Two Merck Scientists are suing Merck who has the patent for the MMR vaccine because they were told to replace the anti-bodies in their study with rabbit antibodies to make the vaccine appear effective.
Here is the court filing
You also have to remember then when mumps outbreaks happen they happen among the vaccinated
Recent Mumps Outbreaks in Vaccinated Populations
Mumps Makes a comeback even among the vaccinated
Harvard Mumps Outbreak: The Real Story Mainstream Media Won’t Tell You
They’ve also had to pull the mumps vaccine in certain countries because the Urabe strain was causing meningitis, now they use the Horatio strain.
Outbreak of Aseptic Meningitis associated with Mass Vaccination with a Urabe-containing Measles-Mumps-Rubella Vaccine: Implications for Immunization Programs
UNEXPECTED PROTEIN FOUND IN MEASLES-MUMPS VACCINE