Dear Editor: Epidemiological records of poliomyelitis in Mexico exist since 1937.¹ To fight against the poliovirus, Mexico manufactured its own Salk vaccine in 1956.² Since 1957 Dr. Sabin collaborated with Dr. Federico Gómez and Dr. Manuel Ramos Álvarez at Hospital Infantil de México. In 1958, a large-scale study was organized in four cities: Mexico City, Guadalajara, Monterrey and Puebla.³ In August 1959, the trivalent oral poliovirus vaccine (OPV) was tested in the city of Toluca, state of Mexico.⁴ By 1962 Mexico had manufactured its own OPV.

In the early 1960s, cases of vaccine associated paralytic polio (VAPP) were documented.⁵

In 1990, Mexico had seven cases of poliomyelitis, two are described here. On September 16^th, a four month old boy from Tecomán in the state of Colima, became sick. On the 25^th, a stool sample was taken and, on the 27^th, he was hospitalized with acute flaccid paralysis. On October 5^th, nerve conduction studies showed damage to the motor neurons of the anterior horn cells, affecting the left leg. The second patient, a 13 month old boy from Tomatlán in the state of Jalisco, on October 16^thbecame sick. On the 23^rd, stool samples were obtained; on October 24^th, because of paralysis of both legs Gillian-Barré considered. On November 7^th, nerve conduction studies showed damage of the motor neurons of the anterior horn cells. The families mentioned that the children were vaccinated. The question was: Were these cases due to vaccine or to wild polioviruses?

On 1990 Dr. Jesus Kumate was the head of the Health Ministry in Mexico. I was sent to the Centers for Disease Control and Prevention (CDC), Atlanta to diagnose the cause of the last two cases of flaccid paralysis. A polymerase chain reaction test (PCR) was performed with primers for the Sabin viruses and the design of genotype-specific PCR primer pairs of the VP1 nucleotide sequences from the PV3/9288/MEX89 were constructed at the CDC.⁶ Table I shows that the viruses were wild polio Leon.