Vaccine Education Center

Questions and Answers About the Vaccine Schedule

The recommended immunization schedule is endorsed by three agencies: the Centers for Disease Control and Prevention (CDC), the American Academy of Pediatrics (AAP) and the American Academy of Family Physicians (AAFP). The endorsement of these agencies is based upon recommendations of groups of scientists, physicians and public health officials. These experts review information related to two main areas of interest:

• Vaccines – Vaccines must be evaluated for how they work (efficacy) and whether they are safe (safety). If they are given with other vaccines, they must also be analyzed to make sure neither interferes with the other; these studies are known as “concomitant use studies." Learn more about how vaccines are made.
• Diseases – Known as “epidemiologic studies,” studies of diseases seek answers to questions such as:
  • Who gets this disease? (susceptibility)
  • When do people get this disease? (seasonality)
  • How many people does this disease affect? (morbidity)
  • How many people die from this disease? (mortality)

By understanding all of these factors, experts can confidently compose an immunization recommendation that is safe and well tested.

How can a single schedule be appropriate for all children?

Although there is only a single recommended schedule, the recommendations incorporate specific information for sub-groups of the population. The schedule informs providers that some people should or should not receive a vaccine based on factors related to:

• Age
• Illness or temporary health conditions
• Allergies
• Chronic health conditions
• Occupation or lifestyle

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.

• Vaccine dosing versus medication dosing: Whereas 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. The purpose of a vaccine is to introduce the smallest amount of a virus or bacteria to cause that memory to develop without introducing enough to make a person sick. It takes about a week 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, the shingles vaccine given to adults contains about 14 times more vaccine virus than the chickenpox vaccine given to children, even though both vaccines are made from the same version of the varicella virus. 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.

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 amount 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 higher and the number of doses to maintain immunity, lower. In contrast, vaccines made from single proteins, polysachharides, or toxoids 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.
• 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 when we are born, as long as there are no genetic problems with our immune system, it begins protecting us from pathogens in the environment almost immediately. 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 is an example because the virus that causes shingles in adults is the same one that causes chickenpox in children. Most of us had chickenpox as children, but what we may not have realized is that even though we got better, some of the virus remained silently in our nervous system. We don’t often 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 virus may reawaken. The result is a bout with shingles.

Wouldn’t it be better to get diseases naturally?

If you could see the world from the perspective of your immune system, you would realize that where the virus or bacteria comes from is irrelevant. Your immune system “sees” something that is foreign, attacks it, disables it and then adds it to the memory bank so it can react more quickly the next time it encounters it.

The differences between a vaccine and getting the disease naturally are the dose and the known time of exposure.

• Dose - When someone is exposed to viruses or bacteria naturally, the dose is often larger, so the immune response that develops will typically be greater (as will the symptoms). However, when scientists are designing vaccines, they determine the smallest amount of virus or bacteria needed to generate a protective immunologic response.
• Time of exposure – Most of the time, we do not know when we are exposed to viruses and bacteria; however, when we take our children to the doctor’s office for a vaccine, we do know. In essence, we are controlling their exposure to the viruses or bacteria that the vaccines protect against because we know when and where they occur. In contrast, and more typical of the norm, we don’t know what viruses or bacteria they might be exposed to from the door knob to the office, the books in the waiting room, or the toddler at the restaurant we go to after the office visit.

So, in summary, a vaccine affords us protection with lesser quantities of virus or bacteria and the control of scheduling the exposure.

When is it okay to change the vaccine schedule?

Children with certain health conditions or illnesses may need to get vaccines at slightly different times than their peers. Healthcare providers refer to these changes as precautions and contraindications:

• Precautions - reasons to delay getting a vaccine. These can include recent blood transfusion, moderate or severe illness, or recent, uncontrolled seizures.
• Contraindications – reasons not to get a vaccine. These can include undergoing chemotherapy or having previous allergic reactions to the vaccine.

If you are concerned about a condition that might be a reason to delay or withhold vaccines, talk to your healthcare provider or contact your local health department.

Why does the vaccine schedule change?

Changes to the immunization schedule can be the result of new (e.g., HPV vaccine for girls) or improved vaccines (acellular pertussis vaccine), changes to the group of people most susceptible to the disease (hepatitis A vaccine), changes to the virus (H1N1 influenza vaccine), or new data about vaccine efficacy (HPV vaccine for boys).

Scientists and public health officials are dedicated to understanding and monitoring diseases and the vaccines being used to prevent them. As a result, the immunization schedule is updated regularly. For this reason, it is a good habit to check with your healthcare provider at every visit about whether you or your family members should get any vaccines.

What is the “catch-up” schedule?

If doses of vaccine need to be delayed or are missed, the catch-up schedule gives healthcare providers the information necessary to safely get a child up to date. Vaccines may have been missed due to vaccine shortages, new or revised vaccine recommendations, or changes in health status that no longer make previous contraindications or precautions necessary. A good habit to develop is to check with your healthcare provider during every visit whether any vaccines can be given.

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