Even as Texas grapples with a measles outbreak that has already left one child dead, the childhood vaccination schedule is coming under new scrutiny by the Trump administration. The recommended list of immunizations starting at birth protects kids against more than a dozen deadly diseases from measles to whooping cough.U.S. Health and Human Services Secretary Robert F. Kennedy Jr., a known vaccine skeptic,included vaccines on his list of things to investigate to see if they have contributed to chronic diseases. And the future of the federal system for weighing the risks and benefits of vaccines in order to make recommendations is in limbo.Yet health experts overwhelmingly credit vaccines for measles and other preventable diseases as huge public health successes. For instance, before a measles vaccine became available in 1963, nearly all children got the highly contagious disease by the time they turned 15 and about 400 to 500 people in the United States died each year from the disease, according to the U.S. Centers for Disease Control and Prevention.Vaccines have eradicated smallpox and have eliminated polio and other diseases from the United States and many other parts of the world.The turmoil currently surrounding vaccines and the diseases they are designed to tackle has left many people confused. Science News sat down with two experts to discuss some of the most frequently voiced concerns and questions.Meet the vaccines expertsAditya Gaur is a pediatric infectious diseases doctor and a clinical researcher at St. Judes Childrens Research Hospital in Memphis. Before coming to the United States in 1996, Gaur worked as a pediatrician in India before vaccines were routinely available. He and colleagues repeatedly treated cases of measles, he says, along with many other vaccine-preventable ailments.I saw tetanus in children and how uncomfortable it was for children, sometimes leading to death, he says. I saw diphtheria and how the throat looks and how difficult it can get to breathe. I saw those with pneumonia and air leaks in the lungs, and then the children that died. Ive seen polio in terms of paralysis. Thanks to vaccines, he hasnt seen a single case of any of those diseases since coming to the United States, he says.Iron lungs, such as this one used in 1938, became a symbol of polio, a viral disease that can paralyze people and leave them unable to breathe without assistance. Now, wild polio circulates only in Afghanistan and Pakistan but falling vaccination rates threaten to undo decades of work to eradicate the disease.Keystone/Getty ImagesKawsar Talaat, an infectious diseases doctor and vaccine safety researcher at the Johns Hopkins School of Public Health, recalls how her father had polio as a child. He was born before the vaccine was available, she says. He was paralyzed as a child. Hes always had a limp, and then as he got older, he got weaker and weaker and weaker, and now hes in wheelchair Its a lifelong debilitating illness, even if you survive it.She has also treated vaccine-preventable diseases. When I was a resident, we would get waves of children in with dehydration due to rotavirus, and we would dread rotavirus season, she says. Now the waves are more like ripples.How do vaccines work?Vaccines train the immune system to fight off diseases. When a baby is born, they get some antibodies from their mothers that may give them some protection against certain illnesses for a few months, Gaur says. Thats why a new vaccine against respiratory syncytial virus is given during pregnancy to protect newborns.As the child grows, the immune system learns to combat illness either by getting infected or through immunization. Vaccines may consist of whole weakened or killed pathogens or parts of those organisms, called antigens. The vaccine doesnt cause disease, but it does teach the immune system what invaders to look out for, Gaur says.Although measles was declared eliminated from the United States in 2000, outbreaks still happen, mostly when infected travelers bring the virus back with them and spread it among unvaccinated people.As of February 28, at least 146 people in Texas had caught measles in an ongoing outbreak that started in late January. Twenty people have been hospitalized with the highly contagious and dangerous disease, and an unvaccinated school-age child has died. The disease also has been reported in eight other states this year.From 2002 through 2016, measles hospitalized 1,018 people in the United States. Of those, 34 died; some others had serious complications including kidney failure, brain swelling, pneumonia, blood clots and eye problems, researchers reported in PLOS One in 2020.In contrast, the side effects from measles vaccines usually consist of a sore arm and sometimes a fever or mild rash. Yes, a child can see natural measles and, if they survive, they have good immunity. But there is no upfront way of saying what will be the outcome, Gaur says. With vaccines, you are controlling that exposure and teaching the immune system how to fight off an infection.Even more benign infections may have serious consequences that vaccines can prevent, Talaat says. For most people, chicken pox is mild, but not for everybody. I have seen kids in the hospital with really severe life-threatening bacterial infections that occurred because their skin was disrupted by chicken pox, she says. Kids have lost so much skin, its as if they were severely burned. Its happened in childrens groins, and so their future fertility and sexual function have been affected.Vaccines not only protect children but also keep them from spreading disease to people in the community at high risk of complications, Talaat says. And even if the disease is mild, it still means that that child will miss a week or more of school, and that their parent will miss a week or more of work.Most of the cases in the Texas outbreak are children who are unvaccinated or have unknown vaccination status. That mirrors previous U.S. outbreaks: Of 285 reported cases in 2024, nearly 90 percent of those sickened were not vaccinated, CDC data show.Losing herd immunityIn the 20092010 school year, at least 20 states reported that 95 percent or more of their kindergartners were vaccinated against measles, mumps and rubella, according to the CDC. That is the threshold that helps protect people in the community who cant be vaccinated because of weakened immune systems or other medical issues. It essentially erects a wall of vaccinated people between the vulnerable and the measles virus. By the 20232024 school year, just 11 states had vaccination rates among kindergartners at or above the 95 percent threshold. Click the arrows to see how MMR vaccination rates among kindergartners has changed between those two school years.MMR vaccine coverage for U.S. kindergartners by school year C. Chang C. Chang Across the United States, vaccination rates have been falling for measles, as well as other childhood diseases. When more than 95 percent of people are vaccinated against measles, there is community or herd immunity that can protect people with weakened immune systems who cant be vaccinated. But measles vaccination rates among kindergartners has fallen from 95.2 percent in the 20192020 school year to 92.7 percent in 20232024. That left about 280,000 kindergartners vulnerable to measles during the last school year, according to the CDC.How many deaths and illnesses are prevented by vaccines?Worldwide, vaccination against 14 pathogens saved 154 million lives over the last half century, researchers reported in 2024 in the Lancet. In the United States, routine childhood vaccinations prevented more than 24 million cases of disease in 2019, including about 1,000 cases of tetanus and more than 4.2 million chicken pox cases, researchers reported in Pediatrics in 2022.Childhood vaccines cover diseases including polio, tetanus, diphtheria, pertussis (whooping cough), measles, mumps, rubella, rotavirus, hepatitis, chicken pox and meningitis. They also cover infections caused by bacteria, including Haemophilus influenzae and Pneumococcal bacteria. All those diseases may cause severe infections that land people in the hospital and can be deadly. Some may have lifelong consequences.Those are all long-lasting vaccines, some even conveying lifetime protection. In addition, yearly vaccines for flu and COVID-19 are also recommended. About 1.2 million influenza cases in the United States were averted in 2019, an estimated 17 percent reduction from what it would have been without vaccination. COVID-19 vaccines are estimated to have saved at least 14 million lives globally in the first year after they were rolled out, researchers reported in 2022 in the Journal of Paediatrics and Child Health. Immunizations against human papilloma virus to prevent cervical, head, throat and other cancers are recommended for older children. Cervical cancer rates have plummeted for young women vaccinated against HPV.Downward trendVaccination rates for kindergartners in the United States have fallen in recent years. During the 20232024 school year, just 92.3 percent of kindergartners were vaccinated against diphtheria, tetanus and pertussis (DTaP) and 92.7 percent were protected from measles, mumps and rubella (MMR). Varicella (chicken pox), polio and hepatitis B vaccination rates have also dropped in recent years.The ability to add new vaccines or update existing ones, such as the flu and COVID vaccines has been jeopardized by the Trump administrations withdrawal from the World Health Organization and cancellation of important meetings of committees that advise the CDC and the U.S. Food and Drug Administration about vaccines. Among other considerations, nixing those meetings may threaten the United States ability to get updated flu vaccines for the next flu season.This decision and other federal efforts to underminewell-establishedscience aboutvaccine safety puts everyone at risk, especially when we are currently experiencingthe worst U.S. flu season in more than a decade, Tina Tan, president of the Infectious Diseases Society of America, said in a statement.The CDC estimates that at least 33 million people have gotten the flu so far this season. An estimated 430,000 people have been hospitalized and 19,000 have died, making this the first high severity flu season since the 20172018 season.How did the childhood vaccine schedule come about?It happened very gradually over time, Talaat says. Advisers for the CDC and FDA carefully weigh the benefits and risks of each vaccine against the harms caused by infections and recommend whether to add a shot to the schedule and when to give it.Once we started making vaccines, children were a natural target because they were the most susceptible to a lot of these infections, Talaat says. For instance, rotavirus infections produce diarrhea that can easily dehydrate young children and land them in the hospital, she says. That vaccine is given when babies are 2 months old.Pertussis vaccines are also among the earliest given because the younger the baby, the more susceptible they are to [whooping cough], Talaat says. Their airways are so small that theyre more likely to die from it.With measles vaccines, Talaat says, theres a sweet spot when you want to get the vaccine into kids to protect them, but you dont want to give it too early, because antibodies passed from the mother to the baby can interfere with the vaccine. So the measles, mumps and rubella shot is typically first given when infants are 12- to 15-months old.Vaccines that protect against HPV and meningococcal disease arent given until kids are preteens or teenagers because they are at higher risk of infection during the teen or young adult years.Is it safe for children to get many vaccines at once?Yes. Many childhood vaccines protect against multiple diseases in a single shot, such as one that combines polio, diphtheria, tetanus and pertussis, hepatitis B and Haemophilus influenzaetype b. Another guards against up to 20 strains of Pneumococcal bacteria. So a 2-month-old might get a couple of shots with protection against more than two dozen pathogens, along with a drink containing the rotavirus vaccine.A baby gets inoculated with the rotavirus vaccine, which is given as a liquid instead of a shot. Thats one of 14 diseases that U.S. children are commonly immunized against.chameleonseye/iStock/Getty Images PlusWe have done studies that show that giving these vaccines together is safe and that the immune responses to the vaccines arent damaged, Talaat says.We are exposed to lots of things every day in our environment. Our bodies and our immune systems are built to handle that, she says. And when we get multiple vaccines and multiple antigens at the same time our bodies can handle that, too.She adds that the reason we give kids a bunch of vaccines all at once is to make sure that they get them. Its hard on families to keep bringing their children back to the doctor to get their shots, she says.What are the side effects or possible harms from vaccines?Sore arms are common since most vaccines are given as shots. Anything which is injectable may cause an immediate owie, and then may cause some swelling and tenderness, Gaur says. Depending on the vaccine, mild side effects might also include short-lived fever, fatigue, muscle or joint pain and maybe a rash.Some people develop rare severe side effects such as allergic reactions. Certain groups have a higher risk of that. For instance, adolescent and young adult males are more likely than other people to develop myocarditis and pericarditis inflammation of the heart or the sac around the heart after a COVID-19 vaccine. But getting a COVID-19 infection is more likely to cause those heart problems than the vaccine, and the vaccine can prevent severe disease and hospitalization, so regulators calculate that benefits of vaccination outweigh the low risks.How are vaccines tested?Vaccines go through many years of development in lab and animal tests before they are tested in people. Clinical trials in people happen in multiple phases.First, vaccines like any treatment are tested in small numbers of people for safety. Usually this involves giving some people in the trial the vaccine while others get a placebo. Thats necessary, Gaur says, because things happen to humans, as in, we may get headaches, we may get fevers. The placebo helps sort out which symptoms come from the vaccines.Additional phases of clinical trials test vaccines in increasingly larger groups of people to look for rare side effects and to determine how well the vaccines prevent disease. New vaccines are tested against placebos in those stages as well. But if there is an existing vaccine, it would not be ethical to leave people unprotected by giving them a placebo, Gaur and Talaat say. Instead, potential vaccines would go head-to-head with existing ones to show that they work at least as well if not better than what is already available.Unlike medications and therapies, which are usually given to sick people to keep them from getting sicker, vaccines are given to healthy people. That means that side effects associated with other treatments would never be tolerated for vaccines. Says Talaat: Vaccines pass a higher bar than most treatments.