SACRAMENTO — California medical regulators have been flooded with complaints about doctors accused of writing improper vaccine exemptions for children, with at least 186 accusations filed in the last four years.

But a large number of those complaints— more than 40% so far — have been closed. Only one doctor has been disciplined.

The Medical Board of California and advocates of tighter vaccine rules say that’s because one group has stymied many investigations: uncooperative parents.

Parents who seek waivers to the state’s mandatory vaccine rules for their children have often blocked the board by refusing to release medical records — denying the state evidence it needs to build a case against a doctor.

“The parents are in cahoots with the doctor who is engaging in unethical behavior,” said Democratic state Sen. Richard Pan of Sacramento, a pediatrician and proponent of stricter vaccine regulations. The board “can’t investigate. They don’t have the ability to.”

Under federal law, parents can decline to release their children’s medical records unless the state goes to court and gets a subpoena.

Public health advocates say the situation highlights a loophole in California’s vaccine law, allowing unscrupulous doctors to sell fake or inappropriate exemptions without much fear of consequences.

Pan is carrying legislation, SB276, to tighten vaccine rules in the aftermath of nationwide measles outbreaks that have coincided with an increase in the number of children receiving medical exemptions to vaccines.

Much of the debate has focused on a provision in Pan’s bill to require the California Department of Public Health to review vaccine exemptions if a single school has an immunization rate below 95% — the threshold for “herd immunity ” — or if a doctor issues more than five in a year. The department could deny exemptions it finds improper.

But the bill’s sharpest teeth could involve a less debated section that would enable the medical board to investigate more doctors — even without the cooperation of parents.

That provision would require parents who seek exemptions to sign a form that gives the state permission, in advance, to access their children’s medical records related to an exemption if the medical board needs them. Those forms would be stored in a state database.

Parents who oppose mandatory vaccination call the bill an assault on medical privacy rights. They’ve stormed the state Capitol this summer to protest the effort.

Allison Serrao, a mother of three who lives in Orange County, said her son received an exemption after he had a life-threatening allergic reaction to a vaccine. She worries she will be required to sign over access to her son’s medical records to keep that exemption.

“Who’s to say they won’t use that information for something else in the future?” Serrao asked. “It’s really scary to me as a parent. It crosses a lot of lines.”

Attorneys working with parents opposed to the bill argue it would fundamentally change the doctor-patient relationship by coercing parents to open their children’s medical records.

Leigh Dundas, an attorney with Advocates for Physicians’ Rights, a group opposed to the bill, said the notion that investigations into bad doctors are being stymied by parents is a “fictional problem.”

If the state wants patient records, the medical board can go to court for a subpoena, she said.

“If they can’t get the records, it’s because they don’t have good enough cause, and that’s as it should be,” Dundas said.

Supporters of the bill stress that the form, to be created by the Department of Public Health, would comply with state and federal privacy laws. The department already tracks a host of other sensitive information related to public health, such as sexually transmitted diseases.

They said the medical board has spoken openly about barriers to its investigations.

In a May report, the board’s staff said that “for most of these medical exemption cases, the board does not have enough evidence to subpoena the medical records.” The state has also sent demand letters to an unknown number of parents whose children have exemptions, attempting to secure records.

Vaccine exemption complaints surge 2016: 12 2017: 43 2018: 41 2019 (as of June 30): 89 Total: 186 Outcome of cases Closed: 77 Pending: 107 Accusation filed: 1 Disciplinary action taken: 1 Source: Medical Board of California

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A spokesman said the board can’t provide a more detailed breakdown of the reasons it has closed investigations; complaints are confidential unless an accusation is filed.

Of the 186 complaints the board has received in the last four years, 77 are closed and 107 remained pending as of July 1. The remaining two involve cases against the lone doctor who has been disciplined.

Leah Russin, a parent from Palo Alto and director of the pro-vaccination group Vaccinate California, said it’s clear the state doesn’t have the tools it needs to crack down on the “side hustles” of a handful of doctors who sell exemptions and advertise their services to vaccine-wary parents online, sometimes for as much as $1,200.

“We’re not trying to put any individual patient or child under the microscope,” Russin said. “We’re trying to get doctors to behave ethically.”

Typically, when the medical board investigates a doctor, it has the cooperation of patients who believe they’ve received improper care. But in the case of vaccine exemptions, it often receives complaints from school nurses or public health officials.

Pan said his bill would help solve the problem by putting exemption records directly in the hands of public health officials, rather than school administrators, who can’t release the full information.

California lawmakers tightened vaccination rules for children entering kindergarten in 2015, when they passed a bill by Pan that removed the state’s former personal belief exemption. The Legislature approved the bill after a measles outbreak traced to unvaccinated visitors at Disneyland sickened at least 147 people.

Since then, the number of children with medical exemptions for vaccines for infectious diseases — such as measles, mumps, whooping cough and rubella — has more than quadrupled.

Bill supporters say that’s concerning because many of those exemptions are concentrated in the same schools, creating pockets of weakened immunity that are at risk for disease outbreak.

The medical board has gone to court with subpoenas for patient records in a handful of cases where a doctor is accused of granting an inappropriate exemption. Several of those cases are now in court, but they represent a fraction of the 186 complaints.

The only doctor disciplined in the past four years was Bob Sears, a pediatrician from Dana Point (Orange County). He was put on probation in 2018 after the board found he had given a toddler an exemption without enough information. He denied wrongdoing.

Sears also faces a second accusation, alleging he gave a pair of siblings childhood-long exemptions for invalid reasons, such as one child’s psoriasis and the assertion that relatives had a history of autoimmune disorders.

In both cases, Pan said, the board was able to investigate Sears because one of the parents objected to the exemption and released the records.

Dr. Dean Blumberg, a supporter of the bill and chief of pediatric infectious diseases at UC Davis Children’s Hospital, said that because the medical board doesn’t have a way to investigate most cases, there are probably many instances in which children are improperly being granted vaccine exemptions.

“That suggests to me that the cases that have been made public are the tip of the iceberg,” he said.

Dustin Gardiner is a San Francisco Chronicle staff writer. Email: dustin.gardiner@sfchronicle.com Twitter: @dustingardiner