This is the third case of crippling type P1 polio in Pakistan this month. The two earlier cases were detected in militant stronghold areas of northwestern Swat valley, where unrest has prevented regular vaccination.
But the latest detection of polio in a middle class central Karachi neighborhood has stunned authorities, who have convened a special meeting on Friday to discuss ways to contain the disease.
"I can confirm detection of type P1 polio virus in an eight-month-old boy in Sadar town," doctor Mazhar Khamisani, project director for Sindh province Expanded Programme on Immunization, told AFP.
He said that samples were taken from the child on June 10 and sent to the National Institute of Health in capital Islamabad but the virus was not detected, after which a sample was sent to a World Health Organization lab in Geneva.
"We got the confirmation from WHO Geneva today," Khamisani said, adding initial investigations showed that the child, who is alive, was regularly vaccinated.
The total number of polio cases in Pakistan this year is now 18, officials said.
The World Health Organization recently listed Nigeria, India, Pakistan and Afghanistan as the only countries in the world where polio is endemic.