VACCINE RESOURCE COMMITTEE

of the Edmonton Zone Medical Staff Association

View Full List of Vaccine Questions Here

measles, mumps, rubella and varicella Vaccine Question #6:

What is the difference between wild-type and vaccine strains of measles?

Vaccine-strain measles virus can be recovered from a nasopharyngeal (i.e. deep nose) swab from a person who has recently received the MMR vaccine, but this does not mean that this person has measles. Vaccine-strain measles is not transmitted from person-to-person and does not cause the complications that frequently result from wild-type measles infection.

Determining whether a fever and rash are due to vaccine-associated symptoms or wild-type measles is extremely important from a public health perspective. Measles cases are tracked closely by epidemiologists and other public health officials. People with measles must be kept isolated from others, and people they have been in contact with require additional isolation and post-exposure prophylaxis if they are not immune to measles.

Measles vaccine is a weakened strain of the virus that creates a very strong protective immune response with much less risk than measles virus infection. About one-two weeks after initial immunization with MMR vaccine, approximately 5% of immunized children experience malaise and fever (with or without rash). Adverse reactions are less frequent after the second dose of vaccine. This is not a public health issue because the vaccine strain is not transmitted person to person and does not cause the severe complications of measles virus infection.  If someone develops a possible measles-like illness after vaccination, swabs can detect whether there is vaccine-strain measles as a cause, which is a very rare occurrence.

The measles vaccine is a live virus vaccine, the attenuated vaccine strain does replicate within a subset of immune cells after vaccination, and as a result it can be detected in nasopharyngeal samples, just like wild-type measles. Real-time PCR-based techniques that can rapidly distinguish vaccine-strain measles from wild-type measles can be extremely helpful in quickly identifying infected children in an outbreak setting without needlessly quarantining uninfected children. This type of testing is not routinely available in reference laboratories; however, so physicians who having difficulty distinguishing between vaccine-associated symptoms and wild type measles infection should contact their local public health department for information on testing options.

Source:

American Society for Microbiology: Measles Vaccination and Infection: Questions and Misconceptions