When will coronavirus cases peak? It’s hard for experts to predict.

The United States passed a tough milestone on Thursday with more than 4 million confirmed cases of coronavirus and approximately 145,000 deaths nationwide. As the virus continues to spread in more than 30 states, experts may say it may take some time before the country sees the end of this surge in infection.

In the early months of the pandemic in the United States, daily deaths peaked in mid-April, gradually declining to less than 1,000 per day after more than 2,300 deaths were reported. Daily incidents that plunged into approximately 20,000 plateaus from mid-May to early June have reached record highs in recent weeks.

Full coverage of the coronavirus outbreak

But the combination of the latest outbreak demographic changes across the country, coupled with state inconsistent mitigation strategies, makes it more difficult for scientists to predict when a new rising curve of concern will begin to level. It has become.

“The trends across the United States don’t seem to be peaking immediately,” said Lauren Lippworth, an epidemiologist at the Vanderbilt University Medical Center in Nashville, Tennessee. “If these trends continue to rise, I think this wave will continue during the winter.”

Earlier in the pandemic, states such as New York, California, New Jersey, Washington, and Louisiana were hit hardest, but new infections and subsequent deaths were seen as cases and deaths diminished or leveled in those areas. Are increasing in other areas. With the proliferation of new cases of COVID-19 in many countries, many states are beginning to see increased hospitalization and mortality.

“Overall, it’s only part of the confirmed COVID cases, and eventually some who died in hospital and then died,” Lipworth said. “But, even at 1% or 2%, the number of cases soars, so that number actually adds up.”

Public health officials warned that there might be a second wave of coronavirus infections in the fall, consistent with the flu season, but Lipworth said Still straight to the first wave By the end of the year.

“I don’t think I can even start thinking about the second wave until I’m sure the first one is over. That’s certainly not the case in most parts of the country,” she said.

Lippworth added that the stage of the pandemic is not strictly defined, but the US needs to put the current outbreak in control before epidemiologists declare the end of the first wave. .. This hasn’t happened yet, she said, because cases and even some of the states that saw deaths leveled or diminished saw both numbers spike again.

California, for example, implemented strict blockades early in the pandemic, delaying statewide transmission. However, new infections are now being reported at record highs, with the state recently surpassing more than 422,000 New York cases with the most confirmed coronavirus cases in the country.

According to Jaline Gerardin, an epidemiologist at Northwestern University’s Feinberg School of Medicine in Chicago who works extensively on domestic modeling and disease prediction, the state’s diverse experience will help model pandemic trajectories nationwide. Has become difficult. Illinois.

However, she said she is likely to see more hospitalizations and deaths within a few weeks as the state is experiencing a surge of new infections.

“The rhythm we saw at the first peak was a few weeks behind,” Geraldine said. “We had a very harsh and sharp intervention on March 21 and were evacuating. Then peaked hospital admission in early April and were admitted a couple of weeks later, and six weeks later. There was no peak death until later in May.”

She said similar delays would be expected this time, but added that it was difficult to predict when this surge would peak as many dynamics changed since March.

One is young people in their 20s, 30s, and 40s. Account for a large proportion of new cases In many states. In Florida, for example, the median age of new coronavirus infections in March was 65. Currently, people aged 25 to 34 Highest percentage of positive cases across the state, The median age of new cases dropped in their thirties.

download NBC News App To completely cover the outbreak of coronavirus

This young demographic could change the predictions for hospitalization and death across the state, but a serious undertesting in March and April, according to Lipworth, suggests that different age groups could see different age groups during the initial surge. Scientists still do not know exactly how it was affected.

“Younger demographics tend to be the groups that are more likely to relax restrictions, which is one reason we are seeing a shift in distribution,” she said. “But now we are testing more people, more asymptomatic people. It is heavily weighted on young people.”

Dr Jennifer Stevens, director of the Center for Healthcare Delivery Science at Beth Israel Deaconess Medical Center in Boston, explains how behavior changes during the pandemic and how that change affects infections. Understanding what to give refines local and regional models.

“It’s a whole bucket of work. Understanding the choices people make and the impact of increased human interaction. Ideally, we’ll have a lead time to prepare as a healthcare professional.” Will give.”

According to Geraldine, fine-tuning the model based on changing behavior is a difficult task. Researchers do not fully understand the role of mild restrictions on slowing transmission.

“I know that hitting things with a big hammer (like installing it in March and April) has a big impact, but I don’t know in detail the different parts of the hammer that caused it. I mean, I don’t really know how the different phases of the restart affect the transmission.”

Still, despite the many unknowns remaining, scientists say that having a more uniform mask policy can go a long way in containing the virus within the community. ..

“If everyone in this country wears a mask, as an epidemiologist we can confidently say that we can influence and flatten these curves.”

Follow NBC health on twitter & Facebook..