(NewsNation) — Poliovirus paralyzed and killed thousands of children in the 1950’s, but all but disappeared in the coming decades when a breakthrough vaccine was introduced to combat the virus. The virus appears to be making a small comeback.

Traces of the poliovirus showed up in the New York City sewer system, indicating there could be transmissions occurring in several New York counties.

The cases showing up in waste water are called vaccine-derived polio, but they are unlikely to result in national transmission because most people are vaccinated against the virus.

Polio was considered eradicated in the United States in 1979, and the U.S. was declared polio-free in 1994.

Infectious disease expert Dr. Amesh Adalja of Johns Hopkins University is alarmed, but not surprised traces of the virus are appearing in sewer waste. He said this poses an opportunity for the virus to circulate among unvaccinated communities.

“It really reflects the fact that their vaccination rates against polio are not high enough in the New York metro area,” Adalja said.

Only 80% of people in New York state are vaccinated against polio. A Rockland County resident was diagnosed with a paralytic case of polio on July 21, the first case in the United States since 2013. Only 60% of Rockland residents were vaccinated against polio as of Aug. 1.

In New York City, 86.2% of children between the ages of six months and five years old are vaccinated against polio.

“You have to remember that what keeps these infectious diseases from being a problem is these are our vaccines and if our vaccination numbers falter, that progress can erode,” Adalja said.

Most infected individuals show few symptoms, if any, but roughly 1 in every 200 cases can be paralytic.

“I don’t think this is going to become a widespread issue,” Adalja said. “But I think we should be prepared to see some paralytic cases resulting.”

Adalja advice is for anyone who is not vaccinated to get vaccinated. The shots are easily available at almost any doctor’s office.