Most people arriving in New York state must quarantine for at least three full days before taking a coronavirus test, Governor Andrew Cuomo announced as he overhauled one of the strictest quarantine regimes for travellers in the United States.
If that test comes back negative, the traveller can leave quarantine.
The requirements, which take effect on Wednesday, will not apply to residents of "contiguous" states, Mr Cuomo told newsmen on Saturday, and there will be different requirements for New Yorkers who leave the state for less than 24 hours.
He named Connecticut, Pennsylvania and New Jersey as examples of contiguous states, home to several commuters to New York City.
But it was unclear whether neighbouring Vermont and Massachusetts would also be exempt from the new regime.
His office did not reply to questions seeking clarification on Saturday.
People will also be required to take a test that comes back negative within the three days prior to heading to New York, the governor said.
If the second test taken at least four days after arrival is also negative, "you can go about your business", Mr Cuomo said in a telephone conference with newsmen.
If it's positive, the person must remain in isolation, he said.
Those deemed essential workers by the state, from nurses to grocery store workers, are exempt from the rules.
The US Centres for Disease Control (CDC) says the virus can incubate for up to 14 days before somebody becomes symptomatic and so recommends a 14-day quarantine period after a possible exposure.
Under New York's rule, a traveller could end his or her quarantine just four days after a possible exposureat the airport, for example, or during a plane flightdespite the CDC noting that coronavirus tests can return a false negative result if taken too early in an incubating infection.
Mr Cuomo's office did not respond to questions about how the rule squares with CDC guidance.
New Yorkers returning to the state within 24 hours of leaving need not quarantine, but must take a test within four days of their return, Mr Cuomo said, though his office afterwards corrected that to say the test must not be taken sooner than four days after returning.
If they are gone longer than 24 hours to non-contiguous states or on a trip abroad, then the general rules apply.
The new rules replace a previous arrangement under which people arriving in New York from a list of states with worsening coronavirus outbreaks had to quarantine in a home or hotel for 14 days, regardless of any test result.
That list grew to encompass nearly every state in the country.
Earlier this year, New York grappled with one of the world's deadliest outbreaks of Covid-19, but now has one of the lowest rates of positive tests of any state in the country even as cases surge in other regions.
The most recent statewide positive test rate was 1.49 per cent, Mr Cuomo said on Saturday.
REUTERS