(CNN) -- Just because you can go to the beach or bars doesn't mean it's time to let your guard down. In fact, some states are seeing new spikes in coronavirus cases.
"With the country starting to open up this holiday weekend, I again remind everyone that the coronavirus is not yet contained," said Dr. Stephen M. Hahn, commissioner of the US Food and Drug Administration.
"It is up to every individual to protect themselves and their community. Social distancing, hand washing and wearing masks protect us all."
Many Americans have flocked to parks, restaurants and beaches to celebrate Memorial Day weekend.
In Alabama's Gulf Shores, "there are literally thousands of people out here on the beach, and what I'm really pleased to see is that many of these folks, almost all of them, are doing a great job with social distancing," beachgoer Steve Ricks told CNN affiliate WPMI.
But some ignored health warnings, despite new information that an estimated 40% of coronavirus transmissions occur before any symptoms.
Video from a pool party at Lake of the Ozarks, Missouri, showed a massive crowd of people crammed together both inside and outside the pool.
Saturday's footage came the same day Arkansas Gov. Asa Hutchinson said several people who attended a swim party contracted Covid-19.
An aide to New York Gov. Andrew Cuomo, Melissa DeRosa, harshly criticized a restaurant in Patchogue that eased restrictions on mask wearing and social distancing by allowing customers to come inside to pick up food on a rainy Friday night.
"That's stupid -- stupid for you, it's stupid for your surrounding patrons, it's stupid for the bar," she said Sunday at a news briefing.
The Dublin Deck Tiki Bar and Grill posted an apology on Instagram, saying, "There are no excuses when it comes to public safety. We should not have allowed anybody inside whatsoever."
Social distancing was forgotten in Daytona Beach, Florida, on Saturday when a man apparently making a rap video threw money from atop a car into the crowd on a main thoroughfare, authorities said. Volusia County Sheriff Mike Chitwood said officers are looking through aerial footage in an attempt to identify and arrest the man.
"Look at the numbers. You'll see that on Thursday, more than 20,000 Americans were infected," said Dr. Seema Yasmin, a former disease detective at the US Centers for Disease Control and Prevention.
On Friday, "that number went up, and there were more than 24,000 Americans newly diagnosed with Covid-19."
Birx has a message for those refusing to wear masks
Wearing a face mask is critical to reducing the spread of coronavirus, especially since many carriers don't even know they're contagious, said Dr. Deborah Birx, the White House coronavirus task force coordinator.
"There is clear scientific evidence now, by all the droplet experiments that happened and that others have done, to show that a mask does prevent droplets from reaching others," Birx told "Fox News Sunday."
She offered a message to those who say they have a right not to wear a mask in public.
"Out of respect for each other, as Americans that care for each other, we need to be wearing masks in public when we cannot social distance," she said.
In an interview with ABC, Birx said "there's asymptomatic spread. And that means that people are spreading the virus unknowingly."
"And this is unusual in the case of respiratory diseases," Birx said. "So you don't know who's infected. And so we really want to be clear all the time that social distancing is absolutely critical. And if you can't social distance and you're outside, you must wear a mask."
Several states see surges in coronavirus cases
States such as North Carolina and Arkansas are seeing major spikes, Yasmin said.
On Saturday, North Carolina reported its highest single-day increase of new coronavirus cases, just a day after the state rolled into its second phase of reopening.
In Arkansas, Gov. Hutchinson said the state seemed to be experiencing a "second peak." He said that rise is due in part to more widespread testing.
And high numbers of new cases have emerged from Virginia, Maryland, the District of Columbia, Nebraska and Illinois, Birx said.
Houston mayor: 'Too many people who are coming together'
Texas beaches also saw crowds over the weekend after bars were allowed to reopen, with restrictions.
Texas is one of a handful of southern states at risk of seeing a rapid surge of new coronavirus cases in some areas, according to a new projection model.
That model predicts that Harris County, which includes Houston, could see more than 2,000 new cases each day by June.
Houston Mayor Sylvester Turner said Sunday the city will enforce the 25% capacity rule in bars and clubs -- two days after saying the rule would not be enforced. The city also will enforce the 50% rule for restraurants.
Speaking at a news conference livestreamed by CNN affiliate KPRC, Turner said he changed course after seeing social media photos of establishments clearly above their occupancy requirements.
"The reality is that there are too many people who are coming together," Turner said. "No social distancing, no mask. And then after this Memorial Day weekend is over they're going to be on somebody's job or in close proximity to somebody else."
The projection model also shows parts of South Florida and parts of Alabama and Tennessee are also poised for spikes in coronavirus cases.
A study leads to encouraging results
There's a bit of good news when it comes to a possible therapy for Covid-19 patients.
A study appeared to show patients who received transfusions of antibody-filled convalescent plasma seemed to fare better. Convalescent plasma is derived from the blood of recovered coronavirus patients.
The study hasn't been peer-reviewed or published in an academic journal, but researchers said the findings are a good sign for plasma therapy.
"We are encouraged that our initial assessment offers evidence in support of convalescent plasma as an effective intervention," said Dr. Nicole Bouvier, an associate professor at the Icahn School of Medicine at Mount Sinai and a co-author on the study.
But more studies are needed to confirm the findings, the researchers said.
CNN's Hollie Silverman, Artemis Moshtaghian, Arman Azad, Melissa Alonso, Dakin Andone, Amir Vera, Nicky Robertson, Ralph Ellis, Barbara Starr and Alec Snyder contributed to this report.