Some people think that vaccines are given without regard to individual differences. These concerns often focus on the idea that the same dose of vaccine may be given to an infant and an adult. But, vaccines are not the same as medicines. Vaccines cause an immune response in the area where they enter the body. Medicines, on the other hand, act systemically, meaning throughout the body.

In a few cases, different doses are used. For example, Tdap vaccine is given to teens and adults and the DTaP vaccine is given to infants. Both protect against the same three bacterial infections: diphtheria, tetanus and pertussis. But teens and adults may experience swelling of the arm after getting numerous doses of the DTaP vaccine. So, Tdap contains lesser quantities of diphtheria and pertussis vaccines to lessen the occurrence of this side effect. 

The influenza vaccine is another example. In this case, older adults require a higher dose to be protected because the immune system doesn’t respond as well with age.

Also, some people take drugs that suppress the immune system for cancer, steroids for asthma, or biologicals for chronic diseases. Specific vaccine recommendations are made for each of these groups.

The good news is that different groups are considered on the vaccine schedule, so “one-size-fits-all” does not apply.

Read more about the recommended immunization schedule.

Vaccines are licensed by the Food and Drug Administration (FDA) before they are given to people in the community. Once licensed, experts use multiple types of information to make the most informed decision about who should get the vaccine. They look at the large amount of data from vaccine trials, which answer the following questions:

• Does the vaccine work?
• How many doses are needed?
• Is the vaccine safe?
• And much more

They also look at what is known about the disease, such as:

• Where is the disease occurring?
• Who is getting sick or dying?
• How many people are sick?
• How is the disease spreading?
• And more

All of this information helps determine whether the vaccine will be added to the recommended immunization schedule, who should get it, and when.

• Why it might be a red flag if a doctor makes their own immunization schedule.

Sometimes more than one dose of a vaccine is needed. This can happen for a few reasons:

• Some vaccines require more than one dose to cause immunity that is strong enough to be protective. Common examples are vaccines made using toxoids, (e.g., diphtheria, tetanus and pertussis vaccines), polysaccharides (e.g., pneumococcal vaccine), or individual proteins (e.g., hepatitis B vaccine).
• More than one dose of vaccine may be needed due to a weakening immune system. One example is aging adults.

An extra dose of vaccine is similar to being exposed to the virus or bacteria in nature. Whether exposure occurs in nature or by vaccination, the child's immune system will respond in the same way. It will recognize the virus or bacteria as "foreign" and make an immune response to prevent it.

A common concern is whether a child who is behind on vaccines needs to start over. The good news is that usually if previous doses of vaccines can be verified, vaccination can pick up right where it left off.

Verification of vaccine history may include:

• Inclusion in a registry
• A medical record
• An immunization record

Sometimes, old day care or school forms will have a record of vaccines. Baby books or other documents from childhood may also be helpful.

People can become immune to a pathogen in two ways. They can suffer the illness or they can be vaccinated.

• Illness — Most people who acquire natural infections, that is infections that result from exposure to pathogens in their communities, suffer illness and recover. But, a small number get very sick, causing them to be hospitalized or die. For many infections, people who have recovered will be protected against getting the disease again.
• Vaccination — When people are vaccinated, they will develop immunity in a more controlled manner because the date, time and quantity of the exposure is predetermined. Most importantly, the quantity of exposure is limited so they will develop immunity, but they will not suffer severe symptoms or complications that may occur during illness.

Some vaccines cause a greater immune response than that which occurs following natural infection. Some examples include HPV, tetanus, Hib and pneumococcal vaccines. So even if you were infected with one of these diseases, the vaccines are still recommended.

Is natural infection better than vaccination?

Some people worry because children get more vaccines today than they did in the past. By 2 years of age, babies can get:

• Vaccines that protect against 16 different diseases (including an antibody preparation [immunization] against respiratory syncytial virus or RSV)
• As many as six shots in one visit
• A total of 29 doses of vaccines or immunizations

But, scientific advances have reduced the number of immunogens in vaccines. Immunogens are parts of a vaccine that induce an immune response. This means that even though babies get more vaccines today than in the 1900s, their immune systems do not need to respond to as many different components.

Number of vaccines and immunogens by year:

1900

• Number of vaccines: 1
• Number of immunogens: ~200

1980

• Number of vaccines: 7
• Number of immunogens: ~3,000

2025

• Number of vaccines: 15
• Number of immunogens: ~160

Resources

• Do babies get too many vaccines?
• Read more about concerns related to too many vaccines.

A recommendation is medical advice about who should get a vaccine. Recommendations are made by groups of experts, including doctors, scientists, public health advisors and others who review and discuss all of the studies that have been done related to particular vaccines. They make decisions based on what is best for the health of individuals and the community. Recommendations are shown on the vaccine schedule.

A requirement, or mandate, is a law about who must get a vaccine. Requirements are decided by state and local governments or in some cases, by employers. These laws describe which vaccines are required in order for a child to go to school or for an adult to work at a certain company. Requirements consider medical, political, economic and other factors. Requirements vary from state to state.

• Find out more about different kinds of recommendations.
• Read more about “Vaccine Science: Licensure, Recommendations and Requirements.”

Some parents concerned about vaccine safety wonder if separating, spacing out or withholding vaccines might make vaccines safer. What they do not realize is that the choice to choose your own vaccine schedule also comes with a certain amount of risk:

• Many vaccine-preventable diseases are still common. So, delaying or withholding vaccines increases the time during which the child is susceptible to diseases that can cause harm.
• Spacing out vaccines requires more doctors' visits. Extra visits increase both the risk for mistakes and stress for the child. This choice may also increase the cost of care, if separate office visits are billed.
• Alternative schedules have not been tested for safety like the AAP recommended schedule. The AAP recommended schedule, on the other hand, stands on a mountain of evidence.

Resources

• What is the harm in delaying or spacing out vaccines?
• Read more about too many vaccines.
• Why it might be a red flag if a doctor makes their own immunization schedule.

It is understandable why parents might want to delay vaccines when their children are sick. But in some cases, children can still get vaccinated, particularly if they have minimal symptoms and seem to be feeling OK.

Mild illness

Children with mild illnesses can typically be vaccinated. Mild illnesses can include:

• Fever
• Diarrhea
• Ear infections
• Upper respiratory infections

Studies have shown that when children with mild illnesses are vaccinated:

• They are not at greater risk of experiencing side effects than healthy children who get vaccinated.
• The level of protection provided by the immunization is equivalent to that of healthy children.

Severe illness

Children with severe illnesses, like meningitis or pneumonia, should not be vaccinated. This way, any symptoms of the infection will not be confused for a vaccine reaction. This allows healthcare providers to:

• Better understand and diagnose the child’s illness
• Rule out the possibility of a symptom being a side effect of vaccination

Read more about fevers.

Some people wonder if babies get too many vaccines at one time. This concern centers on two questions:

1. Can a baby’s immune system respond to so many vaccines at once?
2. Has the current vaccine schedule been adequately tested?

Responding to multiple vaccines at one time

Babies come into contact with germs regularly when they touch things, when they eat and drink, and even when they breathe. So, their immune systems are always busy protecting them. Each vaccine contains only a few parts that will cause an immune response, called immunogens. For that reason, even when babies get multiple vaccines at one visit, the number of immunogens is very small compared with what their immune systems encounter and manage every day.

The immune system has developed the ability to recognize many immunogens at one time in order to survive. Some scientists have figured out how many immunogens people could respond to at a time. One of those scientists is Susumu Tonegawa, who won a Nobel Prize for his work. Tonegawa found that the immune system could make about 10 billion different antibodies. Each different antibody can be directed against one part of the immunogen.

Tonegawa’s findings and the number of immunogens in today's vaccines can be used to estimate the number of vaccines the immune system could handle. When done, it was found that a baby's immune system could theoretically respond to about 10,000 vaccines at one time. So, the vaccines recommended today are not too much for a baby’s immune system to handle.

Adequate testing of the schedule

Before new vaccines can be added to the schedule, they must be tested with any other vaccines given at the same time. These are called “concomitant use studies." These studies test how well existing and new vaccines work and how safe they are when they are given together. The new vaccine can only be added to the schedule if these studies show that it is OK to do so.

Each year, millions of children safely receive vaccines according to the schedule.

Read more about too many vaccines.

Vaccines continue to be studied after they are licensed. For example, two systems that continue to monitor vaccines are:

• Vaccine Adverse Event Reporting System (VAERS) — VAERS reports involve any possible side effect that followed receipt of a vaccine, even if it wasn’t necessarily caused by the vaccine. Reports to VAERS can be submitted by anyone who sees an unexpected side effect following a vaccine.
• The reports are sent to a program co-directed by the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC). While VAERS can detect a potential issue, it is not designed to determine whether the concern was actually caused by the vaccine.
• Vaccine Safety Datalink (VSD) — In contrast, the VSD program is set up to determine if a problem is caused by a vaccine. Scientists compare people who got the vaccine with those who didn’t. If people who got the vaccine experience the side effect more often than those who did not, this offers evidence that the vaccine is likely causing the side effect.

Reports to VAERS can be submitted by anyone who sees an unexpected side effect following a vaccine. Parents should review the Vaccine Information Statements (VIS) given during their child’s vaccine visit. These sheets provide parents with information about:

• Who should or shouldn’t receive a vaccine
• Possible side effects
• What to do if a side effect occurs

Resources

• How are vaccines tested before use?
• How do you know whether a vaccine has been tested long enough?
• Monitoring vaccines after they are in use

Sometimes people wonder why vaccines are still necessary if the diseases aren’t common. Vaccines are still necessary for three reasons:

• Some diseases are still common.
• Some diseases could easily come back.
• Some diseases are common in other parts of the world, so they could easily enter the country.  

Resources

• Why do we still need vaccines?
• Get answers to this and other common questions about childhood vaccines.: English | Spanish | Japanese (PDF)