MMR Vaccine Composition: Live Attenuated Viruses, Not Fragments
The MMR vaccine contains live attenuated (weakened) viruses—not viral fragments—that replicate in the body and produce a subclinical infection, with vaccine virus capable of circulating in the blood (viremia) temporarily after vaccination. 1
Vaccine Composition and Mechanism
The MMR vaccine is a live virus preparation containing three attenuated viral strains: the measles virus (Moraten or Edmonston-Zagreb strain), mumps virus (Jeryl-Lynn strain in the US), and rubella virus (RA 27/3 strain). 1, 2
These are not killed viruses or viral fragments—they are weakened but replicating viruses that produce a mild, subclinical infection in the recipient to generate immunity. 1
The mumps component specifically "produces a subclinical, noncommunicable infection" in vaccinated individuals, demonstrating active viral replication. 1
What Circulates in Blood After Vaccination
Vaccine-induced viremia (virus in the bloodstream) does occur after MMR vaccination, particularly with the rubella component, though it is typically asymptomatic and non-transmissible. 1
The rubella vaccine protects against both clinical illness and viremia in nearly all instances after natural exposure, indicating that vaccine virus itself can cause transient viremia during the initial immune response. 1
Viremic reinfection following natural exposure may occur among vaccinated persons with low antibody levels, demonstrating that even after vaccination, virus (not just fragments) can circulate in blood under certain circumstances. 1
Clinical Implications
The live attenuated viruses replicate sufficiently to stimulate robust immune responses with seroconversion rates of 95-100% for each component. 2
This replication is essential for generating long-term immunity, as the immune system responds to the replicating virus similarly to natural infection, establishing immune memory. 1
The vaccine is supplied in lyophilized (freeze-dried) form and must be reconstituted before administration, with the live viruses remaining viable when properly stored at 2-8°C. 1
Important Distinction from Inactivated Vaccines
Inactivated (killed) measles vaccine was available from 1963-1967 but is no longer used because it provided inferior immunity and put recipients at risk for severe atypical measles upon natural exposure. 1
The current live attenuated vaccines replaced inactivated versions specifically because live virus provides superior, longer-lasting immunity comparable to natural infection. 1