K. E. Stewien, V. Barbosa, and C. P. Rosenburg, “ Immunity levels to poliomyelitis in a sample of school children from the city of S. Paulo, Brazil ”, Rev. saúde pública, vol. 11, no. 2, pp. 270–278, Jun. 1977, doi: 10.1590/S0034-89101977000200011.