The percentage of COVID-19 tests taken in New York state that have come back positive has edged up in a worrisome trend for the former epicentre of the US epidemic.
New York's positivity rate had hovered around 1 per cent for weeks, a hard-won metric after the state tallied thousands of cases per day during the peak of its outbreak in the northern spring.
The rate's uptick to 1.5 per cent now comes with 27 other states recording increases in the number of cases for two straight weeks.
"It's basically Brooklyn, Orange and Rockland that are increasing this number," Governor Andrew Cuomo told reporters on Monday, adding that health officials were looking into COVID-19 clusters in these locations.
He said officials were still trying to determine why the rise in those areas, all in the southern part of the state.
In some Brooklyn neighbourhoods, the positivity rate is as high as 5-7 per cent.
Cases in New York state have risen 4.4 per cent so far in September, one of the smallest increases among US states.
Its positive test rate remains much lower than some midwestern states, where 15 per cent of tests have come back positive.
Cuomo urged New Yorkers to remain vigilant in mask-wearing and warned of consequences if they do not.
"It's not time to get tired because the virus isn't tired," he said.
The number of new cases has risen two weeks in a row in 27 of 50 US states, with North Carolina and New Mexico reporting increases above 50 per cent last week.
The Midwest has emerged as the country's new hotspot, with hospitalisations surging.
Wisconsin set records for new cases twice last week and is now reporting more new daily infections than Florida. South Dakota, Utah and Wyoming have all set records for new daily cases three times this month.
The positivity rate has risen to 26 per cent in South Dakota, up from 17 per cent last week, according to an analysis using The COVID Tracking Project data.
According to the analysis, on Monday, Wisconsin's rate was 19 per cent, Iowa's was 16 per cent, Missouri's was 16 per cent, Kansas' was 15 per cent and Nebraska's 14 per cent.
The World Health Organization considers rates above 5 per cent concerning because it suggests there are more cases in the community that have not yet been uncovered.
Top infectious disease expert Anthony Fauci told Good Morning America on Monday he was concerned about the trend as the weather gets colder and people spend more time indoors, increasing the likelihood of spread.
"We're not in a good place ... because as we get into the fall and the winter you really want the level of community spread to be as low as you can possibly get it," Fauci said.
The United States is reporting 45,000 new infections on average each day, compared with 40,000 a week ago and 35,000 two weeks ago.
Australian Associated Press
