By Christina Maxouris and Holly Yan, CNN

(CNN) -- Just as predicted, the US is now grappling with a new Covid-19 surge -- one that could overwhelm hospitals, kill thousands of Americans a day by January and leave even young survivors with long-term complications.

"We went down to the lowest point lately in early September, around 30,000-35,000 new cases a day. Now we're back up to (about) 50,000 new cases a day. And it's going to continue to rise," said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine.

"This is the fall/winter surge that everyone was worried about. And now it's happening. And it's happening especially in the northern Midwest, and the Northern states are getting hit very hard -- Wisconsin, Montana, the Dakotas. But it's going to be nationally soon enough."

Across the country, 33 states have reported more Covid-19 cases this past week compared to the previous week, according to data from Johns Hopkins University.

In Denver, Mayor Michael Hancock said recent case counts are as "high right now as they were at the height of the pandemic back in May."

He said hospitalizations have also soared, and residents could face tighter Covid-19 restrictions if the numbers keep going up.

In Wisconsin, a field hospital is opening this week to handle a rapid rise in coronavirus patients. The state recently reported record-high numbers of Covid-19 cases, hospitalizations and daily deaths.

If Americans don't turn the tide around, the US could be in for a devastating winter. The University of Washington's Institute for Health Metrics and Evaluation projects more than 135,000 people in the US could die within the next three months.

"This winter -- this November, December, January, February -- could be the worst time in our epidemic," Hotez said. "Get ready to hunker down."

How overwhelmed hospitals can affect all patients

As we've seen throughout this pandemic, surges in new Covid-19 cases lead to increased hospitalizations and deaths in the coming weeks.

At least 10 states have reported record-high Covid-19 hospitalizations since Friday, according to data from the COVID Tracking Project.

"Hospitals could again become overwhelmed," emergency medicine physician Dr. Leana Wen said. "And then we're not just talking about patients with coronavirus who might be in trouble. It's also about other patients who might be coming in for heart attacks and strokes and car accidents who may find a situation that's really untenable."

And some healthcare workers in the US still don't have adequate personal protective equipment and testing supplies, emergency medicine physician Dr. Megan Ranney said.

"We're quite fearful for what we are heading into," she said.

13% increase in child Covid-19 cases in 2 weeks

More children are also being diagnosed with Covid-19, according to the American Academy of Pediatrics and the Children's Hospital Association.

Their latest report found a 13% increase in child cases reported from September 24 to October 8 -- with more than 77,000 new reported infections.

The report looked at data from health departments in 49 states, New York City, the District of Columbia, Puerto Rico, and Guam. Children represented about 10.7% of Covid-19 cases in the states that reported cases by age, the report said.

The news comes as many schools have returned to in-person instruction and are navigating challenges that come with new infections.

There's no "one size fits all" strategy to safely returning to classrooms, said Dr. Anthony Fauci, director of the National Institute for Allergy and Infectious Diseases.

The decisions should be based on how widespread infections are in the community and how much surveillance is required, Fauci said.

"When schools are prepared, when they have a plan, when everyone is universally wearing masks, when they're testing people in a surveillance way to get people who are infected out of the system, (and) they know what to do when they're confronted with a person a child or older students who gets infected -- it can work," he said.

A vaccine trial is paused, but that's 'completely expected.

As Americans wait for a safe and effective vaccine, Johnson & Johnson has paused the advanced clinical trial of its experimental vaccine due to an unexplained illness in one of the volunteers, the company announced Monday.

"Following our guidelines, the participant's illness is being reviewed and evaluated by the ENSEMBLE independent Data Safety Monitoring Board (DSMB) as well as our internal clinical and safety physicians," the company said in a statement. ENSEMBLE is the name of the study.

"Adverse events -- illnesses, accidents, etc. -- even those that are serious, are an expected part of any clinical study, especially large studies."

Johnson & Johnson did not say what the illness was.

This kind of pause isn't immediately concerning, said Dr. Ashish Jha, dean of the Brown University School of Public Health.

"This is completely expected, and it's just a reminder how ridiculous it is to try and meet a political timeline of having a vaccine before November 3," Jha said, referring to President Donald Trump's wishes to have a vaccine ready for the public by Election Day.

"The Johnson & Johnson trial is the biggest trial of the vaccine that I know of -- 60,000 people," Jha said. "Within that trial, you'd expect a few pauses."

This is the second Phase 3 Covid-19 vaccine trial to hit the pause button in the US. AstraZeneca's vaccine trial was paused last month because of a neurological complication in a volunteer. While that trial has resumed in other parts of the world, it remains paused in the US while health officials investigate.