WASHINGTON >> In a matter of days, millions of Americans have seen their lives upended by measures to curb the spread of the new coronavirus.

Normally bustling streets are deserted as families hunker down in their homes. Many of those who do venture out try to stay a safe distance from anyone they encounter, even as they line up to buy now-precious commodities like hand sanitizer. Parents juggle childcare as schools close, perhaps for the rest of the school year. And restaurants and bars sit empty as more and more convert to delivery-only options.

How long will this last? Scientists say there isn’t a simple answer.

“In many ways, this situation is unprecedented — we’re trying to take some actions to curb the spread and timing of this pandemic,” said Stephen Morse, a disease researcher at Columbia University in New York.

Yes, there have been past disease outbreaks that scientists can draw some lessons from but, in those cases, the disease was largely allowed to run its course. “So those models don’t precisely apply,” Morse said.

On Monday, President Donald Trump said the U.S. may be managing the outbreak through July or August. And New York Governor Andrew Cuomo said the state’s number of coronavirus cases may peak — not end — in 45 days.

The overall message is that the country will be fighting the virus outbreak for a matter of months, at least, not days or weeks.

Each model of how the disease could spread relies on data and assumptions about population dynamics, demographics, health care capacity and other factors, said Rebecca Katz, a public health expert at Georgetown University.

The challenge for designing models of what will happen next in the U.S. is that limited testing for COVID-19 means researchers don’t know what the starting point is — how many people are already infected.

In simplest terms, scientists say that the epidemic will slow when people with infections don’t pass the virus on to others.

“Basically, if I infect one other person or more … then the epidemic can take off. If I infect less than one person and everybody infects less than one person, then the epidemic will decline,” said Elizabeth Halloran, a disease researcher at the University of Washington.

Based on data from China and from cruise ships, scientists estimate that unless measures are taken to limit the spread, each infected person will infect about 2 or 3 others, leading to an exponential growth of the virus.

If the virus makes a jump to new person every two to five days, as scientists calculate, then a single infected person could lead to 4,142 total infections within a month — assuming nothing is done to break chains of transmission.

Unless such measures are put in place, scientists estimate that between 40 and 80% of the global population could become infected. Based on an analysis of data from China, scientists found that the majority of new infections are transmitted by people with mild symptoms who may not even know they’re ill, said Jeffrey Shaman, a public health expert at Columbia University.

Even if most people recover and only a fraction of total infections are severe enough to require hospitalization — about 14%, scientists estimate — the sheer scale of the epidemic will put enormous strain on hospitals, healthcare workers and other patients who may see unrelated procedures delayed.

Scientists now agree that measures to break or slow the chains of transmission are crucial to ensure that emergency rooms aren’t quickly overwhelmed by surges in critically ill patients.

If measures like closing schools are successful in slowing disease spread, “we are going to see a hump instead of a peak” in new cases, U.S. government disease expert Dr. Anthony Fauci said in a White House briefing.

In other words, the number of infections will rise more gradually and manageably. But that also means the duration of the outbreak will be prolonged.

“The point of the restrictions we have is actually to stretch this out even longer. We don’t want a big peak to come very quickly,” said Mark Jit, a disease researcher at the London School of Hygiene and Tropical Medicine. “It’s not like a Hollywood movie with a clear ending where everyone is saved, or everyone dies, quickly.”

So how does this end?

Most scientists believe the fight against COVID-19 won’t be over until there’s an effective vaccine. But Fauci and other experts say it will be more than a year before a vaccine can be ready for widespread use.

“The best-case scenario is that we have vaccine in 12 or 18 months and then our lives go back to normal,” Jit said. “The worst-case scenario it takes a long time for a vaccine to be developed, and the world is really changed and our lives aren’t the same again.”

While we wait, are we all shut-ins for more than a year? Almost no one thinks that’s realistic.

“I don’t think we can maintain social distancing as it is right now for the duration of the epidemic,” said Michael Levy, a University of Pennsylvania disease researcher.

What may be more feasible is a plan for intermittent restrictions and enhanced monitoring to control the disease, an idea explored in a new study from researchers at Imperial College London. Once the number of new cases falls below a certain threshold, schools, offices and restaurants could reopen. But if the number of infections spikes again, restrictions would be reinstated.

“The analogy of pumping car brakes on an icy road is what we should be thinking about,” Levy said. “You push on the brakes to slow things down, then ease up — but if you skid, you have to pump the brakes again.”