Gov. Kathy Hochul of New York said on Thursday that everyone who attended a recent anime convention in Manhattan should get tested for the coronavirus, after it was announced that an individual who tested positive for the Omicron variant in Minnesota had attended to the conference.

Ms. Hochul said the individual, a Minnesota resident who was vaccinated and experienced mild symptoms, had attended the Anime NYC 2021 convention at the Javits Center in Midtown Manhattan. She urged people who attended the event, which was held from Nov. 19 to Nov. 21, to get tested and said that health officials would be in contact with attendees. The convention hosted 53,000 attendees over three days, according to a spokesman for the Javits Center.

“We do anticipate there’ll be more cases,” Ms. Hochul said. “This is not cause for alarm; it was foreseen ever since it was first reported out of South Africa, that we knew it would come to New York State at some point.”

Much remains unknown about Omicron, including whether it is more transmissible and capable of causing more serious illness. People had to be vaccinated with at least one dose and wear masks to attend the conference. Proof of at least one dose is required for many indoor activities citywide. New York State and New York City do not have universal indoor mask mandates.

In a statement, Mayor Bill de Blasio said the city had activated its contact tracing program to track down people who attended the event at the Javits Center, adding that “we should assume there is community spread of the variant in our city.”

The Omicron variant still has not been identified in New York, Ms. Hochul said, adding that New Yorkers should continue to wear masks and get tested, while unveiling a new public messaging campaign to encourage people to get booster shots.

Last week, Ms. Hochul declared a state of emergency to give the state more flexibility to respond to the rise in cases and staff shortages in health care facilities. Under an executive order that goes into effect on Friday, for example, hospitals that have less than 10 percent bed capacity will have to temporarily pause elective surgeries.

Ms. Hochul made her remarks on Thursday during a coronavirus briefing in which she introduced Dr. Mary T. Bassett, the newly-appointed New York State health commissioner.

Dr. Bassett steps into the role as parts of upstate New York grapple with a spike in coronavirus cases, while the state contends with the largely unknown Omicron variant during the cold winter months, when people are likelier to gather indoors and travel during the holidays.

Since Nov. 1, daily case counts in New York City have risen quite a bit, reaching 1,500 newly identified cases a day last week. The rise, driven by the Delta variant, has been steepest in Queens, a borough with vaccination rates well above the citywide average.

“The fact that we have not detected it may mean that it’s still extraordinarily rare here, that it is the proverbial needle in the haystack,” Dr. Bassett said of the Omicron variant. “We now have an exposure and we fully expect that it will be detected in the coming days.”

Dr. Bassett, who ran New York City’s health department for four years until 2018, took the helm of an embattled Health Department that suffered from an exodus of health officials under former Gov. Andrew M. Cuomo, who repeatedly interfered in the department’s coronavirus response.

Dr. Bassett replaced Dr. Howard A. Zucker, who announced his resignation in September following political pressure over his role in potentially helping obscure the coronavirus death toll in nursing homes under Mr. Cuomo.

Appointed by Ms. Hochul, who replaced Mr. Cuomo after he resigned in August, Dr. Bassett faces the daunting task of restoring trust in the state’s public health apparatus among experts and New Yorkers alike.

Dr. Bassett won acclaim as the city’s health commissioner for her role in navigating the Ebola scare in 2014, as well as an outbreak of Legionnaire’s disease and the threat of Zika. She most recently served as director of Harvard’s FXB Center for Health and Human Rights.