The person is in their 20s and lives in Whitpain Township, the county health department said in a news release.
It wasn't clear when they tested positive, when they last traveled, or if they were vaccinated.
The department said it is coordinating with the Pennsylvania Department of Health and the U.S. Centers for Disease Control and Prevention.
The news has prompted health officials to warn all Montgomery County residents to take precautions, "given the possibility that Omicron may be more transmissible."
“Individuals who are not fully vaccinated remain at higher risk for contracting COVID-19, and for severe illness or death resulting from a COVID infection, than individuals who are vaccinated,” said Dr. Valerie A. Arkoosh, Chair of the Montgomery County Board of Commissioners.
“The best way to protect you and your family from the Omicron variant is to get vaccinated and get a booster dose if you are eligible.”
The County operates four vaccine clinics that administer the COVID-19 vaccine at no cost.
To view locations and hours, visit the county's official website.
“Getting vaccinated remains the best protection against COVID-19. Viruses normally mutate over time,” said Dr. Richard Lorraine, Medical Director for the Montgomery County Office of Public Health
“A large pool of unvaccinated individuals enables the virus to continue to spread rapidly, and potentially mutate more rapidly. Getting vaccinated is the best way to prevent the spread of the virus and slow the emergence of new variants.”
The first case in Pennsylvania involved a man in his 30s from Northwest Philadelphia, officials said on Dec. 3.
The first case in New Jersey was reported hours later, in a fully vaccinated Georgia woman who had recently returned from South Africa, health officials said.
The World Health Organization classified Omicron as a concern on Nov. 26, and the U.S. declared the same on Nov. 30.
The first nationwide case of Omicron was confirmed on Dec. 1 in California after a person returned to San Francisco from a trip to South Africa.
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