Four months after a measles outbreak at Disneyland, state legislators seeking to tighten immunisation laws across the country are running the gauntlet of anti-vaccination activists who have bombarded them with emails and phone calls, heckled them at public meetings, harassed their staff, organized noisy marches and vilified them on social media.

Three states blindsided by the activists' sheer energy – Oregon, Washington and North Carolina – have either pulled back or killed bills that would have ended a non-specific "personal belief" exemption for parents who don't want to vaccinate their children.

Now the battleground is California, which bore the brunt of the measles outbreak at the beginning of the year and saw school closures, extraordinary quarantine measures and a vigorous public debate lamenting the fact that a disease declared eradicated 15 years ago is once again a public health threat.

A health committee meeting in Sacramento, the state capital, on Wednesday turned into a tense showdown between lawmakers seeking to argue that the science is unequivocally on the side of universal vaccination, and activists accusing them of being in the pocket of unscrupulous big pharmaceutical companies.

One activist, Terry Roark, told the state senate committee her child had died from a vaccine and feared others could be next if parents lost the right to decide what was in their best interests.

"Innocent people will die," she said tearfully. "Innocent children will be killed."

The meeting degenerated at points into yelling and screaming, and two activists were removed.