News & Views: What Should I Know About the New MMR Vaccine, PRIORIX?

Vaccines to prevent measles, mumps and rubella have been available in the U.S. since the 1960s. The three vaccines were combined to form the MMR vaccine in 1971, and since the late 1970s, only one version, known as MMR II, has been used throughout the U.S. Created by Maurice Hilleman and colleagues at Merck, the MMR vaccine has been responsible for dramatic decreases in the rates of infections and deaths associated with each of these diseases. Measles was declared eliminated from the U.S. in 2000, and rubella followed in 2004. While rates of mumps decreased by 99%, it has never been declared eliminated from the U.S. because it has yet to meet the criteria — the absence of continuous disease transmission for 12 months or more — for doing so.

However, in mid-2022, something changed. A new MMR vaccine, called PRIORIX, was approved for use in the U.S. PRIORIX is made by GlaxoSmithKline Biologicals. So, this month, we thought it would be useful to discuss how these vaccines compare.

The active ingredients

Both of the MMR vaccines contain live, weakened viruses that are similar, if not identical, and are delivered in much the same doses:

• Measles component: MMR II contains the modified Enders’ Edmonston strain (Moraten) and PRIORIX contains the Schwarz strain; however, these strains are identical at the level of nucleotide sequences.
• Mumps component: MMR II contains the “Jeryl Lynn” strain of mumps virus, which is composed of two lineages of the virus (JL1 and JL2). PRIORIX contains the RIT4385 strain, which contains only JL1. The JL1 components in the two vaccines are identical at the protein level.
• Rubella component: Both vaccines contain the Wistar RA 27/3 strain developed by Dr. Stanley Plotkin. They are identical at the nucleotide sequence level.

What else is in the vial?

Because in both cases, the vaccines contain live, weakened viruses, neither contains preservatives. Likewise, the viruses are grown in the same types of cells during the vaccine production process. This means that the measles and mumps components are grown in chick embryo cells, and the rubella component is grown in human fetal fibroblast cells. However, the MMR II rubella component is grown in WI-38 cells, and the PRIORIX rubella component is grown in MRC-5 cells. These are the same two cell lines used for all other vaccines grown in fetal cells, except COVID-19 adenovirus-based vaccines, which use fetal retinal cells. (Note: For more information about the history of fetal fibroblast cell lines refer to the resources section of this article.)

MMR II vaccine also contains small quantities of sugars as well as bovine-sourced products (gelatin and fetal bovine serum), human albumin, and neomycin.

PRIORIX vaccine also contains small quantities of sugars, lactose, and amino acids as well as bovine serum albumin, ovalbumin, and neomycin. PRIORIX does not contain gelatin.

Immunogenicity

Antibody responses following receipt of PRIORIX were similar to those found following receipt of MMR II after both the first and second doses. Likewise, persistence of antibody levels measured two years after vaccination were similar.

Safety

Side effect profiles were similar for both vaccines, including the rates and average timing of febrile seizures. Overall, for any MMR vaccine, febrile seizures are estimated to develop at a rate of 3.3 to 8.7 per 10,000 doses, and they are most likely to occur 6 to 11 days after vaccination.

A rare and temporary side effect of measles vaccine is thrombocytopenia, or decreased numbers of platelets. This condition results in decreased blood-clotting efficiency. Comparisons between the rates of thrombocytopenia following receipt of either MMR vaccine found no significant differences.

Recommended use

Because immune responses and safety profiles were similar for MMR II and PRIORIX, the two vaccines are recommended similarly and are considered interchangeable. Dose one of MMR vaccine is recommended at 12-15 months of age and dose two at 4-6 years of age.

Of note, PRIORIX is approved for subcutaneous injection. MMR II is approved for subcutaneous injection and, as of late February 2023, intramuscular injection.

As these diseases still commonly occur throughout the world, having a second supplier of the MMR vaccine in the U.S. is an important development because it gives us another tool for keeping children healthy.

Resources for more information

• Updated recommendations
• MMR II package insert
• Letter of approval for IM use of MMR II
• PRIORIX package insert
• Biographical resources about Maurice Hilleman and his scientific accomplishments, including creation of the MMR vaccine
• Developing mumps vaccine, from HILLEMAN: A Perilous Quest to Save the World’s Children
• Stanley Plotkin: Pioneering the use of fetal cells to make rubella vaccine
• More information about WI-38 and MRC-5 cells, and the history of use of fetal cells in vaccines
• Q&A about DNA, fetal cells and vaccines – Also available in Spanish.
• Webpage about fetal cells in vaccines