(CNN) - As cases spike in parts of New York City, Gov. Andrew Cuomo warned that Covid-19 "remains a force to be reckoned with throughout the country."
"I urge New Yorkers to keep wearing masks, socially distancing and washing their hands, and local governments must continue to enforce state public health guidance," Cuomo said in a statement. "By staying vigilant and smart, we can beat COVID together."
Experts have cautioned the US could see an explosion of coronavirus cases in the fall and winter as people exercise less caution and spend more time indoors. Already the US has reported more than 7.1 million cases and 204,756 deaths since the pandemic began, and 21 states are reporting more new cases in the last seven days compared with the week before, according to a CNN analysis of data from Johns Hopkins University.
Once the epicenter of the pandemic in the US, New York had boasted a test positivity rate -- the percentage of tests being performed that come back positive for the virus -- of less than 1% for more than a month. That rate broke 1% on Saturday as Cuomo reminded New Yorkers "we cannot drop our guard."
Though the rate of positive tests is still low relative to other states, neighborhoods in Queens and Brooklyn are seeing cases "continue to grow at an alarming rate," according to a news release from the city's Department of Health.
But New York is still among the states with the lowest positivity rate in the US. The World Health Organization advised that the rates of positivity in testing should remain at 5% or lower for at least 14 days before businesses reopen. Currently, 22 states and the District of Columbia meet that recommendation, with Vermont holding the lowest rate at 0.53% positive. Twenty-eight states and Puerto Rico have positivity rates higher than 5%. The territory has a 100% positivity rate. Among US states, the rate is highest, 24.64%, in South Dakota, according to Johns Hopkins University.
Public schools in New York City are returning to the classroom for the first time this week, but officials could decide to close schools, limit gatherings and issue fines for not wearing masks.
"For the first time in the city's recovery period, there could be the immediate scaling back of activities in these ZIP codes if progress is not made by Monday evening," the Department of Health said last week.
New York City has reported 237,971 of the state's 455,626 cases. But with a seven-day average of 224 new daily cases last week, the city is still nowhere near its April peak, when more than 5,000 new cases were reported every day.
States reach new highs
New York isn't the only state to report case counts trending in the wrong direction.
Wisconsin reported its highest single-day case increase on Saturday with 2,817 new cases reported, according to the Wisconsin Department of Health. The previous record was 2,533 on September 18, according to the state health department's website.
Last week, Wisconsin Gov. Tony Evers issued an executive order mandating face coverings in an effort to keep infections down. In a news release, his office attributed rising cases primarily to infections among 18- to 24-year-olds and said the state was "facing a new and dangerous phase" of the pandemic.
"We need your help to stop the spread of this virus, and we all have to do this together," Evers said.
Meanwhile, cases in Florida have exceeded 700,000, though it is one of 10 states reporting fewer new cases this week compared to last. It is joined by Arizona, Connecticut, Georgia, Maryland, New Hampshire, Rhode Island, Tennessee, Vermont and Virginia.
The number of new cases reported is holding steady in 19 states. Twenty-one are seeing more new cases reported than last week: Alabama, Alaska, Colorado, Idaho, Massachusetts, Michigan, Minnesota, Montana, Nevada, New Jersey, New Mexico, North Carolina, North Dakota, Oregon, South Carolina, South Dakota, Texas, Utah, Washington state, Wisconsin and Wyoming.
Authorities break up large gatherings
To avoid another surge, infectious disease expert Dr. Anthony Fauci stressed masking, distancing and avoiding crowds. But authorities have had to intervene as some continue to gather in large groups.
On Friday evening, New York City Sheriff's Office deputies broke up a wedding of about 300 people in Queens, New York City Mayor Bill de Blasio's office told CNN. Both the owner and manager of the venue were issued appearance tickets for multiple misdemeanor offenses.
Weddings and other large social gatherings have been the origin of several outbreaks in coronavirus cases in recent months, and authorities have turned their attention to deterring these events and enforcing measures against them.
A Maryland man was sentenced to a year imprisonment Friday after holding two large parties in late March against the state's social distancing rules, which had banned gatherings of more than 10 people, according to the office of Gov. Larry Hogan.
The man became argumentative after he was told to disband his first party of about 50 people, the state's attorney's office said. And five days later, he held a second party of more than 50 people which he refused to end, saying they had a right to congregate and directing his guests to stay, according to a news release.
CNN's Laura Ly, Sheena Jones, Anna Sturla, Dakin Andone, and Chuck Johnston contributed to this report.