That is called scientific theory. Vaccination is trying to replicate the natural process in which the body builds an immunity to viruses i.e. by recognizing the virus, and creating antibodies (However antibodies is just one way the body fights viruses.). Whether vaccination does actually successfully replicate this immunity passed on through the natural process in reality needs to be studied.
A randomized double blind study is required to test this theory. Many times theory does not correspond to real life when put to the test. Can you show a randomized double blind study on measles (as well as mumps and rubella) and mmr? I want to see the data and results for it. Not some relative %, or a review of the study by somebody else. You'll be surprised that when you do look into a positive review of a study, and find that the data does not match or are blown out of proportion. All I see in those two links are the typical rhetoric including the ad hominem of those who decide not to vaccinate.
The burden of proof is on the vaccination promoters (e.g. the pharmaceutical) to prove to the 'asshole' parents that mmr does work against measles, mumps and rubella.
"Efficacy of measles, mumps, and rubella vaccines was established in a series of double-blind controlled field trials which demonstrated a high degree of protective efficacy afforded by the individual vaccine components. 7-12
These studies also established that seroconversion in response to vaccination against measles, mumps, and rubella paralleled protection from these diseases. 13-15"
7. Hilleman, M.R.; Buynak, E.B.; Weibel, R.E.; et al: Development and Evaluation of the Moraten Measles Virus Vaccine, JAMA 206(3): 587-590, 1968.
8. Weibel, R.E.; Stokes, J.; Buynak, E.B.; et al: Live, Attenuated Mumps Virus Vaccine 3. Clinical and Serologic Aspects in a Field Evaluation, N. Engl. J. Med. 276: 245-251, 1967.
9. Hilleman, M.R.; Weibel, R.E.; Buynak, E.B.; et al: Live, Attenuated Mumps Virus Vaccine 4. Protective Efficacy as Measured in a Field Evaluation, N. Engl. J. Med. 276: 252-258, 1967.
10. Cutts, F.T.; Henderson, R.H.; Clements, C.J.; et al: Principles of measles control, Bull WHO 69(1): 1-7, 1991.
11. Weibel, R.E.; Buynak, E.B.; Stokes, J.; et al: Evaluation Of Live Attenuated Mumps Virus Vaccine, Strain Jeryl Lynn, First International Conference on Vaccines Against Viral and Rickettsial Diseases of Man, World Health Organization, No. 147, May 1967.
12. Leibhaber, H.; Ingalls, T.H.; LeBouvier, G.L.; et al: Vaccination With RA 27/3 Rubella Vaccine, Am. J. Dis. Child. 123: 133-136, February 1972.
13. Rosen, L.: Hemagglutination and Hemagglutination-Inhibition with Measles Virus, Virology 13: 139-141, January 1961.
14. Brown, G.C.; et al: Fluorescent-Antibody Marker for Vaccine-Induced Rubella Antibodies, Infection and Immunity 2(4): 360-363, 1970.
15. Buynak, E.B.; et al: Live Attenuated Mumps Virus Vaccine 1. Vaccine Development, Proceedings of the Society for Experimental Biology and Medicine, 123: 768-775, 1966.
A couple of extra studies I just found:
Double blind, placebo-controlled:
A Field Trial with a Live Measles-Mumps- Rubella Vaccine
Simultaneous Administration of Measles-Mumps-Rubella Vaccine With Booster Doses of Diphtheria-Tetanus-Pertussis and Poliovirus Vaccines
But the important ones are in the numbered list above. You'll probably have to go to a medical library or make a subscription online if you want to read them all fully.
And once you get to that point, it would be worth repeating this research exercise with the further studies that indicate one MMR booster dose is required to achieve near 100% immunity.