Vaccine Safety: Dosing Safety

Vaccine doses

How can a baby get the same dose of a vaccine as an older child or adult?

Because vaccines do not work like medications, in many cases the same vaccine dose can be given to different age groups; however, in some cases, different versions of vaccines are available for different age groups.

While medications work via distribution through the bloodstream, the immune response generated by vaccination typically occurs at or near the injection site. When the immune system recognizes a vaccine component as “foreign,” cells of the immune system flock to the area and activate the immune system to respond to the invader. Once the immune response has succeeded in protecting the body from the invader, a small number of cells that would recognize the invader remain and circulate throughout the bloodstream; these are known as memory cells. Find out more about memory cells and other cells of the immune system on the “Parts of the immune system” page.

The purpose of a vaccine is to introduce the smallest amount of antigens, the components of viruses or bacteria that cause illness, to allow a memory immune response to develop without making a person sick. It takes about seven to 10 days after getting a vaccine for this immunologic memory to develop. That is why when you are planning a trip, you are recommended to see your doctor or a travel clinic well in advance of the trip.

In some cases, children and adults need different quantities of a vaccine because our immune system weakens with age. For example, an old version of the shingles vaccine given to adults, called Zostavax®, contained about 14 times more vaccine virus than the chickenpox vaccine given to children, even though both vaccines were made from the same version of the varicella virus. Zostavax has been replaced with Shingrix®, which provides better immunity with the help of adjuvants. Similarly, the adult version of the hepatitis A vaccine contains higher doses than the one for children. In contrast, the Tdap vaccine used in adolescents, teens and adults contains lesser quantities of the diphtheria and pertussis antigens to reduce the chance for swelling of the arm that was common in older children and adults after receiving several doses of DTaP.

Are extra doses of vaccines harmful?

No. An extra dose of any vaccine is similar to being exposed to the virus or bacteria in nature; the difference is that with a dose of vaccine, you know that the exposure happened. If your grandson was exposed to Haemophilus influenzae type b (Hib) in the community, you would not necessarily know about it, but whether he was exposed in the community or by vaccination, his immune system will respond in the same way — it will recognize Hib as “foreign” and make an immune response to prevent disease.

Why are several doses of some vaccines needed?

The number of necessary doses of a particular vaccine depends on many factors including the type of vaccine, the burden of disease in the community, and how the immune system works:

Type of vaccine 

Vaccines made from live, weakened viruses work by reproducing at low levels in the body. As a result, the immune response that is generated tends to be stronger and the number of doses to maintain immunity, fewer. In contrast, vaccines made from single proteins, polysaccharides (a sugar coating found on some bacteria), or toxoids (inactivated bacterial toxins) will only generate antibodies against those parts that were in the vaccine, and because replication does not occur, a less robust and less diverse memory response will result.

Disease in the community 

The amount of disease spreading around a community is directly related to how many people in the community are susceptible. Think about if you were to go to two parties. At the first party, you hardly know anyone and the people you meet do not share your interests; however, at the second party, you know almost every one and they like the same things as you. Which party would you most likely stay at longer? The same is true for viruses and bacteria; the more places they can find to “take up residence” in a community, the more likely they are to stay and spread. 

The amount of disease in a community may change in two ways that can lead to additional doses of a vaccine being necessary for individuals:

  • First, over time, as more people are vaccinated, the amount of disease circulating in the community may decrease. This can result in people being exposed less frequently and, therefore, needing an additional dose of vaccine to maintain immunity. An example of this is the chickenpox vaccine. When it was first introduced, only infants received a dose, but later, a second dose was recommended before starting school to “remind” the immune system of the chickenpox virus.
  • The second scenario during which additional doses of vaccine may be needed is during an outbreak. In this situation, if the infection is spreading through the community, people who were vaccinated several years earlier may need a booster dose. An example of this is adults who may be recommended to get another dose of mumps vaccine if mumps is circulating in their community.

How the immune system works

We are all aware of certain physical indicators of age — weight, wrinkles, even energy levels — but not everyone thinks about the fact that our immune system is also aging. Many people wonder if an infant’s immune system is capable of responding to the immunologic components in vaccines. The fact is that our immune systems begin to protect us from potentially harmful pathogens in the environment almost immediately after we’re born. In contrast, as we age, our immune system is not as robust, and we become more susceptible to diseases we may not have had a problem defending against when we were younger.

Shingles, which is a reawakening of a previous chickenpox infection, is one example. Most of us had chickenpox as children, but what we may not have realized is that even though we got better, some of the chickenpox virus remained silently in our nervous system. We don’t think about it because our immune system keeps the virus in check. However, if our immunity gets low because of illness, stress or age, the chickenpox virus might reawaken. The result is a bout with shingles.

Side effects

Is it true that side effects indicate that a baby’s body is reacting poorly to vaccines?

It is true that side effects are a reaction to a vaccine; however, they do not indicate that the immune system is reacting “poorly.” Quite the opposite, systemic side effects like fever indicate that the child's immune system is responding to the vaccine. While it is reasonable to think that a fever is bad for a child, it actually helps the immune system work more efficiently. The problem with fevers occurs when they rise too quickly, sometimes leading to febrile seizures which, although frightening to see, are benign.

In the case of fevers that result from vaccination, they tend to be what are considered “low-grade” fevers, so the concerns related to fevers from natural infections do not apply. The local side effects, such as pain, redness or swelling near the injection site, are responses to the physical disruption of muscles near the injection or the local immune response when white blood cells penetrate the area to respond to the “invader.”

Find out more about the immune system and how it works.

Alternative to shots

Is there another way to give vaccines besides giving them as shots?

The manner in which a vaccine is administered often takes into account how to generate the most robust immune response. Consider these examples:

  • Rotavirus vaccine – The main reason the rotavirus vaccine is given by mouth is because it protects against an intestinal infection and the best way to induce intestinal immunity is by presenting vaccine viruses to the intestinal surface.
  • 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.

Because many viruses and bacteria cause damage by first entering the bloodstream, the best way to fight off many infections is to vaccinate in a manner to generate antibodies that will be present in the blood.

Necessity of vaccines

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 possible for the disease to come to the U.S. 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.

Learn why vaccines are still necessary by watching this video.

Watch as Dr. Offit talks about the continued need for vaccines in this short video, part of the Talking About Vaccines with Dr. Paul Offit video series.


Reviewed by Paul A. Offit, MD on December 18, 2019

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.