A small but growing percentage of very young children aren't getting critical vaccines that protect them from infectious diseases such as measles and mumps, according to the Centers for Disease Control and Prevention (CDC).

In fact, the rates of unvaccinated toddlers appear to have quadrupled in the past 17 years. When CDC researchers looked at a 2017 national phone survey of families with toddlers, they found that 1.3 percent of the children born in 2015 hadn't gotten their recommended immunizations. In 2001, that was true of only 0.3 percent.

It's not completely clear from the new report why more young children aren't being vaccinated.

"Some children might be unvaccinated because of choices made by parents, whereas for others, lack of access to healthcare or health insurance may be factors,” the researchers write.

Indeed, some parents believe that vaccines cause problems, though there's no scientific evidence to back that up. Vaccinations that help protect children have saved hundreds of thousands of lives in the U.S. and prevented millions of hospitalizations over the years, the CDC says.

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But most kids in the U.S. receive the recommended routine vaccinations, the CDC says.

There's certainly plenty of confusing information online about vaccines for children these days. That can make it tough for parents to know what's true and what's not—and how to keep kids healthy. For instance, should shots be spaced out as much as possible so a child's immune systems doesn't get overwhelmed? 

Here, some common myths about vaccines for kids and what you need to know now. 

Myth 1: The MMR Vaccine Causes Autism

A study published in The Lancet in 1998 purported to link autism to the measles, mumps, and rubella (MMR) vaccine—which children typically receive at 12 months and 4 years of age—generating headlines and fear.

But that study has been widely and soundly debunked (and retracted), and an overwhelming majority of experts agree that vaccines for children aren’t a factor in autism. (In fact, Andrew Wakefield, the lead author of the study, was ultimately forbidden to practice medicine in the United Kingdom, in part because he falsified the study's findings.)

Since that erroneous study was published, numerous studies have found no connection between autism and vaccines, says Wendy Sue Swanson, M.D., a pediatrician at Seattle Children’s Hospital and a spokeswoman for the American Academy of Pediatrics. 

Swanson says she understands why people might think the two are linked. The MMR vaccine is administered at the same phase in children’s lives when they may first begin to show signs of autism spectrum disorder, such as not responding when called by name, seeming oversensitive to noise, and more.

Myth 2: It’s Safer to Space Out Kids' Vaccines

Each year the CDC recommends a vaccination schedule for children. (It has one for adults as well.) This tells healthcare providers and parents when youngsters should receive vaccines for a variety of infectious diseases, such as hepatitis B, rotavirus, human papillomavirus, and tetanus.

But some people worry that having so many vaccines in a short period of time early in life—children can get as many as 29 shots by age 6, not counting a yearly flu vaccine—may overwhelm kids' immune systems.

So some parents request that doctors delay vaccines or spread them out. In a 2012 survey of pediatricians by researchers in Colorado, nearly 93 percent said that in a typical month, at least one parent of a young child asked to stretch out the vaccine schedule.

That's unwise, says Wilbert van Panhuis, M.D., Ph.D., an assistant professor of epidemiology and biomedical informatics at the University of Pittsburgh. The CDC, he says, bases the schedule on disease risks and vaccine effectiveness at specific ages, and the way vaccines may interact with each other.

“To start mixing this up is really complicated and actually can be dangerous,” he says, in part because putting vaccines off can leave kids vulnerable to infectious diseases.

The MMR vaccine, for instance, is timed so that children receive it just as they’ve lost residual immunity from their mothers.

Measles, one of the diseases that the MMR protects against, is highly contagious. If a child isn't vaccinated and comes into contact with a person who has measles—or even walks into a room that the person recently left—he or she has a 9 in 10 chance of contracting it.

A child who has had both recommended doses of the MMR has only a 3 percent likelihood of developing the measles, according to the CDC.

Though it may feel like a lot of shots for young children, each contains only tiny amounts of dead or inactive viruses or bacteria. These help create antibodies, which spur the immune system to fight off illness-causing viruses and bacteria.

So stick to the CDC’s schedule if you can. But if your child misses a dose, don't worry; the CDC makes recommendations for how to catch up. Ask your pediatrician.

Myth 3: Vaccines Can Make Kids Sick

Has your child ever had a shot and then come down with a respiratory infection? This complaint is most common with the flu shot. Doctors often hear, “I got the flu shot and I got sick,” says Pedro Piedra, M.D., a professor of molecular virology and microbiology and of pediatrics at Baylor College of Medicine.

This is probably a coincidence, he says. Most influenza vaccines are delivered in the fall and early winter. “That’s when we have the highest rate of respiratory viruses circulating and causing mischief,” Piedra notes.

The flu shot can’t cause the flu (or another infection) but may spark some mild and temporary flu-like symptoms. This is actually a good sign, according to Swanson; it means the vaccine is effectively building immunity.

None of the other vaccines approved by the Food and Drug Administration (FDA) can cause the illnesses they protect against, either. But like the flu shot, they can cause temporary side effects, such as a mild fever or swelling at the injection site. (Get a rundown of these side effects here.)

If your child does experience a side effect or an adverse reaction to a vaccine, there's no need to avoid the next shot of that type on the schedule. A 2017 study found that the chance of a minor side effect recurring was less than 50 percent, and the chance of a serious reaction occurring again was less than 1 percent.

Myth 4: Vaccines Contain Harmful Chemicals

Some vaccinations for children contain substances that may sound worrisome, notably formaldehyde or traces of mercury. For example, certain formulations of vaccines for hepatitis, flu, meningococcal disease, and polio, among others, include formaldehyde.

But according to the FDA, the quantity of formaldehyde in a vaccine is smaller than the amount that's naturally produced by our bodies.

When it comes to mercury, ethylmercury—the type used in some flu vaccines—is quite different from methylmercury, which is highly toxic and found in some seafood, Piedra notes.

Ethylmercury leaves your body within a few days and poses no danger to children, he says. (But in response to consumer concerns, manufacturers are phasing out the use of mercury in most vaccines altogether.)

Unless you’re allergic to one or more ingredients in vaccines or can’t receive them for other health reasons (talk with your doctor to discuss any potential problems), the substances used to make them can’t hurt you.

Myth 5: Everyone Else Has Vaccinated Their Kids, So I Don't Need to

Theoretically, if everyone in your community is up to date on vaccinations, it will help protect your family. Here's why: For each infectious disease, a certain percentage of people in the area must be vaccinated to keep outbreaks at bay. (That percentage varies from condition to condition.) This is known as herd immunity.  

But if an outbreak does occur, those who aren't vaccinated are more likely to get sick. Which means you can't count on other people's immunity to protect your children if you don't vaccinate them.