Vaccine Education Center

Vaccine Safety FAQs

Are vaccines safe?

Who licenses, recommends and requires vaccines?
Do vaccines overwhelm the immune system?
Do vaccines weaken the immune system?
Can children who are sick receive vaccines?
Can we give vaccines by a method other than shots?
Why do we still need vaccines?
Are children too young to get vaccines?
Can children manage so many different vaccines at the same time?
Why are some vaccines given as a single dose and others as multiple doses?

Are vaccines safe?

The first definition of the word safe is "harmless." This definition would imply that any negative consequence of a vaccine would make the vaccine unsafe. Using this definition, no vaccine is 100 percent safe. Almost all vaccines can cause pain, redness or tenderness at the site of injection. And some vaccines cause more severe side effects. For example, the pertussis (or whooping cough) vaccine can cause persistent, inconsolable crying, high fever or seizures associated with fever. Although none of these severe symptoms result in permanent damage, they can be quite frightening to parents.

But, in truth, few things meet the definition of "harmless." Even everyday activities contain hidden dangers. For example, every year in the United States, 350 people are killed in bath- or shower-related accidents, 200 people are killed when food lodges in their windpipe, and 100 people are struck and killed by lightning. However, few of us consider eating solid food, taking a bath, or walking outside on a rainy day as unsafe activities. We just figure that the benefits of the activity clearly outweigh the risks.

The second definition of the word safe is "having been preserved from a real danger." Using this definition, the danger (the disease) must be significantly greater than the means of protecting against the danger (the vaccine). Or, said another way, a vaccine's benefits must clearly and definitively outweigh its risks.

To better understand the definition of the word safe when applied to vaccines, let's examine four different vaccines and the diseases they prevent:

Is the hepatitis B vaccine safe?

The hepatitis B vaccine has few side effects. However, one side effect is serious. About one of every 600,000 doses of hepatitis B vaccine is complicated by a severe allergic reaction called anaphylaxis. The symptoms of anaphylaxis are hives, difficulty breathing and a drop in blood pressure. Although no one has ever died because of the hepatitis B vaccine, the symptoms of anaphylaxis caused by the vaccine can be quite frightening.

On the other hand, every year about 5,000 people die soon after being infected with hepatitis B virus. In addition, about 10,000 people every year suffer severe liver damage (called cirrhosis) or liver cancer caused by hepatitis B virus. People are much more likely to develop these severe and often fatal consequences of hepatitis B virus infection if they get infected when they are very young children. For this reason, the hepatitis B vaccine is recommended for newborns.

Some parents wonder whether it is necessary to give the hepatitis B virus vaccine to newborns. They ask: "How is a baby going to catch hepatitis B?" But, before the hepatitis B virus vaccine, every year in the United States about 18,000 children less than 10 years of age caught hepatitis B virus from someone other than their mother. Some children catch it from another family member, and some children catch it from someone outside the home who comes in contact with the baby. About 1 million people in the United States are infected with hepatitis B virus. However, because hepatitis B virus can cause a silent infection (meaning without obvious symptoms), many people who are infected with hepatitis B virus don't know that they have it! So it can be hard to tell from whom you could catch hepatitis B virus. Worse yet, many people don't realize that you can catch hepatitis B virus after coming into contact with minute quantities of blood through casual contact with someone who is infected (for example, sharing washcloths or toothbrushes). In fact, a milliliter of blood (about one-fifth of a teaspoon) from someone who is infected can contain as many as one billion infectious viruses, so people can be infected with quantities of infected blood small enough that they are not visible to the naked eye.

Because the benefits of the hepatitis B vaccine clearly and definitively outweigh its risks, the hepatitis B vaccine is safe.

Was the old pertussis vaccine safe?

The old pertussis vaccine had far more risks than the hepatitis B vaccine. The old pertussis vaccine was called the "whole-cell" vaccine and had a high rate of severe side effects. Persistent, inconsolable crying occurred in one of every 100 doses, fever greater than 105 degrees occurred in one of every 330 doses, and seizures with fever occurred in one of every 1,750 doses. Due to negative publicity related to this vaccine, its use decreased in many areas of the world.

For example, the Japanese Ministry of Health decided to stop using the pertussis vaccine in 1975. In the three years before the vaccine was discontinued, there were 400 cases of pertussis and 10 deaths from pertussis in Japan. In the three years after the pertussis vaccine was discontinued, there were 13,000 cases of pertussis and 113 deaths! It should be noted that although the side effects of the pertussis vaccine were high, children didn't die from pertussis vaccine; however, they die die from pertussis infection. The Japanese Ministry of Health, realizing how costly their error had been, soon reinstituted the use of pertussis vaccine.

The children of Japan proved that, despite the side effects, the benefits of the old pertussis vaccine clearly outweighed the risks.

Scientific progress eventually led to the creation of another version of the pertussis vaccine. Known as the “acellular” pertussis vaccine, it was more purified, so instead of containing about 3,000 immunogenic proteins, it only contained two to five proteins. The ("acellular") pertussis vaccine became available in the United States in 1996. Children who received this vaccine had a much lower risk of severe side effects than those who received the old "whole-cell" vaccine. Therefore, the new pertussis vaccine is safer than the old pertussis vaccine. But because the benefits of the old pertussis vaccine outweighed its risks, it too was safe.

Is the pneumococcal vaccine safe?

Let's take a look at the pneumococcal vaccine. The pneumococcal vaccine was licensed in the United States in the year 2000 and was recommended for use in all children less than 5 years of age. Some parents chose to take a "wait-and-see" attitude. They reasoned that because the problems with the rotavirus vaccine were not revealed until the vaccine was given to 1 million children (see below, "Is the rotavirus vaccine safe?"), why not wait and see what happened after the pneumococcal vaccine was given to several million or more children.

However, the choice not to give the pneumococcal vaccine was not a risk-free choice. Before pneumococcal vaccine was first given to infants in 2000, every year in the United States about 700 children (less than 5 years old) got meningitis, 17,000 got bloodstream infections, and 71,000 got pneumonia from pneumococcus. So the choice not to give a pneumococcal vaccine was a choice to risk the severe, often permanent, and occasionally fatal consequences of pneumococcal infection. Parents should be reassured by two facts. First, the pneumococcal vaccine was tested in about 20,000 children before it was licensed for use. Second, the Haemophilus influenzae type b (Hib) vaccine is made in a manner almost identical to the pneumococcal vaccine (see How Are Vaccines Made?) and has been given safely to millions of children since 1990.

Is the rotavirus vaccine safe?

A new rotavirus vaccine was recommended for use in February 2006. This vaccine was tested in more than 70,000 infants. About half of the children received vaccine and the other half received a salt water solution. This large study showed the vaccine to be safe. Children who received the vaccine were not more likely to experience vomiting, diarrhea, fever, irritability or poor feeding than children who did not receive the vaccine.

The first rotavirus vaccine was withdrawn from use because of a problem with safety. The vaccine was found to cause a rare, but potentially very serious, side effect called intussusception. Intussusception occurs when one section of the small intestine folds into another section of the intestine. When this happens, the intestine can become blocked. Intussusception is a medical emergency, and children can die from the disease. The rotavirus vaccine was given to about 1 million children in the United States between 1998 and 1999. About one of every 10,000 children who were given the vaccine got intussusception (a total of about 100 children), and one child died because of the vaccine.

Because of the problem with intussusception in the previous vaccine, any new rotavirus vaccine had to be evaluated for this potential side effect as well. There are currently two rotavirus vaccines (RotaTeq® and Rotarix®) available in the U.S. Studies have shown that children who receive either of the rotavirus vaccines are no more likely to experience intussusception than those who did not receive the vaccine. Intussusception occurs naturally in about 1 of every 100,000 infants.

Systems are in place to protect children against rare side effects from vaccines

The previous rotavirus vaccine is an example of how rare side effects can be detected quickly. The rotavirus vaccine was tested in about 11,000 children before it was submitted to the FDA for licensure. After the vaccine was licensed and recommended for use (see below, "Who licenses, recommends and requires vaccines?"), the vaccine was given to about 1 million children.

A system called the Vaccines Adverse Events Reporting System (VAERS) initially found about 15 cases of an intestinal blockage called intussusception soon after administration of the vaccine. This was worrisome enough to the Centers for Disease Control and Prevention (CDC) to cause them to temporarily suspend use of the rotavirus vaccine until it could be determined whether the vaccine did, in fact, cause intussusception. Their analysis showed that intussusception occurred in about one of every 10,000 children who received the vaccine. Because only 11,000 children were tested before the vaccine was licensed, it was really not possible to pick up such a rare side effect. The result of the rotavirus vaccine experience is that at least 60,000 children had to be tested before the next vaccine was licensed.

Several other systems of study to understand the rate of vaccine side effects, such as the Vaccine Safety Data Link (VSD), are also available. The VSD also allows one to determine the background rate of side effects, meaning the rate of adverse events in children who don't receive a vaccine. So, in many ways, systems like the Vaccine Safety Data Link are better than VAERS because they allow one to determine whether a particular vaccine is the cause of a rare side effect.

In A Look at Each Vaccine, we discuss the risks and benefits of every vaccine in order to determine whether these vaccines are safe. 

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Who licenses, recommends and requires vaccines?

Vaccines are only given to children after a long and careful review by a number of different groups. These different groups either: license, recommend or require vaccines.

Licensure

The first of three processes, licensure, involves gaining approval from the Food and Drug Administration (FDA). As a result, it is the longest of these processes. It can take years, even decades, before pharmaceutical companies can actually start providing the vaccine. For example, the varicella vaccine took about 11 years to be licensed by the FDA.

Vaccines are usually made by first showing that they are safe and effective in experimental animals. Once this is established, the vaccine becomes an Investigational New Drug (IND) and the company is given an IND license to further study the safety and effectiveness of the vaccine in adults, and eventually, children. Again, these trials can take years, giving companies ample time to provide the FDA with proof of long-term safety and effectiveness.

Recommendation

The "recommendation" process begins only after a vaccine is licensed by the FDA. Doctors don't just decide to start giving the vaccine on their own. They seek the recommendations of the Advisory Committee on Immunization Practices (ACIP), which is part of the Centers for Disease Control and Prevention (CDC), the Committee on Infectious Diseases of the American Academy of Pediatrics (AAP), and the American Academy of Family Physicians (AAFP).

While the FDA licensure process involves determining the risks versus the benefits of a particular vaccine, the recommendation process considers the costs versus the benefits. Here's the distinction:

• The FDA examines vaccine safety by considering whether there are any risks (negative effects) associated with the vaccine. They ask: "Do these risks outweigh the benefits of the vaccine or vice versa?"
• In addition to considering the risks and benefits of a vaccine, the ACIP, AAP and AAFP also examine the costs associated with immunizing all or just a segment of the population. For instance, with the chickenpox (varicella) vaccine: What is the cost of immunizing children compared with the potential savings in medical and non-medical costs from immunizing them? (An example of non-medical costs would be money lost by parents who miss work taking care of their unvaccinated child.)
• Another part of the recommendation process is determining which groups of people within the population would benefit from the vaccine, and on what schedule the vaccine should be given based on data previously presented to the FDA.

Requirement

Just because a vaccine is recommended for use doesn't mean that it is required for use. State legislatures and health departments determine whether a vaccine is required. They examine the practicality of requiring it for every child within the state. Factored into this equation is whether a local or state government can afford to pay for the vaccines of children whose parents can't afford it.

However, from the parents' perspective, vaccine requirements shouldn't matter. If a vaccine is considered to be safe and effective by the FDA, and useful for children by the ACIP, AAP and AAFP, then the vaccine is of value and should be given. Local health care budgets, although they determine whether a vaccine is to be required for school entry, have little to do with whether a vaccine is safe, effective and useful.

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Do vaccines overwhelm the immune system?

Studies on the diversity of antibody specificities indicate that the immune system has the capacity to respond to extremely large numbers of immunologically distinct regions of viruses and bacteria. Current data suggest that the theoretical capacity determined by the genes that make different antibodies would allow for as many as 10⁹-10¹¹ different kinds of antibodies (i.e., 1 billion to 100 billion). But this theoretical capacity is limited by the number of circulating antibody-producing cells (B cells or lymphocytes) and the likely redundancy of antibodies generated by one individual.

A more practical way to determine the diversity of the immune response would be to estimate the number of vaccines to which a child could respond at one time. Assuming the quantities of antibodies likely generated by an individual in 1 ml of blood (one-fifth of a teaspoon) during seven days after exposure to a vaccine, and the number of different specificities of those antibodies, then each infant would have the capacity to respond to about 10,000 vaccines at any one time. Using this estimate, one would predict that if 11 vaccines were given to infants at one time, then about 0.1 percent of the immune system would be "used up."

However, because B cells and other lymphocytes are constantly replenished, a vaccine never really "uses up" a fraction of the immune system. For example, the immune system has the ability to replenish about 2 billion lymphocytes each day. This replacement activity illustrates the enormous capacity of the immune system to generate lymphocytes as needed.

Parents may also take comfort in knowing that children are exposed to fewer immunologic components (like proteins and sugars [polysaccharides]) in vaccines today than in the past. The table below summarizes the number of proteins and polysaccharides contained in routinely recommended vaccines administered over the past 100 years. Although we now give children more vaccines, the actual number of immunologic components in vaccines has declined. Whereas previously one vaccine, smallpox, contained about 200 proteins, now the 11 routinely recommended vaccines contain fewer than 130 immunologic components (i.e., proteins or polysaccharides). Two factors account for this decline: first, the worldwide eradication of smallpox obviated the need for that vaccine, and second, advances in protein chemistry have resulted in vaccines containing fewer antigens (e.g. replacement of whole-cell with acellular pertussis vaccine).

References

Offit PA, Quarles J, Gerber MA, et al. Addressing parents’ concerns: Do multiple vaccines overwhelm or weaken the infant’s immune system? Pediatrics. 2002;109:124-9.

Tonegawa S, Steinberg C, Dube S, Bernardini A. Evidence for somatic generation of antibody diversity. Proc Natl Acad Sci USA. 1974;71:4027-31.

Cohn M, Langman RE. The protecton: the unit of humoral immunity selected by evolution. Immunol Rev. 1990;115:9-147.

Ramsay DS, Lewis M. Developmental changes in infant cortisol and behavioral response to inoculation. Child Dev. 1994;65:1491-502.
 

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Do vaccines weaken the immune system?

Vaccinated children are not at greater risk of other infections (infections not prevented by the vaccines) than unvaccinated children. On the contrary, in Germany, a study of 496 vaccinated and unvaccinated children found that children who received immunizations against diphtheria, pertussis, tetanus, Haemophilus influenzae type b (Hib) and polio within the first three months of life had fewer infections with vaccine-related and unrelated pathogens than the non-vaccinated group.

Bacterial and viral infections, on the other hand, often predispose children and adults to severe, invasive infections with other pathogens. For example, children with pneumococcal pneumonia are more likely to have had a recent influenza infection than other children. Similarly, varicella infection increases susceptibility to the 'flesh-eating bacteria (i.e., group A strep).

References

Black SB, Cherry JD, Shinefield HR, et al. Apparent decreased risk of invasive bacterial disease after heterologous childhood immunization. Am J Dis Child 1991;145:746-749.

Davidson M, Letson W, Ward JI, et al. DTP immunization and susceptibility to infectious diseases. Is there a relationship? Am J Dis Child 1991;145:750-754.

Otto S, Mahner B, Kadow I, et al. General non-specific morbidity is reduced after vaccination within the third month of life — the Greifswald study. J Infect 2000;41:172-175.

Storsaeter J, Olin P, Renemar B, et al. Mortality and morbidity from invasive bacterial infections during a clinical trial of acellular pertussis vaccines in Sweden. Pediatr Infect Dis J 1988;7:637-645.

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Can children who are sick receive vaccines?

Some parents may be concerned that children with acute illnesses are less likely to respond to vaccines or are more likely to develop severe reactions to vaccines than are healthy children. Alternatively, some parents may believe that children who are ill shouldn't further burden an immune system already committed to fighting an infection. However, vaccine-specific antibody responses and rates of vaccine-associated adverse reactions of children with mild or moderate illnesses are comparable to those of healthy children. For example, the presence of upper respiratory tract infections, ear infections, fever, skin infections or diarrhea does not affect the level of protective antibodies induced by immunization.

Data on the capacity of vaccines to induce protective immune responses in children with severe infections (such as those with bacterial pneumonia or meningitis) are lacking. Although a delay in vaccines is recommended for children with severe illnesses until the symptoms of illness resolve, this recommendation is not based on the likelihood that the child will have an inadequate immune response to the vaccine. Rather, the reason for deferring immunization is to avoid superimposing a reaction to the vaccine on the underlying illness or to mistakenly attribute a manifestation of the underlying illness to the vaccine.

References

Dennehy PH, Saracen CL, Peter G. Seroconversion rates to combined measles-mumps-rubella-varicella vaccine of children with upper respiratory tract infection. Pediatrics 1994;94:514-516.

Halsey NA, Boulos R, Mode F, et al. Response to measles vaccine in Haitian infants 6 to 12 months old: influence of maternal antibodies, malnutrition, and concurrent illness. N Engl J Med 1985;313:544-549.

King GE, Markowitz LE, Heath J, et al. Antibody response to measles-mumps-rubella vaccine of children with mild illness at the time of vaccination. JAMA 1996;275:704-707.

Ndikuyeze A, Munoz A, Stewart J, et al. Immunogenicity and safety of measles vaccine in ill African children. Int J Epidemiol 1988;17:448-455.

Ratnam S, West R, Gadag V. Measles and rubella antibody response after measles-mumps-rubella vaccination in children with afebrile upper respiratory tract infection. J Pediatr 1995;127:432-434.

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Can we give vaccines by a method other than shots?

By giving vaccines as shots, the body quickly makes antibodies in the blood. Viruses and bacteria usually cause damage by first entering the bloodstream. So, the best way to fight off these infections is to make antibodies that are present in the blood. Hepatitis B, chickenpox and rabies infections can be prevented by giving a vaccine even after the person has been exposed to these viruses. This is because antibodies in the bloodstream are made more quickly after vaccination by shot than they are after exposure to the virus naturally.

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Why do we still need vaccines?

We still need vaccines for the following reasons:

• Some diseases are still so common that a choice not to get a vaccine is a choice to risk natural infection. Examples of these diseases include chickenpox, pertussis, hepatitis B, influenza and pneumococcus.
• Some diseases still occur in the United States at very low levels, so if immunization rates dropped even a little bit, outbreaks of disease would sweep across the country rapidly. Examples of these diseases include measles, mumps and Hib.
• Some diseases have been virtually eliminated from the United States, but still occur frequently in other parts of the world. Because international travel is common, it is quite possible for the disease to come to the US or for us to go to a place where the disease exists. Examples of these diseases include polio, rubella and diphtheria.
• Tetanus is the only vaccine against a disease that is not passed from one person to another. We can never eliminate tetanus from the environment, so this vaccine will always be required. To learn more about tetanus, visit the Diphtheria, Tetanus and Pertussis Vaccines page.
• To watch a video clip about why vaccines are still necessary, visit the Educational Videos and DVDs page, select "Vaccines: Separating Fact from Fear," and watch the three-part response to "Are vaccinations necessary?".

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Are children too young to get vaccines?

No. If children are not too young to be permanently harmed or killed by viruses or bacteria, they aren't too young to be vaccinated to prevent those diseases. Because the diseases that vaccines prevent often occur in very young infants, the only way to prevent them is to give vaccines soon after birth. Fortunately, infants given vaccines in the first few months of life are quite capable of making a protective immune response.

To watch a video clip about whether children are too young to get vaccines, go to the "Vaccines and Your Baby" video on the Educational Videos and DVDs page and select "Can babies handle vaccines so young?".

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Can children manage so many different vaccines at the same time?

A: The mother's womb is essentially a sterile environment. The fluid surrounding the baby is free from bacteria. However, within minutes of leaving the womb, the child must confront thousands of bacteria. By the end of the first week of life, the child's skin, nose, throat and intestines are covered with tens of thousands of different bacteria.

Fortunately, from the moment of birth, infants begin to develop an active immune response to these bacteria — an immune response that prevents these bacteria from entering the bloodstream and causing harm.

The 11 or 12 vaccines that children receive in the first two years of life are just a drop in the ocean when compared to the tens of thousands of environmental challenges that babies successfully manage every day.

To watch a video clip about getting multiple vaccines at one time, go to the video, "Vaccines: Separating Facts from Fear" on the Educational Videos and DVDs page and select "Can my baby get 4 or 5 vaccines at a time?".

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Why are some vaccines given as a single dose and others as multiple doses?

Vaccines may be given multiple times for a few reasons:

• Some vaccines are given more than once so that a greater number of people can be protected against the disease. For example, when adults get the chickenpox vaccine, 78 of 100 will be protected after one dose, but that number rises to 99 of 100 after the second dose. Since the additional dose is not harmful and provides extra protection even in first-dose responders, everyone is recommended to get the second dose.
• Some vaccines provide a low level of protection after a single dose, but additional doses provide a greater response, so that individuals are better protected. This is true of the vaccine for Haemophilus infleunzae type b or Hib.
• Another reason for multiple doses is that immune responses last longer after more than one dose of some vaccines. The pertussis vaccine is an example of this.
• Some vaccines protect against agents that change often enough that older versions would not offer protection. The influenza vaccine is an extreme example of this, requiring new versions every year.

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Reviewed by: Paul A. Offit, MD
Date: March 2012