education.
4 Sept. 2007.
- Many Cases, Real Risks, More Reasons to Protect Yourself. Toronto, Ontario, CA: Sanofi Pasteur, 2006.
- Immunization Action Coalition.
- Matthiessen, Angie, MSW. Personal interview. 22 Aug. 2007.
- Maybury Okonek, Bonnie A. and Pamela M. Peters, Ph.D. "Vaccines: How and Why?" Access Excellence. 25 Aug 2007
- Malaria Vaccine Initiative.
- "Measles Deaths Down Worldwide." CNN Online: Health. 19 January 2007. 3 September 2007
- National Institute of Allergy and Infectious Diseases.
- Petplace Veterinarians. "Canine Vaccine Recommendations." PetPlace. 4 Sept 07
- Schaffner, William, M.D. Personal interview. 18 Sept. 2007.
- "Vaccination Fact vs. Fiction." Philadelphia, PA: Wyeth Pharmaceuticals, Inc., 2005.
- World Health Organization.