The backlash against parents who don’t vaccinate their children is reaching the halls of Congress as well as state legislatures.

This week, California senators Barbara Boxer and Dianne Feinstein asked their state’s secretary of health and human services to reconsider public policy allowing vaccine exemptions for non-medical reasons.

“While a small number of children cannot be vaccinated due to an underlying medical condition, we believe there should be no such thing as a philosophical or personal belief exemption since everyone uses public spaces,” Boxer and Feinstein wrote. “As we have learned in the past month, parents who refuse to vaccinate their children not only put their own family at risk, but they also endanger other families.”

Their letter came a day after two California legislators announced a bill to eliminate non-medical exemptions from school vaccination requirements.

“We are offering legislation that will abolish the personal belief exemption that currently allows children who have not received vaccinations needed to protect the public health to enroll in our schools,” said co-author Richard Pan (D-Elk Grove) at a news conference. “As a pediatrician, I have witnessed children suffering lifelong injury or death from vaccine-preventable infection. This does not have to happen.”

Meanwhile, the University of California announced today that beginning in 2017 all incoming students at its 10 campuses must be vaccinated against measles, mumps, rubella, chicken pox, meningococcus, tetanus, and whooping cough.

Other States Are Tightening Vaccine Rules

The National Conference of State Legislatures (NCSL) noted other states are taking similar steps to tighten vaccination exemptions.

Parents in Washington and Vermont who want to exempt their children from vaccination must now get approval from a health professional.

Oregon requires parents to get a signature from their primary care doctor. Oregon parents can also attend an online education session on the benefits and risks of vaccination.

Colorado requires schools to collect and publish information on vaccination and exemption rates. California already publishes childcare and school immunization levels.

“If there is a good side to this current measles outbreak, it is that it forces parents and lawmakers to take another look at the question of vaccination and come up with better policies to protect our children,” said Diane Peterson, associate director for immunization projects at the Immunization Action Coalition. “We are seeing that letting kids go without vaccines and depending on the herd to protect them doesn’t work.”

Read more: Backlash Grows Against Vaccine Opponents »

What The Current Laws Require

More than 100 people in 14 states and Mexico have contracted measles in recent weeks. The outbreak began at two Disney resorts in Southern California.

All 50 states require children attending public and private schools to be vaccinated against measles and other diseases. All allow exemptions for children who cannot be vaccinated for medical reasons. The most common medical reasons are diseases such as HIV and cancer or drug treatments such as high-dose steroids that weaken the immune system

Pan wants California to become the third state to require all children be vaccinated unless they have a valid medical reason. According to the NCSL, only Mississippi and West Virginia now have the strict medical exemption policy.

“This [California] proposal is terrific,” Dr. James Cherry, a professor of pediatrics at the University of California, Los Angeles told Healthline. “The states with the toughest laws have the highest immunization rates. The higher the immunization rate, the better we protect infants and others who cannot be vaccinated.”

For now, 48 states allow vaccination exemptions for religious reasons. Twenty states allow exemptions for parents who object to vaccination because of any personal beliefs.

In an email to Healthline, the California Department of Public Health said about 0.5 percent of California kindergartners are not vaccinated because of their parent’s religious beliefs.

Another 2.5 percent of kindergartners are not vaccinated because parents used the personal belief exemption.

Read more: The Truth About the MMR Vaccine »

The personal belief exemption requires a health practitioner to counsel parents about vaccination. The counseling requirement was added in 2013 in a bill authored by Pan, who was then a state assemblyman.

Cherry cautioned that some parents can, and do, ignore sound medical advice. And the required counseling can be provided by doctors who reject conventional medical treatments.

Vaccine Opponents Tend to Live in Clusters

The problem is children who are not vaccinated tend to live and attend school in clusters. The result is that community immunity, sometimes called herd immunity, breaks down. The lower the vaccination rate, the quicker measles and other infectious diseases can spread.

A recent analysis by the Los Angeles Times found that some Los Angeles preschool centers have measles vaccination rates as low as 51 percent. Vaccination rates were lower in private institutions and in higher income areas. Vaccination rates were higher in public schools and in lower income areas.

Kaiser Permanente found similar patterns in Northern California. Families with children who are not vaccinated tend to cluster together.

School immunization reports showed that 87 percent of Los Angeles-area preschoolers in private centers had all their required vaccines. Public preschools had a 90 percent vaccination rate, and Head Start centers had a 96 percent vaccination rate.

Medical experts say at least 92 percent of children must be immunized to keep diseases such as measles from spreading quickly. Children are best protected if at least 95 percent of people are vaccinated.

Read More: What Does Measles Look Like? »