(CNN) Some states have reopened too quickly, allowing the coronavirus pandemic to come roaring back, Dr. Anthony Fauci said Thursday.
"There are some times when despite the guidelines and the recommendations to open up carefully and prudently, some states skipped over those and just opened up too quickly," Fauci said.
Fauci said on Podcast-19, FiveThirtyEight's weekly podcast on Covid-19, that he thought in some respects Florida and Arizona's reopening plans have contributed to the uptick in cases in those states.
"Certainly Florida I know, you know, I think jumped over a couple of checkpoints," Fauci said.
Fauci told the podcast that some governors and mayors did it perfectly when doing away with some restrictions and stay-at-home orders.
Earlier Fauci, , director of the National Institute of Allergy and Infectious Diseases, said states with spiking coronavirus cases still can contain them by pausing their reopening processes, rather than shutting down a second time.
"Rather than think in terms of reverting back down to a complete shutdown, I would think we need to get the states pausing in their opening process," Fauci told The Hill's Editor-at-Large Steve Clemons on Thursday.
Fauci said the range of people the virus affects -- from those with no symptoms to those who end up in intensive care or die -- makes the pandemic really difficult to get under control.
"Not to be hyperbolic about it -- it really is the perfect storm and (an) infectious disease and public health person's worst nightmare. It's a spectacularly transmissible virus," he said. "The efficiency with which this transmits is really striking."
Four states that account for about 50% of new infections -- Arizona, California, Florida and Texas -- need to aggressively keep people socially distanced, including by closing bars and preventing crowds, he said.
"If we can do that consistently, I will tell you, almost certainly, you're going to see a down curve of those infections," Fauci said.
The country and some states are setting records for average daily officially reported cases, ICUs in hot spots are reaching capacity, and most states are seeing spikes, recalling the uncertainty of months ago when the virus first broke out.
Another health expert echoed Fauci's initial comment about second shutdowns.
"If you're not doing the ... things we've talked about in the past to get this outbreak under control, starting with test and trace ... your only option is to shut down," Dr. Ali Khan, former director of the CDC's public health preparedness office, told CNN's "New Day" Thursday.
New York Mayor Bill de Blasio said large events in the city will be canceled through September 30, according to a news release from his office.
"It's all about health and safety first," de Blasio told CNN's Wolf Blitzer. "This is obviously the thing that President Trump doesn't understand. We don't just decree that we want things to happen regardless of the human impact. We actually look at the science, we look at the data. The data is telling us it is not time for large gatherings."
Events of one city block or smaller can apply for a permit. Demonstrations and religious events are exempt from the mayor's order.
"This is a historic moment of change," he said. "We have to respect that but also say to people the kinds of gatherings we're used to -- the parades, the fairs -- we just can't have that while we're focusing on health right now."
The US reached more than 3 million coronavirus cases this week, according to data from Johns Hopkins University. At least 33 states as of Thursday morning have seen an upward trend in average daily cases -- an increase of at least 10% -- over the past week.
Some states have recently reported alarming rates at which people are testing positive: 28% in Arizona; 26% in Texas; and 19% in Florida, according to The Covid Tracking Project.
Here are some significant numbers reported Thursday:
Many states are feeling the effects of the Covid-19 surge.
In South Texas' Brooks County, anyone who is positive for Covid-19 can be arrested if they appear in public, other than to receive medical care, without first being cleared by the state's health services department, the county attorney said. The county, with about 7,000 residents, says this week it has 23 active cases.
The South Texas Health System's hospitals have more than doubled its usual amount of ICU beds to deal with an influx in Covid-19 patients, Wesley Robinson, the system's assistant chief nursing officer, said Thursday.
They've put patients in conference rooms and surgical wings, Robinson told CNN. At least one of the system's hospitals, in the city of Weslaco, has set up tent outside for coronavirus patients.
"Everyone is exhausted," emergency department nurse director Pablo Laredo said Thursday. "Patients here are very sick."
The University of Mississippi Medical Center has more patients than rooms, the center's vice chancellor said Thursday in a briefing with state health officials.
Still, Dr. LouAnn Woodward, said she doesn't advocate a new statewide shelter-in-place order but does encourage mask-wearing to reduce transmission. "We cannot shelter in place through the duration of this pandemic," she said.
In Florida, 48 hospitals on Thursday had no more capacity in its intensive care units -- down from 56 two days earlier, the state Agency for Health Care Administration said. The group has said hospitals can add ICU beds in a surge situation when necessary.
As cases and hospitalizations rise, an answer to the question of whether children will return to school in the fall becomes less clear.
Fauci, in an interview scheduled to air Friday on Sirius XM Radio, said, "We should try to get the schools open," but he emphasized grade schools could have different approaches than universities.
"If you keep children out of school, the unintended negative ripple effect of consequences can be profound with regard to, what do the parents do, that they then stay off of work to take care of their children?" Fauci said, in recordings released Thursday.
"So, the broad approach would be obviously ... paying attention to the safety of the children, which is always paramount, but within the context of doing whatever you can to safeguard the health and the welfare of the children, we should try to get the schools open," he said.
President Donald Trump said he will pressure governors to reopen schools in the fall and has threatened to cut federal funding of schools that don't reopen.
The White House has also claimed the CDC's guidelines for reopening schools is too strict. On Thursday, agency Director Dr. Robert Redfield said the CDC will not revise its guidelines.
Thursday night, he said the guidelines are not mandatory.
"I want to really stress that the purpose of the CDC guidelines are to provide a variety of different strategies for schools to use to help facilitate the reopening of schools. I can tell you that the guidance that we put out are out, and they stand," he said at the "CNN Global Town Hall -- Coronavirus: Facts and Fears" on Thursday night.
Redfield said the CDC will continue to provide additional material and update the guidelines as warranted.