By Theresa Waldrop, Christina Maxouris and Jason Hanna, CNN

(CNN) -- Several states on Tuesday announced they are loosening restrictions put in place to stop the spread of Covid-19, even as health experts warn against doing just that.

Texas and Mississippi, for example, are lifting mask mandates and allowing businesses to open at full capacity.

It's too soon, health experts say. After the number of new cases declined from unprecedented highs recently, the downturn appears to have stagnated at a high level -- seven-day average case levels are as high as they were last summer. And more transmissible variants are spreading.

"This is not a time to relax restrictions -- especially the governors who are all high-fiving themselves saying we're out of this. We're not," said Dr. Peter Hotez, co-director of the Center for Vaccine Development at Texas Children's Hospital.

The country is at risk of losing "the hard-earned ground we have gained" in the battle against Covid-19 as highly contagious variants take advantage of Americans getting lax with safety measures, said Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention.

"I am really worried about reports that more states are rolling back the exact public health measures we have recommended to protect people from Covid-19," Walensky said Monday. "Please stay strong in your conviction."

"If we were to hang on a few weeks, at least, or maybe more than that, we'd learn a lot more, we'd figure out what this variant is actually doing," Hotez told CNN's Jake Tapper on Tuesday.

"And we'd have a higher percentage of the US population vaccinated. And then we could revisit it, but not now," Hotez said of states reopening.

President Joe Biden said there will be enough vaccine for every adult American by the end of May, speeding up by months that expected timeline.

But, the President warned: "It's not over yet. Stay vigilant."

Highly transmissible variants that threaten to create another surge in the US include the B.1.1.7 strain, which was first identified in the UK. The CDC warns it could become the predominant variant in the US this month.

Dr. Zeke Emanuel, who was a member of the Biden Transition Covid-19 Advisory Board, is another who warned against opening up too early.

"We should not ease up, allow indoor dining, big groups ... getting rid of mask mandates. We have to hold on for another two or three months in this condition," Emanuel told CNN. "We're still having, on average, 2,000 (Covid-19) deaths a day. We cannot become inured to that."

If Americans relax too early, Covid-19 numbers are "going to reverse and go up," he added.

Other states loosening restrictions

In Louisiana, most businesses and restaurants will be allowed to increase capacity to 75% and there will be no restrictions on religious services as of Wednesday, Gov. John Bel Edwards said.

In Michigan, restaurants can open at 50% capacity, up from 25%, on March 5, Gov. Gretchen Whitmer said.

Kentucky Gov. Andy Beshear said many businesses can increase capacity to 60% -- including barbershops and hair salons, bars and restaurants, bowling alleys, fitness centers, movie theaters and government offices.

Pennsylvania state officials announced they had revised maximum-occupancy limits for indoor and outdoor events and eliminated out-of-state travel restrictions. And in Chicago, officials said they will allow restaurants to operate indoors at the lesser of 50% capacity or 50 people, raising the previous limit of 40%.

Dr. Ashish Jha, dean of the Brown University School of Public Health, said these moves are coming at the wrong time.

"We've got states easing restrictions -- which they should not be doing at this moment -- and we've got the variants that are circulating that are a lot more infectious. Put all that together, and (I) can't help but worry about where we're going to be for the next couple of months," Jha said Tuesday.

'We're not in great shape' with case numbers similar to last summer

The University of Minnesota's Center for Infectious Disease Research and Policy recently warned the proportion of cases of the B.1.1.7 variant was rapidly increasing in parts of the US and a "major peak in cases, hospitalizations and deaths in the near future remains a strong possibility."

Covid-19 cases, hospitalizations and deaths have declined over many weeks, though some numbers have recently wobbled, and experts say they are still high contextually:

-- Cases: The US has averaged about 67,760 new cases a day over the last week -- far below the nation's pandemic peak average of more than 249,700 daily on January 8, according to Johns Hopkins University.

The rate of decline recently slowed -- the average was just above 70,000 a week ago, and it even rose a bit before falling again late last week. Some of that may be attributable to stalled case reporting by some states hit by severe winter storms last month.

But the current seven-day average still is above the highest average seen last summer: around 67,030 on July 20.

"So we're not in great shape," and if highly contagious variants take hold from this level, "I think we can absolutely see a huge spike that will really lead to a lot more suffering," Jha said.

-- Hospitalizations: More than 46,700 Covid-19 patients were in US hospitals on Monday -- the lowest number since October 29, according to the COVID Tracking Project

-- Deaths: The country has averaged 2,046 Covid-19 deaths a day across the last week -- a number that has hovered above and below 2,000 for the past couple of weeks. The country is nearly seven weeks removed from its peak daily average of 3,354, according to Johns Hopkins data.

A third vaccine now being administered in the US

Barbara Schmalenberger, 86, on Tuesday was the first to receive Johnson & Johnson's single-dose vaccine -- after it was authorized days earlier -- at the Schottenstein Center, in Columbus, Ohio, an arena where mass vaccines are taking place.

Johnson & Johnson's is the third vaccine authorized for use in the United States, and almost 4 million doses will be available in the first shipments, with a goal to ship 20 million doses by the end of the month, the White House has said.

Other states are expecting to receive their first doses of the Johnson & Johnson vaccine this week.

Biden on Tuesday announced that Merck & Co. will partner with Johnson & Johnson to help manufacture its vaccine, which allowed for the earlier May goal of having enough vaccines for all adults in the United States.

Before now, the only Covid-19 vaccines authorized in the US -- from Pfizer/BioNtech and Moderna -- have been two-dose products.

Three Federal Emergency Management Agency vaccination sites in Texas are expected to receive about 24,000 doses of the Johnson & Johnson vaccine Tuesday, according to Lara Anton, press officer for the Texas Department of State Health Services.

More than 200,000 Johnson & Johnson doses are expected to be available to the state for next week's allocation, Anton said.

In Houston, Mayor Sylvester Turner said he expects the first delivery of the Johnson & Johnson vaccine.

"It certainly will be the best option for our transient populations," he said of the one-dose vaccine. "For example, our homeless population, which is very difficult to get back for their second shot."

Track US Covid-19 vaccinations here

In Colorado, health officials said they expect to receive more than 45,000 doses of the Johnson & Johnson vaccine by Friday.

The authorization of the vaccine will "make it easier for the state to reach its vaccination goals as more people become eligible in the weeks to come," said Dr. Eric France, the chief medical officer for the Colorado Department of Public Health and Environment.

Ohio Gov. Mike DeWine announced the state will receive more than 96,000 doses of the vaccine this week, boosting the state's total number of doses expected this week to more than 448,000.

Because of the additional doses, DeWine said the state will be expanding its vaccine eligibility to Phase 1C on March 4: people who are living with Type 1 diabetes, are pregnant, are bone marrow transplant recipients or living with ALS, as well as people working in childcare services, funeral services and law enforcement and corrections officers.

The state will also start its Phase 2 on the same day, lowering age eligibility to people 60 or older.

Across the US, more than 51.7 million people have so far received at least their first dose of a Covid-19 vaccine, according to CDC data. More than 26.1 million have received both doses.