Susceptibility to rubella infection in females at high risk. Immune protection associated to population density.

Susceptibility to rubella in 428 Mexican females of childbearing age from four sanitary areas confined to a Mexican State (Queretaro) was determined. Members of the group were residents of urban and rural communities and selected by random sampling. Anti-viral antibodies were determined by inhibition of haemagglutination. Concentration was expressed as International Units of IgG anti-rubella haemagglutinin (IU/ml). Antibody concentrations lower than 15.6 (IU/ml) were regarded as non-protective. The percentage of women immune-protected to rubella in the areas varied from 28.8 to 75.6 with an average of 61.9. The difference in percentages of immune-protected females within the areas was statistically significant (chi 2 = 48.26 and p < 0.001). Immune protection was associated to population density, with less protection in less populated areas. Our results differ from the reported values of a serosurvey performed in the same state one year before: immune protection 61.9% versus 79.96%, respectively.