A city of nearly 7 million people in southern India has declared a "high alert" for polio after an active strain of the virus was found in samples of sewage water, an official said Wednesday.

A vaccination drive will be launched after tests revealed a vaccine-derived poliovirus in sewage samples taken in Hyderabad, the capital of Telangana state, Rajeshwar Tiwari, the state's top health official, told reporters. About 350,000 children ranging from 6 weeks to 3 years old will be vaccinated during the weeklong campaign, which will start on Monday.

Tiwari said that "a high alert" was declared in 24 sections of Hyderabad that had been identified as "most-sensitive areas."

India fought a decades-long battle to eradicate polio in the country, with the federal and state governments, the World Health Organization, UNICEF and several volunteer groups joining hands to fight the crippling disease. India was formally declared polio-free in 2014. Its last case was detected in the eastern state of West Bengal in 2011.

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The virus strain was found last month in a water sample traced to a sewage treatment plant in the Amberpet section of Hyderabad, Tiwari said. It was detected during countrywide random tests of sewage water that have been held regularly since the disease was eradicated in India.

Hyderabad, whose population is about 6.8 million, is often referred to as "Cyberabad" because several international information technology giants, including Accenture, Microsoft, Verizon and Oracle, have their India headquarters in the city.

Tiwari sought to reassure city residents by saying that the vaccination drive would be repeated so that no child is affected. "There is no need to panic," he said. "As India was declared polio-free five years ago, tests are carried out regularly to ascertain if there are traces of polio virus in the environment."

State health authorities have decided to set up more than 750 vaccination centers in Hyderabad and another 122 in nearby Ranga Reddy district to ensure that all children are covered during the vaccination drive.

Apart from state government employees, volunteers from WHO, UNICEF and Rotary Club will also participate in the drive, Tiwari said.