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By the age of 2, children in the U.S. are supposed to have gotten a grand total of 24 vaccines. While individual vaccines have repeatedly been proved safe and effective, some parents worry that the vaccination schedule can be dangerous.

As NPR's Patti Neighmond reports, the Institute of Medicine is now weighing in on the debate with a comprehensive review of the evidence.

PATTI NEIGHMOND, BYLINE: The Institute of Medicine was asked to examine the safety of the timing and number of vaccines children receive against diseases like measles, polio and whooping cough. Dr. Allan Berg is a family physician at the University of Washington. [POST-BROADCAST CORRECTION: The doctor's first name was misstated. He is Alfred Berg.]

DR. ALFRED BERG: Children sometimes can get as many as five immunizations at a single visit, and the total number of vaccines, the timing of the vaccines, so we were looking at elements of the entire schedule rather than adverse effects of individual vaccines.

NEIGHMOND: Berg was also a member of the IOM investigative committee. He says after a yearlong review of all available scientific evidence, parents should be reassured. There was no evidence of any adverse health effects related to the vaccine schedule.

BERG: That included things like autoimmune diseases, which even captures diabetes, asthma, hypersensitivity - that's allergies - seizures, epilepsy, child developmental disorders and that includes things like autism, other kinds of learning disorders, communication disorders, other kinds of intellectual disabilities, even rare things like tics or Tourette syndrome.

NEIGHMOND: But at the same time, Berg says, when it comes to the timing and number of vaccines given, current research isn't as comprehensive as it could be. For example, there's been little research into whether it's still safe and just as effective when vaccines are spread over a longer period of time than is currently recommended.

BERG: Once the schedule has been set, though, no one has studied whether moving a vaccine forward two weeks or backward two weeks or adding two vaccines together or splitting them apart would be better than the current schedule.

NEIGHMOND: So the committee suggests federal researchers could collect data and follow health outcomes among children whose vaccines are delayed for any number of reasons. In some cases, children get sick and can't get their vaccine on schedule. In other cases, parents choose to space out vaccines because they're worried about the safety of getting a number of vaccines at the same time. Even though this review finds no reason to worry and that the current recommended schedule is completely safe, Berg says it still might be useful to know what happens to children who don't get their vaccinations on time by comparing them to children who do.

Preliminary research does show children who don't get vaccinated on time are hospitalized more often than children who are vaccinated on time. Further monitoring and research is needed. In any case, some parents remain skeptical. Barbara Loe Fisher is president of the nonprofit advocacy group National Vaccine Information Center.

BARBARA LOE FISHER: We do not have the scientific evidence to assure doctors and parents that the current vaccine schedule is safe. They make statements like we are reassured, but there are less than 40 studies that they had to look at. That's not nearly enough science to give parents confidence that the current vaccine schedule is safe.

NEIGHMOND: Despite Fisher's concerns, federal health officials say most parents adhere to the recommended vaccine schedule and that 90 percent of all children are fully vaccinated by the time they enter kindergarten. Only about 1 percent of parents refuse all vaccines.

Patti Neighmond, NPR News.

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