On July 21 of this year, public health officials a case of paralytic polio in a young adult in Rockland County, New York, northwest of New York City—the first reported instance of polio in the US .

Although was caused by vaccine-derived poliovirus, the patient was unvaccinated and had not traveled to a country where polio is a risk. Genomic sequencing suggests that the virus has been circulating locally under the radar for up to a year, according to the Centers for Disease Control and Prevention (CDC).

Infection with poliovirus, an enterovirus that primarily infects the intestinal tract, is only about 25% of the time, typically causing a flu-like illness. Depending on the virus type, 1% to 5% of people with infections will develop meningitis. About 0.5% to 0.05% of those who are infected—1 in 200 to 1 in 2000 individuals—will develop paralysis after the virus infects the spinal cord, and a small proportion of them will die as a result of breathing problems, according to the CDC.