The Democratic governor quietly acted less than an hour after lawmakers sent him changes he demanded as a condition of approving the bills, even as protesters outside his office chanted for him to veto the measures. 

Despite protesters' outrcry, the bill is widely regarded by health advocates as a necessary step to keep vaccination rates high enough to sustain herd immunity, which is threatened in the US by poor vaccination rates and measles outbreaks. 

Up until now, a handful of doctors have circumvented even California's relatively strict state laws, which allow only medical exemptions, by selling slips that claimed children were exempt for 'allergies' or 'family history.'

Legislators approved the changes as protests by hundreds of emotional opponents boiled over, with dissenters delaying Senate debate for nearly two hours by shouting and pounding on walls and doors.