Comparison of seroconversion rates with CAM-70 measles vaccine at different dosages

Objectives To compare seroconversion rates induced by Biken CAM-70 measles vaccines at different viral concentrations. Methods Healthy children aged 9 to 18 months from a primary health care uni t in Rio de Janeiro, Brazil, and whose guardians agreed with their particip ation, were randomly assigned to receive one of the following vaccine formu lations: 5,000, 1,000 or 200 CCID50 (50% Tissue Culture Infective Dose). Th e research team, participants, and data analysts were blinded to the type o f vaccine administered Pre- and post-vaccination antibody levels were asses sed through Plaque Reduction Neutralization Test. Two interim data analyses were planned to assess unequivocal evidence of the superiority of one of t he vaccine types. Results From 223 recruited children, 84% completed the whole course. Of the m, 79% were less than 10 months of age, and 93% did not show detectable mea sles antibodies in pre-vaccination serum. Seroconversion (four-fold increas e in antibody levels) in groups vaccinated with 5000, 1,000 or 200 CCID50, were 82%, 55%, and 37% (p<0.0000), respectively. Differences in the mean co ncentration of post-vaccination antibodies were also substantial and statis tically significant (p<0.000). Seroconversion rates (pooling data from all vaccine formulations) were 73% to children aged 10 months or more, and 53% in those below 10 months. Conclusions Vaccines with concentrations below 5,000 CCID50 did nor produce satisfactory seroconversion rates. The vaccine performance by age was cons istent with that seen in other studies using Biken CAM-70 strain in which a sizable proportion of 9-month-old children failed to achieve full immunolo gical response.