The Evidence For the Childhood Vaccine Schedule, Explained

Before their immune systems mature, young children are especially vulnerable to infections — and to falling far more ill than adults might with certain common illnesses.

That’s why childhood vaccination programs have been such a boon for public health. In the past 30 years, recommended childhood vaccines have prevented an estimated 1.1 million deaths and 32 million hospitalizations in the United States.

The recommended schedule for these vaccines is first set by an advisory committee to the Centers for Disease Control and Prevention, which reviews the recommendations and decides whether to adopt them. The committee’s members — which include experts in vaccination, infectious disease, pediatrics and public health — meet several times each year to review the latest clinical trials as well as health and safety data from the general population.

Today, children vaccinated in line with these recommendations are protected from 16 diseases, including infections that can cause paralysis, brain damage and deadly fevers. Those who aren’t given recommended shots or follow an alternative schedule — which research suggests is the case for an estimated 27 percent of U.S. children — may be at higher risk of developing these preventable illnesses.

“When we start spacing things out, the danger is that either the child gets exposed, or we never get them caught up and they miss out entirely,” said Dr. Beth Thielen, a pediatric infectious disease physician at the University of Minnesota.

Why is the vaccine schedule set the way it is?

To understand why doctors recommend vaccinating on a specific schedule, it’s helpful to understand what protection babies need and when. “We want them to have maximal protection when they need it, and when they need it is when they’re at the highest risk of serious infection and before they are exposed,” said Dr. Leila Posch, a pediatric infectious diseases physician at Children’s Hospital Los Angeles. “Second, we want to give vaccine doses when their immune system is mature enough to respond to them well.”

The hepatitis B vaccine is recommended at birth because the infection, which can lead to permanent liver damage and cancer, is highly transmissible during delivery. It can also spread through drops of blood on surfaces or skin. Before the vaccine, about 18,000 children were infected each year, about half of them at birth. Today, hepatitis B is vanishingly rare among U.S. children.

At a baby’s eight-week checkup, doctors recommend shots that protect against whooping cough, tetanus and polio, along with other infections that can cause pneumonia, brain swelling and heart damage. At this point, a baby’s immune system will have matured enough to effectively make antibodies, proteins that protect the body from specific infections, in response to these shots.

These vaccines require additional doses over the next year and a half to maximize protection, explained Dr. Ben Hoffman, the president of the American Academy of Pediatrics and a pediatrician at Oregon Health and Science University. At around six months, babies should also start receiving annual vaccines to protect against flu and Covid-19, both of which can be deadly for infants.

Thanks to protective antibodies that they receive from their mothers in utero, babies are born with immunity to a few dangerous infections, including hepatitis A, measles, mumps and rubella. That means certain vaccinations can wait until that immunity wanes — around age one, Dr. Thielen said. Babies also receive vaccines against chickenpox at this age.

Right before kindergarten, children receive one more round of booster shots to give the immune system another update.

What happens when parents stray from the recommended schedule?

Parents who choose not to follow the recommended schedule often do so because they have concerns that babies receive too many vaccines at once, Dr. Thielen said, or they may worry that certain ones are given too early. Some public figures, including Robert F. Kennedy, Jr., have also cast doubt on the vaccines included in the schedule.

There is no evidence that the number of vaccines children receive is harmful, she said. And all of the vaccines in the schedule have been rigorously tested for safety — both individually and to make sure they’re safe and effective in combination with one another, Dr. Posch said.

On the other hand, there are only limited studies on alternative schedules. Those that exist include much smaller numbers of patients.

What pediatricians do know is that these alternative schedules, which may forgo certain vaccines or stagger the recommended shots, leave children vulnerable for longer periods.

“I have seen families who have unfortunately missed opportunities to get vaccines, and those vaccines then can’t do the job of protecting their kid,” Dr. Hoffman said.

Doctors are also seeing more children become sick with infections that they once saw only rarely. A few years ago, a child arrived at Oregon Health and Science University with a tetanus infection, Dr. Hoffman said. “Before that, I don’t think a single person at OHSU had ever seen tetanus,” he said. The child was in the intensive care unit for months.

Dr. Thielen said she had cared for children with measles and permanent brain injuries from meningitis. All of them, she said, had fallen ill with diseases that vaccines could have prevented.