Understanding the Vaccines on the Childhood Vaccination Schedule
Early vaccination can prevent severe disease, hospitalization, and death in children.
Most kids in the U.S.—93% of them—get a standard set of vaccinations as part of their childhood pediatrician visits. Perhaps the most well known protect against measles, mumps, and rubella (MMR); diphtheria, tetanus, acellular pertussis (DTaP); poliovirus; and varicella—diseases that can cause severe, potentially life-threatening, illness. Some of these vaccines are given just months after birth, while others are typically given at school age.
The timing depends on what will produce the greatest immune response, says Bill Moss, MD, MPH, a professor in Epidemiology and executive director of the International Vaccine Access Center. That’s just one of the factors that goes into the logic of the childhood vaccination schedule.
In this Q&A adapted from the March 11 episode of Public Health On Call, Moss discusses the different types of vaccines, the timing of the schedule, and how vaccinating kids helps protect the entire community.
This is part of a series of podcasts from Public Health On Call about vaccine basics from the molecular level to global policy.
What different types of vaccines do kids receive?
There are several basic classes:
- Live attenuated vaccines use a weakened version of the viral pathogen that causes the disease to induce a protective immune response against that virus. Examples of attenuated viral vaccines are the MMR vaccine and the varicella vaccine. Sometimes they can cause very mild illness. Children who get the measles vaccine, for example, may get a fever or a slight rash, but they will not get seriously ill.
- Inactivated vaccines use the killed version of the pathogen that causes a disease. Examples include the flu shot and polio shot.
- Subunit vaccines, like the hepatitis B vaccine or the HPV vaccine, use a specific part of the pathogen—usually a protein on the surface of the virus that our immune system will recognize and fight. We also do that with some bacterial pathogens like Hib, pneumococcus, and Bordetella pertussis.
- Toxoid vaccines, such as for diphtheria and tetanus, target toxins produced by the bacteria rather than the bacteria itself. Your immune response is therefore to the toxin, not to the bacteria.
Why do some vaccines require more doses than others?
Attenuated viral vaccines tend to induce a stronger immune response, so you need fewer doses. For example, one dose of the rubella vaccine is 97% effective at preventing the disease. Toxoid vaccines and subunit vaccines, on the other hand, require multiple doses to induce a strong immune response, which is why young children get those vaccines multiple times.
What is the logic behind the timing and types of vaccines on the childhood vaccination schedule?
A number of factors go into designing an optimal childhood immunization schedule. First is disease burden: In what age group is this disease most common and most likely to result in severe disease?
A second consideration is immunogenicity: At what age is the vaccine going to induce a strong and protective immune response? A good example of this is the measles vaccine. Very young infants still have maternal antibodies that protect them from any pathogens the mother was exposed to or vaccinated against. If you give the measles vaccine too early, those maternal antibodies will inhibit that vaccine, and the child’s immune response will not be as strong.Those maternal antibodies will wane in the first year of life, which is why the measles vaccine is given between 12 and 15 months of age.
The third consideration is trying to make the vaccination schedule easier to maximize coverage. You don't want to have so many visits required that children end up missing out on vaccinations.
Does vaccinating children younger than 1 year against measles protect them?
As we see with the large measles outbreak in West Texas and New Mexico, the risk of measles is greater than we have traditionally assumed with our current measles vaccination schedule. Parents should talk with their child’s pediatrician or health care provider about their individual risk for acquiring measles.
The measles vaccine can be given to children as young as 6 months of age. The CDC also recommends that children between the ages of 6 and 11 months who travel internationally be vaccinated against measles. But those children will still need the recommended doses between 12 and 15 months, and at 4 to 6 years of age.
Is it dangerous to get more than one vaccine at one time?
It's not dangerous to receive multiple vaccines at the same time, and vaccines are often studied in combination to make sure that they don't interfere with one another. It is true that fever and soreness at the site of injection are common after vaccination, regardless of the number administered.
Is immunity from vaccination better than infection-acquired immunity for these diseases?
Yes, because vaccines are deliberately designed to train your immune system without causing disease. If a child is infected with measles virus rather than vaccinated against measles, for example, they are at risk of serious complications of the disease, which aren't raised by the vaccine.
How does the childhood immunization schedule benefit the larger community?
The reason why we don't see a lot of measles in young infants in the U.S., apart from beneficial maternal antibodies, is that most of them are growing up in communities and households where people are vaccinated, so their chance of encountering someone with measles is very low.
This is herd immunity: where people who are not vaccinated can actually be protected by people who are vaccinated around them. If a lot of people are protected through vaccination, the probability of a single unvaccinated or susceptible individual encountering someone with the disease is very low.
We have vulnerable populations in our communities—older adults, pregnant people, immunocompromised people, or people with underlying conditions—and we protect those people through herd immunity. This breaks down if you have a large number of unvaccinated, susceptible individuals interacting with each other.
What things need to happen to have a measles outbreak in the U.S.?
To have a measles outbreak in the U.S. right now you need two things: a community that is susceptible because of low vaccination rates, and an infectious person who comes into that community from another country where the measles virus is circulating.
What I'm concerned about going forward is that we're going to see more measles cases in other countries and perhaps declining measles vaccine coverage here in the U.S. That's going to be a recipe for more frequent and larger measles outbreaks.
Why do you think it's important for people to understand vaccines and the immunization schedule?
Vaccines are one of humanity's greatest accomplishments—in my mind, far greater than going to the moon. We have saved millions of lives through vaccination globally, through a relatively simple technology. To see us slide back because people don't understand the value of vaccines or what they do, or because people don't recognize the severity and the complications of vaccine-preventable diseases is really tragic.
This interview was edited for length and clarity by Morgan Coulson, an editorial associate in the Office of External Affairs at the Johns Hopkins Bloomberg School of Public Health.
