Cláudio S. Pannuti, Ricardo José Morello, José Cássio de Moraes, Suely Pires Curti, Ana Maria S. Afonso, Maria Cláudia Corrêa Camargo, and Vanda A. U. F. de Souza

• Instituto de Medicina Tropical de São PauloLIM-HC, Departamento de Moléstias Infecciosas e Parasitárias, Faculdade de Medicina
• Departamento de Microbiologia, Instituto de Ciências Biomédicas, Universidade de São Paulo
• Secretaria de Estado da Saúde de São Paulo, Centro de VigilÂncia Epidemiológica Professor Alexandre Vranjac
• Laboratório de Vírus Exantemáticos, Instituto Adolfo Lutz, São Paulo, Brazil4

Identification of Primary and Secondary measles vaccine Failures by Measurement of Immunoglobulin G Avidity in measles Cases during the 1997 São Paulo Epidemic. Clin Diagn Lab Immunol. 2004 January; 11(1): 119-122.

Despite almost universal use of measles vaccines in recent decades, epidemics of the disease continue to occur. Understanding the role of primary vaccine failure (failure to seroconvert after vaccination) and secondary vaccine failures (waning immunity after seroconversion) in measles epidemics is important for the evaluation of measles control programs in developing countries. After a measles epidemic in São Paulo, Brazil, 159 cases previously confirmed by detection of specific immunoglobulin M (IgM) antibodies were tested for IgG avidity, and a secondary immune response, defined by an IgG avidity index of at least 30%, was established in 30 of 159 (18.9%) patients. Among the 159 patients, 107 (67.3%) had not been vaccinated and 52 (32.7%) had received one or more doses of measles vaccine. Of the 107 unvaccinated patients, 104 (97.2%) showed a primary immune response, defined as an IgG avidity index of less than 30%. Among the 52 patients with documented vaccination, 25 (48.1%) showed a primary immune response and 27 (51.9%) showed a secondary immune response, thereby constituting a secondary vaccine failure. Primary vaccine failure was observed in 13 of 13 patients vaccinated prior to 1 year of age and in 43.5 and 12.5%, respectively, of patients receiving one or two doses after their first birthdays. These results provide evidence that measurement of IgG avidity can be used to distinguish between primary and secondary vaccine failures in vaccinated patients with measles; the method can also be a useful tool for the evaluation of measles control programs.

Almost all of the unvaccinated patients (over 97%) showed a primary immune response whereas those who had been vaccinated showed half that amount (at 48.1%). A secondary vaccine failure is represented by waning immunity (showing that vaccination tends to delay the disease occurrence to exposed individuals and does not offer true natural long-term immunity).