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Alyssa Hernandez often worries about her 2-year-old son, Noah, when they leave their Sacramento, California home. The little boy, who had a liver transplant when he was 6 months old, cannot get vaccinated against a number of diseases, including highly contagious measles, because his immune system is suppressed due to the transplant.

“I’m scared to take him out,” Hernandez said. “I’m scared to have him go to school, because you don’t know what’s around.”

As parts of the U.S. experience some of the worst measles outbreaks in years — largely due to parents who don't vaccinate their children — that fear is understandable. The World Health Organization has ranked resistance to vaccinations as one of the top 10 threats to public health in 2019.

In California, an increasing number of parents are finding ways to avoid immunizations for their children, with the the surprising assistance of medical doctors, a recent study found.

Noah Hernandez on the day he was born in 2017. Noah was born with a serious liver disease and had a transplant when he was six months old. Alyssa Alimena

Despite a California law passed after the 2015 Disneyland measles outbreak that got rid of the "personal belief" vaccine exemptions for children entering school, pockets of low vaccination rates have developed in the state. A number of counties are reporting rates lower than 90 percent, the number needed to achieve herd immunity, which occurs when enough people are vaccinated against an infectious disease to protect others in the community who are not.

That is likely due to a surge in medical exemptions— a doctor's note allowing a child to go to school without the required vaccinations, according to research published October in the journal Pediatrics. In some schools the medical exemption rate is as high as 20 percent, according to the California Department of Public Health.

We’ve known people to call some of these doctors who don’t even require an exam or evaluation. You pay the money and they will give you the medical exemption.

California parents opposed to vaccinations have found a way around the law, with help from doctors willing to write medical exemptions for kids who don’t need them. The Pediatrics study found some medical exemptions were being given with inadequate justification, such as “family history of allergies and family history of autoimmune disorders.”

'Selling' medical exemptions

The study revealed that the exemptions were being generated by doctors who don’t normally treat children and were “coming from physicians who were charging fees.”

Noah, seen here the day after his transplant. Because Noah has to take immunosuppressant medications to help his body accept his transplanted liver, he can't get vaccinated against many infectious diseases. Alyssa Alimena

While some doctors appeared to charge a single fee for a permanent exemption, the researchers discovered that some were giving temporary exemptions, say for three months at a time, and then charging a new fee for each additional exemption.

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State Sen. Dr. Richard Pan, a pediatrician who represents California’s 6th district, blames a “handful of doctors” for eroding the impact of the vaccination law through the exploitation of a loophole.

“To grant a medical exemption, you don’t actually have to give the very specific reason,” Pan explained. “You have to cite which vaccines you’re giving the exemption for, and the period of time the exemption actually lasts. We’ve known people to call some of these doctors who don’t even require an exam or evaluation. You pay the money and they will give you the medical exemption.”

Even when specific reasons are provided in the medical exemption, the conditions cited don’t jibe with the science on vaccines, said Dr. Erica Pan, interim Alameda County Health Officer and a clinician professor of pediatric infectious diseases at the University of California, San Francisco. Pan is not related to Dr. Richard Pan.

When vaccination rates fall, children like Noah Hernandez who can’t get their shots either because of compromised immune systems or young age are at risk, she said. In the case of whooping cough, “infants don’t get fully vaccinated until after infancy,” Pan said. “They are at the highest risk for hospitalization or even deaths. They need for the general community to be protected.”

Noah at 18 months old. Because of his liver transplant, he is vulnerable to infectious diseases like measles or chickenpox. “I’m scared to have him go to school, because you don’t know what’s around,” said Noah's mother, Alyssa Hernandez. Alyssa Alimena

Richard Pan doesn’t mince his words when he talks about the doctors “selling” exemptions: “The thing we need to recognize is that many of the physicians who have broken their oath, they’re doing it for their own pocketbook. It’s not based on their expertise. They’re monetizing their license,” he told NBC News.

An internet search found a number of physician sites aimed at parents opposed to vaccinations.

One listed “vaccine choice doctors by state,” while others invited parents to learn more about exemptions.

A site run by Dr. Kelly Sutton claims: “Finally…The Step-by-step Program to Help Protect Your Child from the ‘One Size Fits All’ California Vaccine Mandate!”

The site offers parents “the tools and knowledge you need to protect your rights as a parent to choose the healthcare of your children,” and advertises Sutton’s “EXCLUSIVE program for just $97.”

In a video on the website, Sutton asks, “With passage of SB277 into law are you overcome with fear that your children will be forcibly vaccinated?”

Sutton maintains she isn’t selling exemptions, but instead is giving a more complete visit that includes a detailed exam along with vaccine education.

“Due to pervasive underreporting of vaccine reactions in the United States, a physician should not presume that a previous vaccine reaction or injury has been documented in a patient’s chart,” Sutton said in an email to NBC News. “Accordingly, it is incumbent upon physicians to thoroughly investigate these matters.”

Vaccine and public health experts are disturbed by the new developments in California.

“When the legislature closed the [personal belief exemption] loophole I was very excited,” said Dr. Peter Hotez, co-director of the Texas Children’s Hospital Center for Vaccine Development and author of “Vaccines Did Not Cause Rachel’s Autism: My Journey as a Vaccine Scientist, Pediatrician, and Autism Dad.”

“One thing that never occurred to me was that physicians would write phony medical exemptions,” Hotez said. “That’s very scary.”

A solution may lie with state medical boards, Hotez said.

The Medical Board of California looks into allegations that physicians are inappropriately issuing exemptions for required vaccinations, said spokesperson Carlos Villatoro, in an email.

“One of the challenges that the board is facing with these types of complaints is that oftentimes the patient’s parents do not approach the board with the complaint because they are seeking the medical exemption," said Villatoro. "In these instances, the board must be able to prove good cause to issue a subpoena for the medical records.”

The very idea that some physicians could be “selling” exemptions to anti-vax parents, “makes me feel embarrassed for the profession,” said Dr. Albert Wu, an internist and professor of health policy and management at the Johns Hopkins Bloomberg School of Public Health.

“Before there was a vaccine, there were 3 to 4 million people infected with measles, 50,000 hospitalized, 500 who died and many more disabled from conditions like encephalitis,” said Wu.