When an adolescent is administered the measles vaccine, two types of immune mechanisms play a role in protecting against measles infection, namely cell-mediated and humoral immunity. Cell-mediated immunity plays a role in providing long-term immunity against measles and also ensuring recovery from the infection. Usually, the cell-mediated responses were better sustained and more effective in protecting against the measles virus than the antibody-mediated responses (Ovsyannikova, 2003). Generally, 15 to 25 year olds who receive at least 2 does of the MMR vaccine during infancy and childhood are well-protected against the infection and develop protective immune responses whenever the virus enters the body (Ovsyannikova, 2003).
Individuals that are highly sero-positive develop better lympho-proliferative responses to the virus than those who are less sero-positive (depends on the sero-conversion rates). The T-cells (mainly the CD-4 and CD-8 memory T-cells lymphocytes) secreted certain substances such as cytokine IFN-gamma and cytokine IL-4 when stimulated antigenically by the measles virus (Ovsyannikova, 2003). The secretion of IFN-gamma is usually much higher than IL-4. These cytokines activates the lymphocytes present in the body to protect against measles (Ovsyannikova, 2003). The ability to recognize the virus antigens was possible through stimulation by vaccination (NHS, 2004).
The occurrence of asymptomatic infection in individuals fully vaccinated against measles is relatively less (less than 10 %). Individuals who have developed asymptomatic infection have a significant rise in the certain specific antibodies against measles such as immunoglobulin IgG and IgM (Huiss, 1997). However, the IgM level rise is low compared to the IgG level. Usually, the IgG-1 subclass of antibodies is significantly raised after asymptomatic infection compared to other subclasses (Huiss, 1997). This antibody is usually present along with several other proteins including nucleoproteins and hemagglutinin proteins in the blood (Huiss, 1997). Once the vaccinated individual is exposed to the measles virus, it takes about 30 weeks for the antibody levels to reduce to normal. A secondary immune response to the virus can only occur if the IgG levels are present below a certain level known as ‘threshold level’ (Huiss, 1997).
Huiss, S., Damien, B., Schneider, F., & Muller, C. P. (1997). “Characteristics of asymptomatic secondary immune responses to measles virus in late convalescent donors.” Clin Exp Immunol, 109(3), 416-420. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?itool=abstractplus&db=pubmed&cmd=Retrieve&dopt=abstractplus&list_uids=9328115
NHS (2004). Vaccine development and testing – How does the vaccine work?, Retrieved on May 5, 2007, from NHS Web site: http://www.mmrthefacts.nhs.uk/library/howitworks.php
Ovsyannikova, I. G., Dhiman, N., Jacobson, R. M., Et al (2003). “Frequency of Measles Virus-Specific CD4+ and CD8+ T Cells in Subjects Seronegative or Highly Seropositive for Measles Vaccine.” Clin Diagn Lab Immunol, 10(3), pp. 411–416.