The world greets newborn babies by bombarding them with everything from dust to disease-causing microbes. Babies can deftly parry exposures to certain harmful substances, thanks to antibodies passed onto them in the womb. But this kind of inherited immunity is short-lived, so a child must develop her own immune system response to combat life-threatening diseases. That's where vaccines come into the picture. By age two, most children will receive almost 30 shots designed to boost a child's natural defenses against disease. Yet at the same time, parents who take their children for those recommended vaccinations might be inundated with Web site and celebrity-espoused rumors making false claims that shots are not necessary or cause autism.

At best, navigating this landscape can be confusing. But when weighing the risks of encountering life-threatening disease against the benefits of receiving a vaccine there's no contest. The vast majority of children do not experience anything worse than short-lived redness or itching at the spot of the injection. So why is there so much misinformation?

One reason is developmental timing. Children receive a bevy of vaccines before age two, roughly overlapping with when children with autism spectrum disorder may noticeably develop symptoms such as the inability to gain new skills. That coincidence has helped the autism-vaccine connection myth persist. And what may have once been passed around by neighborhood gossip is now amplified worldwide by Web sites and social media. But the science simply does not bear out such fears. In August 2011 the Institute of Medicine (IOM) released a report that examined eight childhood vaccines and potential side effects. It found that vaccines are largely safe and that side effects are usually very rare and minor.

Side effects

The overwhelming medical evidence finds that most vaccine side effects among newborns and young children are mild—swelling, redness and a small, hard lump at the site of the injection—and typically pass within a couple of days. A far less common but serious vaccine side effect, occurring in fewer than one in a million cases, is an immediate allergic reaction that can be treated with common medications to ease itching or swelling or, in more serious cases, by administering epinephrine. Rarely, with certain vaccinations there can be other problems. After receiving the first shot of the measles-mumps-rubella (MMR) vaccination, for example, a child has a roughly one in 3,000 chance of developing a fever that leads to a seizure. Such seizures do not lead to any permanent neurological damage. Moreover, they also occur more generally when kids develop high fevers—afflicting up to 5 percent of young children. That does not mean it is not upsetting to see. “It's hard to watch your child seize,” says Paul Offit, a pediatrics professor at Children's Hospital of Philadelphia. His own daughter had a seizure after receiving the combo vaccine against diphtheria, tetanus and pertussis but he says it's important to remember that this type of seizure does not cause long-term consequences. And scientific evidence finds that MMR-related seizures are actually less frequent than ones that occur as a direct result of the measles infection itself.

What's in a vaccine?

When a person encounters a pathogen, there is no way to predict whether it will induce a mild or severe form of disease or how the body will react. But vaccines are selected to contain a weakened or dead form of the disease-causing germ that can spark an immune response. Exposure will thus allow the body to build up immunity in a controlled way. The immune system reaction stops this compromised foreign invader in its tracks at the site of injection. And the key to vaccine success is that, afterward, the immune system starts to create fast-response infection fighters called memory cells that will circulate throughout the body and be able to recognize (and fend off) that same pathogen in the future.

These weak or killed stimulants, called antigens, are grown in a lab setting, isolated and then mixed with preservatives, stabilizers and a substance like aluminum that will trigger the immune system to vigorously respond to the vaccine. The process saves lives: among children born in the past two decades, vaccinations will prevent more than 20 million hospitalizations and 732,000 deaths, according to U.S. Centers for Disease Control and Prevention estimates.

Why do vaccines cause any side effects?

Like any medicine, immunizations can lead to a negative reaction. That's not surprising because every individual is different. Genetic variations, immune deficiencies and environmental exposures all contribute to how a person's body reacts to inoculations against disease.

Is such a high number of shots dangerous?

Expert medical bodies, including the American Academy of Pediatrics (AAP), urge parents to vaccinate their kids against 16 diseases. The CDC schedule for childhood vaccinations, which relies on recommendations from the Advisory Committee on Immunization Practices and is endorsed by the AAP, is drawn up based on data detailing when the body's immune system will mount the best response to the vaccine and, second, balanced against the need to protect kids at the earliest age possible. There are no scientific data suggesting a medical benefit from spacing out vaccines over a longer period than the official recommendations.

Vaccines are also regularly evaluated for safety. Before they are added to the recommended regimen, they must undergo testing to make sure they won't interfere with one another. Multiple studies have also evaluated vaccines' cumulative effects.

Still, pediatricians face continual pressure from some worried parents who may want to spread out the recommended inoculations or postpone certain vaccinations, decisions that can prolong the period of vulnerability to disease. For example, a nationally representative study published in the April 2015 Pediatrics found that in an average month 93 percent of surveyed doctors received at least one request from parents to spread out shots. And it happened fairly frequently: over a fifth of the 534 surveyed physicians reported that 10 percent of parents made such requests. Parents' wishes were then often honored—most of the surveyed clinicians ultimately agreed to do so, at least some of the time.

Yet fears about a child's body not being able to handle potent vaccines are misguided. Children's immune systems respond to several hundred foreign substances that trigger an immune response every day. In contrast, the complete schedule of recommended childhood vaccinations includes less than 200 antigens.

Do vaccines cause autism?

Some concerns about vaccines stem from discredited work that suggested they lead to autism. (The findings were subsequently retracted, and the physician who did that small study was barred from practicing medicine because of ethical lapses.) More than a dozen studies have added to the body of evidence that this link does not exist. Expert groups, including the IOM and the AAP, agree that vaccines do not contribute to children's autism rates. The vaccine additive thimerosal, a preservative that contains a form of mercury (organomercurcial, not methylmercury) also does not cause autism. It is now only included in pediatric vaccines in trace amounts (or not at all), but there is no medical evidence that it causes autism in patients.

Can't children thrive without vaccines?

We live in a crowded, fast-moving world, and disease travels easily. The data are clear: failure to immunize a child comes with a much more formidable risk—leaving children vulnerable to contracting a potentially debilitating or lethal illness. Some children are too sick or too young to receive inoculations, so they remain at risk. If those children or other unvaccinated kids come into contact with someone else who was not protected against certain microbes, that can set off a wave of disease such as the measles outbreak in the U.S. in the summer of 2017. Maladies that have become uncommon, such as polio and measles, can also quickly reappear if we stop vaccinating against them, particularly when they are unintentionally imported across geographic borders. The 2015 measles outbreak that rippled through the U.S., for example, had genetic markers that suggest it came from an overseas traveler. Protecting kids actually helps protect everyone.