City and state health officials made the alarming announcement Friday that the polio virus has been detected in New York City’s sewage, indicating that it has been spreading locally for some time. The country’s first polio case in almost a decade was found in Rockland County, just north of New York City, in late July but wastewater analysis later showed the virus was circulating in the area weeks earlier. “For every one case of paralytic polio identified, hundreds more may be undetected,” New York State Health Commissioner Mary Bassett said Friday. Officials urged unvaccinated New Yorkers, especially children, to get a polio shot immediately. Most adults were vaccinated against polio as children and don’t need to get another shot, but around 14 percent of kids in New York are believed to be unvaccinated.