The immunization schedule is dynamic, meaning it changes over time as we learn more and as new vaccines are developed. For parents, the immunization schedule can seem overwhelming, even confusing, because infants get several vaccines, often in the form of multiple doses, during the first few years of life.

You can think of the schedule like a map. Whereas a map helps people navigate a new area, the immunization schedule helps people navigate vaccine recommendations. As different versions of a map contain varied levels of detail based on their purpose, so do different visualizations of the schedule. For example, sometimes, parents are offered simplified versions of the recommended schedule to make navigating vaccinations during the first few years of life easier. Often, parents don’t see a version of the schedule as their children get older, and adults may not even realize a version exists for them. While simplified versions of the schedule or verbal descriptions of which vaccines are due at an appointment may make it easier for families to make vaccine decisions on a given day, these approaches don’t tell the whole story, sometimes leaving people to wonder whether there is any rhyme or reason to the schedule at all.

On this page, we take a deeper dive into how science comes together to inform the immunization schedule. 

Characteristics of pathogens inform the schedule’s design

When a map is made, the characteristics of the area inform its design. Are there mountains? Are there roads? What about buildings? 

In the same manner, the immunization schedule is based on characteristics of pathogens. Who is most susceptible to the disease? When do people get sick? Where are people getting infected? This understanding of the pathogen is derived from epidemiologic studies. If we look at different age groups, infants tend to be the most susceptible to many diseases because, unlike older children and adults, they haven’t been exposed to many pathogens. It’s like when someone starts a new job: In the beginning, they have lots of different tasks to learn because they know very little about the job, but over time, they have fewer tasks to learn because they have become more experienced. The same is true of our immune systems.

Vaccines give us a way to more quickly educate a baby’s immune system against several of the most dangerous pathogens early in life. Likewise, to be most effective, vaccination should occur before most recipients are exposed to the pathogen. This combination of increased risk during infancy and the need to protect before exposure explains why so many vaccines are given in the first few months of life. 

Some may wonder about protection offered by maternal antibodies delivered to the baby before birth (via the placenta) or in breast milk. While this “passive immunity,” meaning immunity generated by someone else’s immune system, is helpful and provides us with a window of time to vaccinate, three points are worth considering: 

1. Protection from passive immunity will be variable among babies because of things like the antibodies present in the maternal immune system, the gestational week of delivery (preterm babies tend to have lower levels of maternal antibodies), and more.
2. Protection from passive immunity will go away over time because it was not generated by the infant’s own immune system.
3. Some vaccines (like diphtheria, tetanus and pertussis) require multiple doses to become protective, so the window during which a baby is protected by maternal antibodies should be one during which we take advantage of the time available to build the baby’s own immunity or as the saying goes, we should “make hay while the sun shines.”

So, with all of this in mind, anticipated placement of the vaccine on the schedule will take several factors into account, such as who will be vaccinated and when. To inform decisions related to placement on the schedule, the vaccine will be tested in the target group at a time when they would be vaccinated (e.g., at a particular age). The vaccine will also be tested with any other vaccines that would be given at the same time. When new vaccines are tested with existing vaccines on the schedule, these are called concomitant use studies; these studies ensure both that the new vaccine works as anticipated when given with existing vaccines and vice versa.

Characteristics of individuals inform the schedule’s design

Like a simplified map does not relay all of the information about an area, neither does a simplified schedule. Sometimes, especially if they see a simplified schedule, people think vaccine recommendations are made in a “one-size-fits-all” approach, but several elements of the schedule demonstrate that this is not an accurate assessment:

Vaccines

Some vaccines are only given to children, while others are only given to adults. As described previously, vaccines are given in advance of increased risk. So, adolescents can’t get HPV vaccine before 9 years of age, and most adults can’t get shingles vaccine before 50 years of age.

Types of recommendations

Some recommendations are for “everyone,” such as the birth dose of hepatitis B vaccine or three doses of diphtheria-, tetanus-, pertussis-containing vaccines at 2 months, 4 months and 6 months of age. However, other recommendations are made for specific subgroups of a population, such as meningococcal vaccine at 2 months of age only for certain babies, such as those with sickle cell disease or HIV, among other conditions. Similarly, only subgroups of adults between 19 and 49 years of age are recommended to get the pneumococcal vaccine based on an increased risk of severe disease if infected (e.g., this list is long, but two examples include people who smoke and those who have chronic diseases involving the heart, liver or lungs). Other examples of subgroups that you might have come across include travelers or those who are pregnant. The schedules and recommendation publications describe this nuanced guidance. (See “Related resources” section for links.)

Exceptions

Even when “everyone” in a group is recommended to get a particular vaccine, the schedule provides guidance regarding exceptions, such as the precautions and contraindications described in the “Appendix” section of each schedule as well as in the “General Best Practices for Immunizations” guidance. (See more about the parts of the schedule and the guidance document lower on the page.)

Doses

Some vaccines are available in different doses for different groups. For example, the COVID-19 mRNA vaccine made by Pfizer comes in two doses based on age. While both are administered in a volume of 0.3 milliliters, the version for children between 6 months and 11 years of age has only one-tenth the dose of that given to those 12 and older (3 micrograms versus 30 micrograms). The COVID-19 mRNA vaccine made by Moderna also has two different doses for those 6 months to 11 years of age and those 12 years and older (25 micrograms versus 50 micrograms, respectively), but the doses are also contained in different volumes (0.25 milliliters for children and 0.5 milliliters for those 12 and older). Other examples include hepatitis B vaccines and influenza vaccines. In the case of influenza vaccines known as “Fluzone,” three different versions are available. Anyone 6 months and older can get 45 micrograms in 0.5 milliliters. However, there is a lower dose for infants between 6 and 35 months of age (22.5 micrograms in 0.25 milliliters), and you may have heard of the “high-dose” formulation for the elderly (those 65 years and older), which delivers 180 micrograms in 0.5 milliliter. These doses have all been tested in the age groups for which they are used.

What is important to realize when it comes to dosing is that vaccines are different than medications. Whereas medication dosing often relies on the physical body size to ensure that the medication is disseminated at appropriate quantities throughout the body, vaccines are processed by the immune system near the injection site and then the educated cells of the immune system travel throughout the body to monitor for future infections. This is why in the example above, “Fluzone” has one dose that can be given to anyone 6 months and older. 

The considerations related to dosing of vaccines are two-fold. First, it is necessary to determine the dose that will generate protective immunity without causing illness. To determine the best dose, early vaccine studies will often include different doses to figure out the least amount of antigen that produces protective immunity. In some cases, this amount does not vary between children and adults. The second consideration is the volume of the vaccine. Since many vaccines are given in the muscle, and babies are smaller, a lower volume is easier to administer (and more comfortable for the baby). While 1-milliliter dosing is often OK for adults, 0.25 or 0.5 milliliters is generally better for infants and children. In some cases, these smaller volumes are used for all age groups.

Recommendations inform the detail behind the schedule

To make maps, scientists, engineers and mathematicians generate and use data that are converted to scale and designed to accurately reflect a three-dimensional location into a two-dimensional document; this area of science is called cartography. 

In a similar manner, immunization schedules are designed to accurately reflect the recommendations for each vaccine. Recommendations are based on a wealth of data related to the pathogen, vaccine studies, and understanding of the recipients of each vaccine. These data are reviewed by work groups and members of the Advisory Committee on Immunization Practices (ACIP). When the ACIP votes to recommend a vaccine, the details are published in the Centers for Disease Control and Prevention’s (CDC’s) Morbidity and Mortality Weekly Report (MMWR). The complete set of recommendations for each vaccine is listed under “current recommendations” in the “Vaccine-Specific Recommendations” section of the CDC’s website. The vaccine-specific recommendations for each vaccine average about 34 pages in length and typically include two to four links to different issues of MMWR (as updates are made related to new information or versions of a vaccine). 

Archived recommendations are also often available, so healthcare providers (or others who are interested) can go back and review how recommendations evolved for different vaccines. 

The published guidance includes: 

• How the recommendations were developed
• Background information about the pathogen
• Descriptions of the vaccine or vaccines available to protect against it
• Details related to vaccine effectiveness and safety
• Detailed recommendations (including for specific subgroups)
• Precautions and contraindications (reasons people should delay or avoid the vaccine, respectively)
• References
• Committee members and any conflicts of interest
• Future directions

In addition to the vaccine-specific recommendations, another document also informs vaccine administration. It is called “General Best Practices for Immunization.” This document includes information related to:

• Timing and spacing of vaccines and related products
• Additional information about contraindications and precautions
• Preventing and managing adverse reactions
• Vaccine administration
• Storage and handling
• Altered immunocompetence
• Special situations
• Vaccination records
• Vaccination programs
• Glossary

This information adds another 75-80 printed pages of details related to the administration of vaccines.

Various components help with reading the schedule

In the same way that reading maps takes practice, so does reading the immunization schedule. The childhood immunization schedule used by healthcare providers is much more comprehensive than most simplified versions demonstrate. The comprehensive schedule includes several pages, tables and notes. The same is true of the adult immunization schedule. As an example, the 2025 childhood immunization schedule is 17 pages, and the adult schedule is 15 pages. To help with understanding them, maps typically include a legend, compass and scale. The comprehensive immunization schedules also have several different components to help in understanding them:

• List of vaccines, including the abbreviations used on the schedule and the trade names of available vaccines. The child schedule also includes the same information related to combination vaccines.
• Instructions for how to use the schedule most effectively.
• List of professional organizations who reviewed and approved the schedule (Nope, it’s not just the CDC.). On the 2025 childhood schedule, seven groups are listed, and for the 2025 adult schedule, eight groups are listed. Each organization that reviews the schedule represents groups of healthcare professionals who are involved in the vaccination of the individuals represented on the schedule (children or adults).
• Instructions for how to report outbreaks or adverse events, how to contact the CDC with questions, and where to access related resources with additional information.
• Table of vaccines recommended by age (Table 1 on both schedules). This table is also color coded to identify whether all individuals of that age are recommended to get the vaccine or only certain subgroups.
• Table for catch-up immunizations (Table 2; only on the children’s schedule).
• Table for recommended vaccines by medical indication (Table 2 on the adult schedule and Table 3 on the childhood schedule); this table also includes color coding.
• A general notes section (called “Additional information”) and notes sections for category of vaccine (e.g., measles, mumps and rubella or MMR, influenza [all types], etc.).
• A guide to contraindications and precautions by category of vaccine (“Appendix” on both schedules).
• An addendum listing any changes to the schedule since it was released.

You can find the current schedules used by healthcare providers on the CDC’s website: Childhood schedule | Adult schedule

Changes to the schedule indicate progress

When new roads or structures are built, maps need to be updated. Likewise, when new vaccines become available or recommendations change, the immunization schedule needs to be updated. In the same way that more roads or structures are viewed as progress by some, but not others, the same is true of vaccines. Those who view vaccine changes negatively tend to have concerns related to too many vaccines, exposure to increased vaccine ingredients, and more cynically, a schedule informed solely by profit motive. The data don’t support any of these concerns.

Too many vaccines

One of the most common concerns about the number of vaccines relates to whether vaccines overwhelm an infant’s immune system. However, we know that this is not the case. First, infants are exposed to bacteria from the moment of birth, becoming colonized with tens of thousands of different bacteria in the early days after birth, and they continue to be immunologically challenged constantly. While babies get some protection from maternal antibodies, and their exposure to potential pathogens can be limited by the number of people who visit with them, the reality is, their immune systems need to be ready to start protecting them from birth. As with all people, these 24/7 protection operations are like the heart pumping and the lungs breathing — they happen without us even thinking about them. The only time we really know that our immune systems aren’t perfect is when we get sick, but we never have a good sense of how much work is being done to protect us day in and day out. 

The second part of our understanding related to the number of vaccines comes from the vaccines themselves. Antigens are the parts of a vaccine that our immune systems respond to; most often these are parts of the pathogen (e.g., proteins and sugars) or modified toxins produced by the pathogen (called toxoids). The first vaccine was the smallpox vaccine; it contained about 200 antigens. By 1914, the pertussis vaccine had been developed; it was called a “whole-cell” vaccine because it contained a killed version of the bacteria that causes pertussis. As such, our bodies made immune responses to about 3,000 antigens when the vaccine was administered. Over time, we eradicated smallpox (eliminated it from the globe), and we learned more about pertussis, including which parts of the bacteria we needed to make a protective immune response. With improved lab techniques and this increased understanding, the acellular pertussis vaccine was created. It contains two to five antigens (depending on brand). As a result, by the 21st century, and even as this is written in 2025, all the vaccines given during the first two years of life contain fewer antigens than either the smallpox vaccine (200 antigens) or the whole-cell pertussis vaccine (about 3,000 antigens). Said another way, the antigenic challenge to an infant’s immune system by vaccination has significantly decreased over time.

Exposure to increased vaccine ingredients

Another source of concern about the increased number of vaccines relates to ingredients other than the antigens in vaccines. The non-antigenic components of vaccines can be sorted into four categories: adjuvants, stabilizers, preservatives and residual byproducts. Each has a particular role, and vaccines do not always contain all four categories of ingredients. You can learn more about specific vaccine ingredients and see the ingredients in each vaccine in the “Vaccine Ingredients” section of our website. 

Importantly, when thinking about vaccine ingredients, it is useful to remember a few rules of thumb:

• The same as any chemical, including water, the famous quote by Paracelsus, “The dose makes the poison” is relevant. Most of the ingredients in vaccines are minimal in quantity, so even if several vaccines are received on a single day, they are not too great for the person, even if it is a baby, to manage.
• Likewise, from a manufacturing standpoint, it does not benefit the producer to put more of an ingredient, like an adjuvant, stabilizer or preservative, than is necessary into a dose of vaccine because every ingredient increases the cost to make the vaccine.
• Conversely, while residual byproducts may be measurable, those quantities are typically negligible because over time, purification techniques have improved.
• In most cases, ingredients are present in microgram or nanogram quantities. To get a sense of these quantities, consider that a single raisin weighs about one gram. If you cut that one raisin into 1 million (1,000,000) equally sized pieces, one of the million pieces would be a microgram. If you cut a single raisin into a billion (1,000,000,000) equally sized pieces, one of the billion pieces would be a nanogram. Even if a vaccine contains 50 or 100 of those pieces in either micrograms or nanograms, the amount is extraordinarily small, and our bodies are well equipped to handle them.

Profit motive

Unfortunately, the increased number of vaccines over time is often associated with a profit motive purported to prioritize greed over health. While people seem to be OK with companies making money on other products, like cell phones, social media ads, and even dietary supplements, they view profits from vaccines as the driving force behind increased numbers of vaccinations over time. That framing, however, fails us in a few ways. First, it outsizes the profits on vaccines compared with other products. In 2023, the worldwide vaccine market was estimated to be $77 billion — while that sounds like a lot, it is less than Apple cell phones (estimated at almost $192 billion in 2023), less than Meta, the world’s largest social media company (almost $135 billion in 2023), and less than the global market for dietary supplements ($146 billion in 2023). For additional perspective, consider that this $77 billion was estimated to represent about 5% of the total global pharmaceutical revenue for that year. So, it may be fair to say that pharmaceutical companies are making a lot of money, but it is also fair to say that vaccines are not the products “keeping the lights on” at those companies. 

Second, the framing of greed over health also overlooks the real battle, which is the fight to survive against nature’s pathogens. Related to this, such framing fails to consider — or learn from — our collective historical experience. The costs of the battle against nature’s pathogens are paid by the whole of society, both physically and financially. Historically, people feared infectious diseases because their effects were a scary, but all too common, part of life. For example: 

• In Africa, parents didn’t name their children until the threat of measles had passed because so many children died of the disease early in life.
• Mirrors were not hung in the homes of smallpox survivors, so they would not see their permanently scarred faces.
• Children were not able to go swimming in the summer because it was unclear how and where they got polio.
• In the mid-1960s, so many children were born deaf because their mothers were infected with rubella during pregnancy that special classrooms and staff were developed to accommodate these students, known as part of the “rubella bulge,” as they moved through elementary and secondary school.
• Three times more people are estimated to have died from influenza during the 1918 influenza pandemic than from WWI (1914-1918).

The financial costs associated with infectious diseases are also considerable — typically greater than the costs associated with preventing them. For example, a modeling study by Carrico and colleagues considered the lifetime of the 2017 U.S. birth cohort with and without the availability of vaccines. They estimated that routinely recommended vaccinations would prevent more than 17 million cases of disease and 31,000 deaths. The relative costs of vaccination were $8.5 billion, and costs associated with cases of disease that occurred despite immunization (since vaccines are not 100% effective), lost productivity (due to infections, resulting disabilities and death), and vaccine side effects were $3.2 billion for a total of $11.7 billion. On the other hand, if no vaccines were available, the costs associated with disease (including healthcare and losses in productivity) were $66.8 billion, so overall the vaccination program netted a savings to society of $55.1 billion. While this study offers a single example, many other studies have demonstrated the health and financial savings that vaccines help societies to realize. 

In the big picture: The schedule and questions of chronic disease

The larger the area a map covers, the fewer details the map can reveal about what is happening in communities on the ground; it is not until a person gets to the location that they can see the whole picture. Similarly, when people look at the immunization schedule, they cannot realize the whole story either. Unfortunately, what some people focus on when looking at the schedule over time is that the number of vaccines has increased, and they suggest that because rates of chronic diseases have also increased, more vaccines may have led to the increase in chronic diseases. However, as discussed in the Parents PACK article, “Do Vaccines Cause Chronic Diseases?” just because two things happen coincidently in time does not mean they are causally related. This incorrect logic is known as a causal fallacy. 

Claiming that vaccines cause chronic diseases is easy to do and difficult to disprove because of the non-specific nature of the suggestion. Which diseases? Which vaccines? Some suggest a giant tent — all diseases and all vaccines, leading to calls for studying the entire vaccination schedule for increases in any or all diseases. This would be like drawing conclusions about a community while looking at it on a map or flying over it in an airplane. In the same way that learning the details about a community could not happen with this approach, a well-constructed scientific study could not reasonably test such a broad and sweeping hypothesis about the immunization schedule. Some suggest that a “vaccinated-unvaccinated” study could test this, but that type of study would be difficult, if not impossible:

• First, it would be unethical to randomly assign newborns to an unvaccinated group when we know that vaccines are lifesaving.
• Second, some say, let people self-select, but this would be problematic because there would likely be other differences in the health-seeking behaviors of people who opt in to a “vaccinated” group and those who opt in to an “unvaccinated” group. These differences would be “confounding” factors, meaning they would make it difficult to draw conclusions about the relationship between vaccines and chronic diseases.
• Third, we also have information about the causes of many chronic diseases. Sometimes genetics; environmental factors, including infections; or a combination of these can cause chronic diseases. For example, autism can result from genetics, paternal age or rubella infection during pregnancy, among other things. Arthritis can result from genetics, diet or Lyme infections, among other things. Type 2 diabetes can result from lifestyle choices, genetics or, possibly, COVID-19 infections. Although this latter relationship needs further exploration. The takeaway here is that since many chronic diseases have been linked with multiple potential causes, even if a difference was found between vaccinated and unvaccinated individuals, it would be difficult to conclude that vaccines were the cause.

In the same way that getting off the plane and exploring the community would help a person learn about different aspects of the community, scientists can evaluate more discrete questions about the schedule, like whether a vaccine or group of vaccines is related to a particular disease. That has been done for many, but not all, vaccine-condition pairs. For example, we know that MMR vaccine does not cause autism. We know that several vaccines given in the first year of life don’t cause asthma or allergies. We know that vaccines given in the first few months of life don’t cause sudden infant death syndrome (SIDS). 

Science can also study chronic diseases to learn more about their causes, prevention and treatment. Said another way, prevention of infectious diseases and chronic diseases are not mutually exclusive propositions. We can — and should — work toward a population that is not characterized by health effects from either infectious or chronic diseases. With that in mind, two final points are important to consider. First, studying vaccines as they relate to chronic diseases that have already been exhaustively studied is not a good use of limited resources. This would be like repeatedly going to the same restaurant in a town you are exploring. To use your time in the best way to learn more, you need to explore other places. Second, unless we learn new information that changes what we currently know, withholding vaccinations does not reduce the risk of developing a chronic disease. It simply leaves the individual at an increased risk for infectious diseases.

Final thoughts

Science is cumulative. One study (e.g., vaccinated-unvaccinated) will not provide all the answers in the same way that using only one map will limit what we can learn about an area. What makes the knowledge gained from science strong is that it doesn’t come from one single person or one single study. Studies done in different ways by different groups add pieces to the puzzle. When it comes to vaccines and the immunization schedule, tens of thousands of studies have contributed to what we know.

Related resources

• Current immunization schedules (webpage)
• ACIP recommendations by vaccine (webpage)
• General Best Practices for Immunization (webpage)
• Vaccine Science: Process of Vaccine Development (webpage)
• Vaccine Science: Licensure, Recommendations and Requirements (webpage)
• Vaccine Policymaking – Behind the Scenes with the ACIP Consumer Rep (videos)
• Immune System and Vaccines (webpage), part of website section “Human Immune System”
• Vaccine Ingredients (website section)
• Value of the Immunization Program for Children in the 2017 US Birth Cohort (modeling study)
• Vaccines and Other Conditions (website section)
• Do Vaccines Cause Chronic Diseases? (videos)
• Vaccine Safety References (webpage)

Reviewed by Paul A. Offit, MD, on April 1, 2025