MMR Vaccine Composition: Live Attenuated Viruses, Not Fragments
The MMR vaccine contains live attenuated (weakened) viruses, not viral fragments. This is a critical distinction that affects how the vaccine works and the immune response it generates.
Vaccine Composition and Mechanism
- MMR is a live virus vaccine preparation containing attenuated strains of measles, mumps, and rubella viruses 1, 2
- The vaccine viruses are grown in chick embryo fibroblasts and remain viable after reconstitution 1, 3
- These live attenuated viruses produce a subclinical, noncommunicable infection that stimulates immunity 1
- The vaccine must be stored at 2-8°C and protected from light specifically because the live viruses can be inactivated by improper storage 1
How Live Virus Vaccines Differ from Fragment-Based Vaccines
- Live attenuated vaccines replicate in the host to produce immunity, which is fundamentally different from inactivated or subunit vaccines that contain only viral fragments 1, 2
- Vaccine virus replication and stimulation of immunity typically occur 1-2 weeks after vaccination 1
- The live viruses can produce measurable antibody responses in >97% of susceptible persons through active viral replication 1
Clinical Evidence of Live Virus Activity
- Approximately 5% of vaccinated persons develop a transient measles-like rash 7-10 days after vaccination, demonstrating active viral replication 4
- The rubella vaccine virus can be excreted in breast milk and transmitted to infants (though remaining asymptomatic), further confirming the presence of live virus 1
- Enhanced replication of vaccine viruses may occur in immunocompromised persons, which is why MMR is contraindicated in severely immunosuppressed individuals 1
Important Safety Implications
- Because these are live viruses, MMR vaccine should not be administered to severely immunocompromised persons due to risk of vaccine-associated infection 1
- The vaccine is contraindicated in pregnancy due to theoretical risk of fetal infection from live virus 1
- Recent administration of immune globulins can inhibit the immune response to the live vaccine viruses for 3 or more months 1
Common Pitfall: Confusing MMR with inactivated vaccines (like influenza or hepatitis A vaccines) that contain killed viruses or viral fragments. The live nature of MMR is essential to its high efficacy but also determines its contraindications and precautions 1.