When should children get each vaccine dose? And who gets to decide?

The federal government wants to change the number of times children visit their doctors to receive vaccines, beginning with some of childhood’s earliest shots. 

Earlier this month, President Donald Trump signed an executive order outlining his administration’s “Gold Standard Childhood Vaccine Recommendations.” The order called for all childhood immunizations to be administered at separate medical visits “to the maximum extent feasible.”

The administration has not yet released a schedule detailing what separate vaccine visits would look like in practice. But if implemented broadly, the policy would dramatically change not just how many times children visit the doctor for vaccines, but when they receive them. 

Separating those vaccines across different appointments raises a fundamental question at the heart of the administration’s proposed overhaul: Why do children get certain vaccines when they do in the first place?

Straight Arrow examined the evidence behind the timing of three common childhood vaccines — MMR, hepatitis B and the diphtheria, tetanus and pertussis (or DTaP) shot — to understand why children receive them when they do, how precisely scientists can identify the “best” age to vaccinate and where biology ends and public health judgment begins.

The schedule that predated the Trump administration recommended newborns receive the hepatitis B vaccine almost immediately after birth. Then, at 2, 4 and 6 months, children received a range of shots protecting against diseases from pertussis and polio to pneumococcal disease and rotavirus, before receiving their first routine MMR dose around their first birthday.

The recommended timing of vaccine doses reflects a mix of biology, disease risk and practicality. 

“The core of it has to do with balancing the trade-off between when the burden of disease starts increasing or is greatest — because obviously you’d want to vaccinate before the burden of disease increases — and the immune response to the vaccine,” said William Moss, a pediatrician and infectious disease epidemiologist at Johns Hopkins Bloomberg School of Public Health and the executive director of the International Vaccine Access Center.

“There are also other factors, including the logistics,” Moss said. “So trying to coordinate the administration of a vaccine dose with either other vaccines or with other child health interventions that are given at a particular time.

Who decides when vaccines are given?

Since 1964, the federal government has relied on the Advisory Committee on Immunization Practices, or ACIP, to help determine how vaccines should be used in the U.S. The committee was created as vaccines against diseases like polio and measles became available and federal officials concluded that national vaccination policy had become too complicated to handle through separate expert panels convened for individual vaccines.

For decades, ACIP and the American Academy of Pediatrics (AAP) developed their own recommendations. They didn’t always agree. After years of disagreement over when children should receive hepatitis B, polio and a second MMR dose, ACIP, the AAP and the American Academy of Family Physicians published the first harmonized childhood immunization schedule in 1995. This created a single national calendar, designed both to identify when doses should be given and to provide acceptable age ranges and limit unnecessary visits.

Traditionally, ACIP considers multiple factors as it puts together national vaccine schedules: At what age are children most vulnerable to a disease? When will their immune systems respond effectively? How many doses are needed and how far apart should they be? Can the resulting schedule realistically be followed? Its formal framework also weighs the strength of scientific evidence, potential benefits and harms, cost and equity. 

But ACIP’s recommendations aren’t the first step in vaccine oversight. The Food and Drug Administration must first approve a vaccine for use. Manufacturers submit clinical-trial data examining safety, immune response and effectiveness at particular ages and dose intervals. The FDA reviews that evidence and determines whether to license the vaccine, including the ages for which it can be used.

Clinical trials aren’t designed to identify one biologically perfect day to vaccinate. They establish that particular ages and intervals — chosen based on biology, disease risk and practical considerations like existing well-child visits — are safe and effective.

“They obviously can’t in the trials evaluate all the different, you know, the whole universe or landscape of schedules,” Moss said.

Why do hours-old babies get the hepatitis B vaccine? 

Prior to the current Trump administration, the Centers for Disease Control and Prevention and ACIP recommended that nearly all newborns receive their first hepatitis B vaccine within 24 hours of birth — earlier than any other routine childhood vaccine. (One exception was babies weighing less than 4.4 pounds whose mothers were hepatitis B-negative; for these newborns, vaccination was typically delayed until hospital discharge or about one month of age because of a weaker immune response at birth.)

The primary reason was the risk of transmission from mother to baby during childbirth. While hepatitis B infection is often short-lived in adults, about 90% of infected infants develop chronic hepatitis B, which can eventually lead to serious liver disease. Babies born to mothers with hepatitis B were typically given both the vaccine and hepatitis B immune globulin within 12 hours of delivery to reduce the risk of infection.

The universal birth dose also served as a kind of public health safety net. Not every pregnant woman is tested for hepatitis B during her pregnancy, maternal infections can be missed, test results can be delayed or incorrectly documented and babies can later encounter the virus through infected household members. Vaccinating before a newborn leaves the hospital also takes advantage of one of the few times nearly every infant is already in contact with the healthcare system.

“From a public health point of view, it makes sense to recommend universal birth dose of hepatitis B vaccine, just so you don’t miss anyone,” Moss said.

But Moss also acknowledged why some parents of low-risk infants may reach a different conclusion: If a mother has tested negative for hepatitis B and there are no other known risk factors, that baby’s immediate risk of infection is likely very low. In those circumstances, he said, delaying vaccination until around 2 months can be a reasonable individual decision.

That distinction illustrates one of the tensions underlying vaccine schedules: What makes sense for an individual child may differ from what provides the greatest protection across an entire population. While longstanding federal recommendations generally erred on the side of maximizing protection across the population, the Trump administration has shifted more of that calculation to the individual child. Under its new approach, parents and clinicians are increasingly being asked to weigh a child’s particular risk of infection against the benefits and risks of vaccination and decide whether — or when — to vaccinate.

For hepatitis B, that meant ending the decades-old recommendation that nearly every newborn receive the vaccine at birth. In December, the CDC began recommending the birth dose for babies whose mothers test positive for hepatitis B or whose infection status is unknown. But for babies born to a mother who tests negative, parents and clinicians should decide whether to vaccinate the baby at birth or begin the series later based on the child’s individual risk. If they defer the birth dose, federal guidance suggests starting no earlier than two months.

Supporters of the change argued that widespread prenatal screening now makes it possible to identify the infants at greatest risk. Vicky Pebsworth, a member of ACIP, which has been reconfigured under this administration, said babies whose mothers test negative face an “extremely low risk of hepatitis infection during childhood and especially during the first month of life.”

Critics counter that universal vaccination provides protection when screening or follow-up fails. 

“There are moms who are unaware of their hepatitis status,” Indiana family physician Teresa Lovins previously told Straight Arrow News. “I have seen where it gets missed.” 

She also noted that some babies may miss subsequent pediatric visits, losing another opportunity to vaccinate.

READ MORE: New hepatitis B recommendation reveals cracks in system

The debate over the birth dose, then, isn’t primarily about whether a healthy newborn’s immune system can respond to the vaccine. It’s about how much protection to build into a population-wide schedule for an infection that may pose little immediate threat to an individual low-risk baby, but can have lifelong consequences when infection does occur.

The two-month surge

For some parents, one of the most alarming parts of the childhood vaccine schedule comes between 2 and 6 months of age.

At a typical 2-month check-up, a child receives the first of several doses of vaccines against rotavirus, diphtheria, tetanus, pertussis, Haemophilus influenzae type b (or Hib), pneumococcal disease and polio. Many of those vaccines are given again at 4 months and some again at 6 months.

The sudden surge of shots concerned Brianna, a mother of seven who was expecting her eighth child earlier this year. After fully vaccinating her first two children, she began questioning the schedule.

“Why do kids need so many vaccines? Why do they receive so many at once?” she recalled wondering.

DTaP — a combination vaccine that protects against diphtheria, tetanus and pertussis — is one of those given at 2, 4 and 6 months, followed by two additional doses later in childhood.

There are good reasons to start the vaccine series early. Pertussis can be particularly dangerous for young infants, and one dose of DTaP isn’t enough to build the desired protection. Multiple doses train and reinforce the immune response during the period when babies are especially vulnerable to severe disease.

But there is nothing biologically special about 2, 4 and 6 months exactly. DTaP can be given beginning at 6 weeks, and the first three doses can be separated by as little as four weeks. The routine schedule instead spaces them about two months apart. Other countries use different schedules — including doses at 6, 10 and 14 weeks or at 2, 3 and 4 months — demonstrating that the U.S. timing is not the only schedule capable of providing protection.

Logistics also play a role. Vaccine schedules were designed around safety as well as opportunities to actually get children vaccinated. Pediatricians already routinely see babies around 2, 4 and 6 months for well-child visits, when they track growth, screen for potential health or developmental concerns and assess developmental milestones such as head control, movement and communication. Scheduling vaccines during those visits takes advantage of appointments families are already expected to attend. Giving several recommended vaccines at once also avoids requiring a separate appointment for each shot and reduces the chance a child misses or delays a vaccine.

Scientific evidence supports that approach. Clinical studies and decades of experience found that, with limited exceptions, administering commonly used vaccines during the same visit produced similar immune responses and rates of adverse reactions as administering them separately. During one measles outbreak, researchers estimated that roughly one-third of measles cases among unvaccinated but vaccine-eligible preschoolers could have been prevented if children had received MMR when they were already at the doctor receiving another vaccine.

The clustering of vaccines is intentional: Once scientists established that vaccines could safely be given together without generally compromising their effectiveness, administering several at the same appointment offered a way to protect children sooner while requiring fewer visits.

And practical considerations are important because a vaccine schedule only works if people actually follow it, said Walter Orenstein, a professor emeritus in the infectious disease division at Emory University School of Medicine.

“Vaccines don’t save lives; vaccinations save lives,” said Orenstein, who previously worked for 26 years at the CDC’s Immunization Program.

Measles

Typically, a child receives a first dose of the MMR vaccine around their first birthday. There’s a good biological reason for that.

Babies are born with some temporary protection against measles. During pregnancy, mothers who have immunity to the virus pass antibodies — proteins the immune system makes to recognize and fight infections — across the placenta to their babies. 

While those antibodies protect young babies, they can also interfere with vaccination. MMR is a live-attenuated vaccine, meaning it uses a weakened form of the measles virus to trigger an immune response without causing measles. The virus from the vaccine needs to replicate enough for an infant’s immune system to recognize it and build its own protection. But maternal antibodies can recognize and neutralize the virus before that process gets underway.

Maternal antibodies gradually wane after birth. In one study, researchers estimated that by 6 months, more than 95% of infants had lost maternally derived measles immunity. As those antibodies disappear, a window of opportunity opens to vaccinate without maternal antibodies interfering with the immune response. Researchers have found that only 74% of 6-month-old babies given the measles vaccine developed detectable neutralizing antibodies, compared with 100% of 15-month-olds. The younger infants who responded also produced fewer antibodies.

That’s one of the main reasons infants typically don’t receive their first MMR dose until they’re older. But waiting longer comes with a trade-off. As maternal antibodies fade, babies become increasingly susceptible to measles.

“If you wait longer, the child’s going to have a stronger immune response,” Moss said. “But you don’t want to wait too long because then the child is at risk of the disease, kind of waiting for the vaccine.”

That immune response-risk balance can change when a baby faces a higher risk of measles. During an outbreak or before travel to an area where measles is circulating, pediatricians may recommend vaccinating an infant as young as 6 months, Moss said. But that early shot is considered a “zero dose” rather than part of the routine series because the immune response at that age may be less reliable. 

In those cases, the immediate benefit of providing some protection against measles outweighs the benefit of waiting for a stronger immune response.

The longstanding federal vaccine schedule calls for children to receive a second dose of MMR between ages 4 and 6. 

Unlike some vaccines, where subsequent doses act as boosters to strengthen or restore immunity that has waned over time, the second MMR dose provides another opportunity to immunize the small percentage of children who didn’t develop adequate protection after their first dose.

“That second dose is really to immunize that small proportion of children who didn’t respond to the first dose,” Moss explained. “Then you get up to like 97, 98% population protection with that second dose.”

Mumps and rubella

The mumps and rubella components of the MMR vaccine complicate the picture. A second dose of rubella is generally not needed, because a single dose of the shot invokes a sufficient immune response. 

“We really only need a single dose. We give a second dose of rubella vaccine because it’s part of the MMR, the combination vaccine,” Moss said. Immunity to mumps can wane over time, which is why there are outbreaks among highly vaccinated groups such as college students. During some outbreaks, officials may recommend an additional dose of a mumps-containing vaccine. But because a standalone mumps vaccine isn’t available in the U.S., the MMR vaccine is given.

There is a kernel of truth in the Trump administration’s argument that having individual vaccines available could offer more flexibility. During a mumps outbreak on a college campus, for example, officials might want to vaccinate specifically against mumps rather than administer another dose of measles and rubella. 

But that raises a different question from whether combination vaccines should routinely be separated: Is there evidence that receiving an additional, potentially unnecessary vaccine component is harmful?

Not that Moss is aware of. 

“There’s no harm in giving the combination vaccine,” he said.

Orenstein agreed. 

“The data are very clear. There’s no advantage to separating measles, mumps, and rubella,” he said.

Decades of research support that conclusion. The AAP  says administering multiple vaccines at the same time is safe and generally does not interfere with the immune response to individual vaccines. Studies comparing vaccines including MMR, chickenpox, polio and DTaP given simultaneously versus at separate visits have found similar immune responses and rates of adverse events.

There is also a practical obstacle to creating separate vaccines: none are currently licensed or manufactured in the U.S. Merck stopped producing its individual measles, mumps and rubella vaccines nearly two decades ago, and both Merck and GSK — the two manufacturers of MMR vaccines licensed in the U.S. — have said there is no scientific evidence supporting separating the vaccines. Experts have warned that bringing individual products back to market could take potentially as long as a decade, because manufacturers would need to develop manufacturing processes, conduct clinical studies and obtain regulatory approval.

That process can be expensive. A 2024 analysis by researchers at the Department of Health and Human Services estimated that developing a preventive vaccine costs roughly $133 million in direct development costs, excluding major expenses such as manufacturing facilities and supply chains. After accounting for development failures and the cost of capital, the researchers estimated an average cost of about $887 million — and 10 years — to bring a novel vaccine to market. Those figures aren’t estimates for resurrecting the individual measles, mumps or rubella vaccines specifically, which could be substantially different because the antigens are already well established. But they illustrate the scale of investment vaccine development can require. 

Still, Moss questioned whether manufacturers would see enough demand to justify that investment. 

“I don’t know if a manufacturer is going to want to go through all the trouble of getting these individual vaccines approved and licensed and manufactured,” he said, noting that companies would face additional research and development and clinical trials. “That will be a hurdle.”

Could the vaccine schedule be different? Should it be?

Because the childhood vaccine schedule isn’t set in stone, there is room for legitimate debate over how the schedule is designed.

But the country’s overall approach has been remarkably effective at preventing disease. Before measles vaccination, an estimated 3 to 4 million Americans contracted the virus each year; by 2000, the U.S. had eliminated continuous domestic transmission. Pertussis vaccination reduced cases by more than 90% compared with the pre-vaccine era. In 2024, CDC researchers estimated that routine childhood vaccination among children born from 1994 through 2023 will prevent roughly 508 million illnesses, 32 million hospitalizations and 1.1 million deaths.

Now, some of those gains are showing signs of vulnerability. During the 2025–26 school year, MMR coverage among kindergartners fell to 92.4%, while exemptions from at least one required vaccine reached a record 4.2%. At the same time, the U.S. is experiencing its largest measles resurgence in decades, with more than 2,500 cases reported so far in 2026.

The current debate over childhood vaccination is also being driven by persistent concerns that vaccines may contribute to autism, despite large studies finding no causal association. And as Straight Arrow has previously reported, many of the top autism researchers in the country believe the increase in autism diagnoses largely reflects changes in awareness, screening and diagnostic practices rather than a true increase in its prevalence. 

Still, national vaccine recommendations should be continually monitored and revised as new evidence emerges — whether that evidence points to a better dosing schedule, new safety concerns or changes in disease risk. 

“We have to be open to new data that would change things,” Orenstein said. But he said the available evidence supports the longstanding recommendations that preceded the Trump administration’s overhaul. “The data we have supports the current recommendations.”

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