Why Should My Baby Get So Many Vaccines before 24 Months of Age?

New parents have barely had a chance to “meet” their baby before they are asked to permit that baby to get a hepatitis B vaccine. Then, often while still on maternity leave, a slew more — up to six doses at 8 weeks of age. By the baby’s first birthday, more than 20 doses. Many of these doses are to protect against diseases parents have never had, or even known others who have. It is no wonder some parents balk at these requests, or at least ask, “Why should we consent to this?”

Understanding why vaccines are given so early in life and why so many doses of some vaccines are necessary can help make sense of this important parental decision. At the heart of both of these questions is the immunization schedule, so let’s take a closer look.


Hepatitis B

Hepatitis B vaccine is recommended for most babies within 24 hours of birth. Because hepatitis B is often considered to be a sexually transmitted disease, parents wonder why a newborn needs it. Three factors make hepatitis B a risk for anyone, including newborns:

  1. Hepatitis B is transmitted through blood. When people are infected with hepatitis B, they have up to 1 billion infectious virus particles in one-fifth of a teaspoon of their blood. This large quantity of the virus means that even a very small quantity of blood — less than can be seen with the naked eye — can result in transmission. In fact, hepatitis B is significantly more easily transmitted than HIV.
  2. Hepatitis B virus is hardy, meaning that it can survive outside of the body on harsh surfaces, or those not typically considered “friendly” to viruses, such as tables, clothing or toothbrushes, for up to seven days. So people, including babies, can be exposed to the virus if invisible amounts of blood from someone infected with hepatitis B remains on surfaces or items, such as washcloths.
  3. Many people do not realize they are infected, so it is difficult to protect children from exposure when you don’t know that they are being exposed.

Before widespread vaccination, every year about half of the children younger than 10 years old who were infected with hepatitis B were known to be infected during birth because their moms were infected. Unfortunately, the other half, about 9,000 children every year in the U.S., were infected in some other way. Many never figured out how or when.

Vaccines at two-, four-, and six-month visits

The bulk of vaccine doses during the first two years of life are given during these three visits. In addition to one or two more doses of hepatitis B vaccine, infants receive vaccines to protect against seven other diseases — rotavirus, diphtheria, tetanus, pertussis, pneumococcus, Haemophilus influenzae type b, and polio. So, why are these vaccines given in multiple doses at these ages?


Rotavirus vaccine is the only vaccine routinely recommended in the U.S. that is given by mouth. This vaccine is best started by the time an infant is 15 weeks old. The reasons for the timing of this vaccine is that rotavirus is most likely to infect young children, particularly infants. Because it can quickly lead to dehydration, it can result in a medical emergency, so it is better to prevent this infection if possible.

Diphtheria, tetanus, and pertussis

These vaccines have been given in combination since the late 1940s. The version used for babies is referred to as DTaP. It made sense to put these vaccines together, reducing three shots to one, because they are made in the same way, and they protect against these diseases in a similar way.

Diphtheria, tetanus, and pertussis are each caused by bacteria that make people sick by producing harmful proteins, called toxins. These toxins act like poisons, causing illness. By using inactivated toxins, called toxoids, as the vaccine, people develop antibodies that protect them if they are infected.

Of these three diseases, a baby is most likely to be exposed to pertussis, and pertussis is also the most dangerous because it causes a narrowing of the windpipes that makes it difficult for babies to get enough oxygen when they experience repeated bouts of coughing. Pertussis tends to be under-diagnosed in older children and adults, who frequently transmit it to babies. Unfortunately, of these three vaccines, pertussis is also the one that is least effective. Nonetheless, those immunized with pertussis vaccine are seven times less likely to be infected during an outbreak than those who aren’t immunized.

In addition to the three doses of DTaP vaccine at 2, 4 and 6 months of age, young children receive two additional doses before starting school. The first three doses all contribute to the development of primary immunity, meaning a baby needs all three of these before parents can feel confident that the baby is likely to be protected against these diseases. The latter two doses are considered booster doses, meaning they remind the immune system to recognize the toxins, thereby strengthening immunologic memory.

Haemophilus influenzae type b and pneumococcus

Although these vaccines are not given together, both protect against bacteria that can make children younger than 2 years of age very sick with meningitis (an inflammation of the brain), pneumonia, or bloodstream infections. Young babies have a limited ability to make antibody responses to bacteria like Haemophilus influenzae type b (Hib), pneumococcus, and meningococcus.

Hib and pneumococcus were the most frequent causes of severe illness in young children before vaccines were available. Because of the limitations of making an immune response against them, vaccine technology had to be developed that could overcome this. Researchers found a solution by adding a harmless protein, called a conjugate protein, to the parts of the bacteria that cause disease. Healthcare providers were ecstatic when they had a way to protect babies from the illnesses caused by these two diseases.

Because meningococcus does not occur as frequently, only babies considered to be at highest risk typically receive a meningococcal vaccine in infancy.


Although polio has almost been eradicated, or eliminated from the entire world, it still occurs in a small number of countries. The disease is highly contagious, and while most recover, not all do so without consequence. Before a vaccine was available, every year, some children were paralyzed following polio infections. Parents in the 1950s were so scared of their children getting polio during the summer that they would keep them from swimming in pools, going to church, etc.

Because we don’t see polio in the U.S., it would be easy for a parent to consider forgoing this vaccine. But polio is highly contagious and could easily re-emerge if immunization rates decrease. In fact, in September 2019, the first case of polio in the Philippines in nearly two decades was announced. Unfortunately, this development is a reminder of how quickly polio could re-emerge if immunization rates are not high enough.

Vaccines at 12- to 15-month visits

In addition to third or fourth doses of several of the vaccines mentioned above, babies receive vaccines to protect them against five additional diseases around 1 year of age — measles, mumps, rubella, chickenpox, and hepatitis A.

Live, weakened viral vaccines: MMR and varicella

Four of the five vaccines given at this age are live, weakened viral vaccines, including measles, mumps and rubella in the MMR vaccine, and varicella, more commonly known as chickenpox. This means that immunity is the result of the vaccine virus replicating after the vaccine is given. Because the vaccine virus has been grown in the laboratory, it does not replicate as efficiently. The result is development of a robust immune response without actually being ill.

Because these vaccines rely on viral replication, the timing for their receipt has been carefully determined. Like threading a needle, public health officials have to, on one hand, protect babies before they are likely to be exposed, while on the other hand, delay vaccination until maternal antibodies are less likely to interfere with the development of immunity. This balance is one of the reasons healthcare providers were so scared during the recent measles outbreaks. They understand just how vulnerable their patients less than 1 year old are. Measles is one of — if not the most — contagious of infectious diseases, making it very adept at finding the non-immune among us.

Of the viral infections, chickenpox presents its own conundrum. While this virus was never the killer that measles was, it still killed about 50 children every year in the U.S. before a vaccine was developed. While widespread vaccination can help stem transmission of this disease, the reality is that most adults have the virus living in nerve cells, having had chickenpox as children. The result is that the virus has a natural reservoir for survival and can re-emerge at any point. The good news is that if non-immune people avoid touching the sores, they are less likely to become ill. Further, with the advent of effective shingles vaccines, fewer people will experience flare-ups. But, the disease can still spread; we just don’t know for whom it will be severe; and if infected as an adult, illness is likely to be worse. For these reasons, getting the chickenpox vaccine before a child can be exposed is important.

Experience has also shown that people who received the chickenpox vaccine are less likely to develop shingles as adults. And if they do, their cases are less severe because the virus that is reactivating is the vaccine strain, which is less damaging.

Hepatitis A

This infection is similar to hepatitis B in that it infects the liver, but it differs in how it is transmitted. Specifically, hepatitis A virus is transmitted primarily through contaminated food and water. Historically in the U.S., the virus was most often associated with contaminated food or food handlers, such as at restaurants. More recently, the virus has been spreading from person to person among those who are homeless and those abusing drugs, specifically as a result of the opioid epidemic.

Parents may wonder, then, why their babies should get this vaccine. Unfortunately, every year before widespread vaccination, children were infected with this virus and often did not know when or from where. Further, when babies and young children are infected with hepatitis A, they often do not have symptoms. While that may seem like a good thing, it is not ideal for a few reasons. From an individual standpoint, families do not know the child is infected, so the child can be chronically infected with the result being liver disease or liver cancer years later. From a public health standpoint, not knowing the child is infected means that they can unwittingly spread the disease to others.

Influenza vaccine

Depending on when a baby is born, the influenza vaccine may be offered at the six-month or one-year visit. The first time a baby gets this vaccine, two doses, separated by four weeks, are necessary to allow the immune system to generate protective immunity. Like virtually the rest of the population, the baby will then go on to get a single dose every year before the start of influenza season. Although the vaccine is not always a perfect match to circulating strains of the virus, some immunity is better than none.

Influenza is a respiratory illness, and every year, babies and young children are hospitalized with influenza. Sadly, every year, some die from their infections. One of the most important points about influenza is that when it infects a person, it makes the respiratory tract particularly susceptible to other potentially severe infections. For this reason, vaccinating against influenza may also indirectly protect against secondary respiratory illnesses as well.

In summary

When babies are born, they emerge from the sterility of the womb into a world fraught with germs. As parents, we do our best to keep them in clean and safe environments, but the reality is that, like anyone at any age, they are constantly exposed to agents that could make them sick. Most often, their immune systems are effective at staving off illness, but not always.

Before vaccines, parents wanting to do the best for their children clamored for ways to protect them from diseases that could leave them disabled, or worse. Healthcare providers sought ways to prevent, rather than treat, those illnesses after spending their careers watching children succumb to infections that they had limited tools to fight. Public health officials watched as waves of illnesses spread through the communities they were meant to protect. Vaccines changed all of that.

Today, we don’t see the diseases, so we may be lulled into the comfortable notion that they are not present. But, they are still present, and they wait for opportunities to make themselves known. The vaccines we have protect against the worst of these. And while the immunization schedule can seem daunting, it is set up as it is quite intentionally — to position babies against potential attacks. Knowing this, maybe the question should be, “How can I not give my baby all doses of these vaccines before 24 months of age?”


Read the full article from CHOP here.