A polio case identified in New York last month is "just the very, very tip of the iceberg" and an indication there "must be several hundred cases in the community circulating," a senior official with the US Centers for Disease Control and Prevention told CNN on Wednesday.  

The case was found In Rockland County, which has a stunningly low polio vaccination rate. Dr. José Romero, director of the CDC's National Center for Immunization and Respiratory Diseases, noted that the majority of people with polio don't have symptoms and so can spread the virus without knowing it. 

"There are a number of individuals in the community that have been infected with poliovirus. They are shedding the virus," he said. "The spread is always a possibility because the spread is going to be silent." 

A team of CDC disease detectives traveled last week from agency headquarters in Atlanta to Rockland County, and they are "quite nervous" that polio "could mushroom out of control very quickly and we could have a crisis on our hands," said a community health leader who has met with the team.  

"They are -- what is the opposite of cautiously optimistic?" said another community leader, an expert in vaccine education, who has also met with the CDC team in Rockland County. Both leaders requested anonymity because they are not authorized to speak publicly. 

Polio can cause incurable paralysis and death, but most people in the US are protected, thanks to vaccination. Others, however, may be vulnerable to the virus for a variety of reasons. 

Unvaccinated and undervaccinated people are vulnerable, and polio vaccination rates in Rockland County and neighboring Orange County, just north of New York City, are about 60%, compared with 93% nationwide. Immune-compromised people can be vulnerable even if they are fully vaccinated. 

Romero said the CDC is considering a variety of options to protect people from polio, including offering children in the area an extra shot of the vaccine, as UK health authorities are doing now in London, or recommending extra doses to certain groups of adults.  

"We're looking into all aspects of how to deal with this. At this point, we don't have a definitive answer," he said.