California's fight to vaccinate 95 percent of its population against measles --thereby gaining "herd immunity" against this very contagious disease -- is being thwarted by doctors that continue granting medical exemptions preventing children from being vaccinated with the measles vaccine MMR.

Measles is one of the most contagious diseases as California found to its sorrow in 2014 when a single person with measles visited Disneyland. This lone carrier infected 131 Californians and was fueled by low vaccination rates, according to doctors.

California legislators tightened the state’s vaccination laws over the next few months. They barred parents from opting out of vaccines because of their personal beliefs.

Now, parents in California can excuse children from vaccines only if their child has a medical reason not to be vaccinated. But even this exemption is now being abused wholesale as five California doctors are under investigation over student vaccine exemptions.

These doctors are among a number who wrote suspect medical exemptions from vaccines. State authorities have accused some physicians of writing exemptions because a child has asthma or psoriasis. They note that while vaccination rates have gone up, so have medical exemption rates.

California legislators will likely pass a bill forcing the state to track each medical exemption in an attempt to crack down on fraudulent vaccination exemptions. Public health officials say the state needs to maintain high vaccination rates because measles has again become a major problem in other parts of the world despite announced as eradicated in the year 2000.

On Thursday, the California Medical Board (CMB) announced the investigation of two more doctors over allegations they signed unnecessary vaccine exemptions for students.

Sonya Yanchuk, age 1, sits in her mother Nadia Yanchuk's lap as nurse Mariana Gonchara (L) administers a measles vaccine shot in health clinic № 10 on May 15, 2019 in Kiev, Ukraine. Brendan Hoffman/Getty Images

Being investigated are Sacramento-area physicians Kelly Sutton and Michael Fielding Allen. The other doctors under state investigation for the same offense are Bob Sears of Orange County; Ron Kennedy of Santa Rosa and Kenneth Stoller of San Francisco.

CMB alleges these five doctors wrote a third of all exemptions revealed in an investigative report on exemptions in eight Bay Area school districts.

CMB in June requested a court order to enforce a subpoena filed to Kaiser Permanente. The subpoena seeks to order Kaiser Permanente to turn over copies of medical exemptions signed by Sutton and Allen.

CMB recently won a similar court battle to review exemptions signed by Kennedy. In June, CMB accused Sears of writing unnecessary exemptions. His license has been on probation since last year for the same reason.