(CNN)As scientists look into ways to stop or prevent Covid-19, other researchers have found evidence the disease caused by the new coronavirus was in the United States weeks before cases were reported.
Researcher Francois Balloux of the University College London Genetics Institute and his colleagues in the United Kingdom pulled viral sequences from a giant global database that scientists around the world are using to share data.
They looked at samples taken at different times and from different places, and said they indicate that the virus began infecting people at the end of 2019.
"Our results are in line with previous estimates and point to all sequences sharing a common ancestor towards the end of 2019, supporting this as the period when SARS-CoV-2 jumped into its human host," the team wrote in a report, published in the journal Infection, Genetics and Evolution.
Balloux told CNN his team is "really, really, really confident" about when the host jumped.
They also found genetic evidence that supports suspicions the virus was infecting people in Europe, the United States and elsewhere weeks or even months before the first official cases were reported in January and February.
One US community is checking to see whether there were cases there that went undiagnosed in 2019.
In Chicago, the Cook County Medical Examiner's Office said it will review deaths involving heart attacks and pneumonia for indications of Covid-19 as far back as November, Cook County spokesperson Natalia Derevyanny told CNN.
The first known coronavirus death in the county was March 16.
The office will look at viral pneumonia cases along with heart attacks caused by arteries being blocked (arterial thrombosis), as opposed to cases brought on by heart failure.
"The goal is to see if this virus was present before we knew of it," Derevyanny said.
While Derevyanny called the decision to look back to November an arbitrary timeframe, if a positive case is discovered it will prompt the office to look back even further.
The investigation may include additional testing of preserved tissue samples, Derevyanny said.
Cuomo: It comes down to how much you value life
New York Gov. Andrew Cuomo on Tuesday said debates on how soon states should ease social distancing restrictions come down to the value of human life -- and that policymakers are avoiding saying so explicitly.
Cuomo, whose state by far has the most recorded coronavirus cases, reacted Tuesday to projections that the country's coronavirus death rate will soar because many states are easing restrictions now.
"The fundamental question, which we're not articulating, is how much is a human life worth?" Cuomo said at a news conference.
"The faster we reopen, the lower the economic cost. But, the higher the human cost, because (of) more lives lost," Cuomo said in a news conference. "That ... is the decision we are really making."
In the past two weeks, governors across the country introduced plans for phased reopenings amid mounting pressure from residents and businesses who are fearful of devastating economic impacts of lockdowns.
But easing restrictions now may come with a heavy price.
"It's the balance of something that's a very difficult choice," Dr. Anthony Fauci, the country's leading infectious disease expert, told CNN Monday night. "How many deaths and how much suffering are you willing to accept to get back to what you want to be some form of normality, sooner rather than later?"
At least 42 states will have eased restrictions by Sunday, ranging from simply opening state parks to allowing some businesses to restart. That includes California -- the first state to implement a sweeping stay-at-home order -- where some stores will be allowed to reopen this week.
So far, the US has recorded more than 1,200,000 infections and at least 71,043 deaths.
Poll: Majority prioritizes preventing illness over economy
A majority of Americans who answered a Monmouth University poll, meanwhile, indicated they prioritize preventing illnesses over long-term economic concerns.
In the poll, conducted Thursday though Monday, adults were asked which should be the more important factor in deciding whether to lift outbreak restrictions -- ensuring as few people as possible get sick from the coronavirus, or ensuring the economy doesn't enter a deep and lengthy downturn.
About 56% answered the former; 33% said the latter; 9% said both equally. The poll of 808 adults in the United States has a margin of error of +/- 3.5 percentage points, Monmouth said Tuesday.
More vaccine candidates tested in the US
Researchers continue to race for a potential coronavirus vaccine -- and another group of candidates is being tested on people in the United States.
US pharmaceutical giant Pfizer and German biotechnology company BioNTech have begun testing four coronavirus vaccine candidates in humans in New York and Maryland, the companies said Tuesday.
The trial had already started in patients in Germany last week. The Phase 1/2 study is designed to test the safety, effectiveness and best dose level of the four vaccine candidates.
The first stage of the US trial will enroll up to 360 healthy adults, starting with ages 18 to 55 and eventually including ages 65 to 85, the companies said.
These companies aren't the first with a vaccine program this far along.
The US National Institutes of Health started human testing of one vaccine candidate in the United States in March. In the United Kingdom, Oxford University's Jenner Institute began testing its vaccine candidate in humans in April.
The World Health Organization says 108 potential Covid-19 vaccines are in development around the world -- up from 102 on April 30. Eight of the potential vaccine programs have been approved for clinical trials, WHO says.
Fauci, a member of the White House's coronavirus task force, last week suggested January as a potential date for a vaccine, but vaccines typically take years to produce.
How governors are moving forward
California was one of the states where crowds gathered over the weekend as thousands of protesters descended on the state's Capitol and an Orange County beach to protest social distancing orders.
The governor on Monday announced retail shops in the state -- including clothing stores, florists and bookstores -- can begin to reopen Friday, after health officials said the state was meeting important metrics including sufficient test and tracing capacity.
Los Angeles Mayor Eric Garcetti said he didn't think his city would reopen this week, saying Monday that despite the governor's announcement, different parts of the state may see different timelines for reopening.
In Michigan, Gov. Gretchen Whitmer said the lockdown will continue "until at least May 15," warning that reopening the state too soon could lead to a second shutdown.
Kentucky Gov. Andy Beshear, who will let construction and manufacturing businesses reopen May 11 as part of a phased reopening plan, said Monday the numbers in Kentucky are "really steady," even with more testing.
In Mississippi, Gov. Tate Reeves has amended his "safer at home" order, allowing outdoor gatherings of up to 20 people starting Thursday.
Saying he knows the virus doesn't do well in the sun or heat, Reeves added, "to be outdoors is about the safest place you can be." While lab tests in the US have shown the virus "dies the quickest in the presence of direct sunlight," the World Health Organization has said "you can catch COVID-19, no matter how sunny or hot the weather is."
Reeves' plan also allows dining service in restaurants, as long as the institutions follow guidelines provided by the state, including a mandatory deep cleaning.
"I don't want to wait if there are steps that we believe we can safely take now to ease the burden on Mississippians fighting this virus," he said.
Protests against masks
As health officials and businesses navigate safe reopenings, many communities -- and the federal government -- have urged Americans to wear face coverings when they're in public. In parts of the US, it's required.
But those guidelines have also seen pushback -- including last week at Michigan's Capitol, where hundreds of protesters showed up, most of whom were not covering their faces.
On Friday, a security guard was shot in the head and killed after telling a customer at a Michigan Family Dollar store to wear a face mask. The governor required face masks in enclosed public spaces in late April. Three people have been charged.
In Stillwater, Oklahoma, an emergency proclamation issued to require face masks in stores and restaurants was amended a day later after store employees were "threatened with physical violence and showered with verbal abuse," Stillwater City Manager Norman McNickle said in a statement.
And in San Diego County, a supermarket customer wore a Ku Klux Klan-style hood to cover his face and only removed it at the cashier's desk, despite having been asked to do so multiple times before, CNN affiliate KSWB reported.
