There are two circumstances when a third dose of MMR is recommended. ACIP recommends that women of childbearing age who have received 2 doses of rubella-containing vaccine and have rubella serum IgG levels that are not clearly positive should receive 1 additional dose of MMR vaccine (maximum of 3 doses). Further testing for serologic evidence of rubella immunity is not recommended. MMR should not be administered to a pregnant woman.
In 2018, ACIP published guidance for MMR vaccination of people at increased risk for acquiring mumps during an outbreak. People previously vaccinated with 2 doses of a mumps virus–containing vaccine who are identified by public health authorities as being part of a group or population at increased risk for acquiring mumps because of an outbreak should receive a third dose of a mumps virus–containing vaccine (MMR or MMRV) to improve protection against mumps disease and related complications. More information about this recommendation is available at www.cdc.gov/mmwr/volumes/67/wr/pdfs/mm6701a7-H.pdf.