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The true number of Americans who've been infected with COVID-19 may top 20 million, according to new estimates from the Centers for Disease Control and Prevention.

"Our best estimate right now is that for every case that's reported, there actually are 10 other infections," Dr. Robert Redfield, director of the CDC, said on a call with reporters Thursday.

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The assessment comes from looking at blood samples across the country for the presence of antibodies to the virus. For every confirmed case of COVID-19, 10 more people had antibodies, Redfield said, referring to proteins in the blood that indicate whether a person's immune system has previously fought off the corona virus.

Those samples aren't just from people who have had antibody testing. They also come from testing performed on donated blood at blood banks or from other laboratory testing of blood.

Currently, there are 2.3 million COVID-19 cases reported in the U.S. The CDC's new estimate pushes the actual number of corona virus cases up to at least 23 million.

The estimation comes amid rises in cases across the Southeast and Western U.S., particularly among younger adults in their 20s, 30s and 40s.

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Also Thursday, the CDC expanded its list of who is at greatest risk for COVID-19 complications, removing the age cutoff of 65.

"There's not an exact cutoff of age at which people should or should not be concerned," Dr. Jay Butler, head of the COVID-19 response at the CDC, said. Rather, a person's risk increases with age, but that doesn't preclude younger adults from complications.

Indeed, people of any age with certain underlying health conditions have a higher risk, though the likelihood of having these conditions increases with age. At risk are those with heart disease, chronic kidney disease, chronic obstructive pulmonary disease, obesity, type 2 diabetes, sickle cell disease and anyone with a compromised immune system.

CDC also clarified the list of other conditions that might increase a person’s risk of severe illness, including asthma, high blood pressure, neurologic conditions such as dementia, cerebrovascular disease such as stroke, and pregnancy.

Research published on Thursday from the CDC specifically addressed the risk in pregnant women. When compared to non-pregnant women with the virus, pregnant women with COVID-19 were more likely to be hospitalized, admitted to the intensive care unit and put on a ventilator.

Death rates between the two groups of women, however, were similar.

Redfield also urged Americans to be vigilant about behaviour measures known to minimize spread of the corona virus, particularly as the country heads into the July Fourth holiday.

The coronavirus spreads mainly from person to person through respiratory droplets from coughing, sneezing, talking and singing.

"The most powerful tool that we have is social distancing," he said. That means maintaining a physical distance of at least 6 feet in public, wearing face coverings and regular hand-washing.

"If you must go out into the community, being in contact with fewer people is better than many," Redfield added.

© Global News Wendell Potter

The stark differences in how the coronavirus pandemic has unfolded in the U.S. and in Canada show the flaws in for-profit health care, according to a former U.S. health insurance executive who now advocates for medicare.

"You learn a lot about a health-care system when a global crisis hits (and) different nations have different results. Canada’s single-payer system is saving lives," Wendell Potter said in a series of tweets Thursday.

"The U.S. profit-driven corporate model is failing. I'll regret slandering Canada's system for the rest of my life."

Read more: Amid COVID-19, Americans have more faith in Canada than themselves: poll

Potter, a former communications vice-president with U.S. insurer Cigna who now leads an organization called Medicare for All Now!, said insurance companies have spent big on attempts to discredit the Canadian system.

"It was a lie (and) the nations' COVID responses prove it," he wrote. "The truth: Canada's doing much better than the U.S. when it comes to COVID-19 testing and treatment.

"On a per capita basis, more Canadians are being tested (and) fewer getting sick (and) dying. This may shock Americans who still believe the lies I told about the Canadian health care system."

Cigna did not immediately respond to a request for comment on Thursday afternoon. Nor did the industry group America's Health Insurance Plans, which Potter accused of supplying insurance executives with "cherry-picked data (and) anecdotes" to encourage the perception that Canadians face unreasonable wait times.

In an interview, Potter said he wanted to draw attention to the reality north of the border, and "what can happen when you have a system that is better organized" than in the United States.

COVID-19 has claimed more than 8,500 lives in Canada as of Thursday. In the U.S., which has a population nearly nine times greater than Canada's, there have been more than 122,000 fatalities, according to figures compiled by Johns Hopkins University.

Read more: How many Canadians have the new coronavirus? Total number of confirmed cases by region

For every 100,000 people, Canada has had 23 deaths, while the U.S. has had about 37, according to the university researchers' calculations.

Canada has also tested a greater proportion of its residents. On average, the U.S. has tested 56 people per 100,000 daily, while Canada has tested 65 per 100,000, Johns Hopkins data shows.

Read more: Comparing coronavirus responses: What did Canada and the U.S. do differently?

In past interviews with Global News, experts have pointed to a number of factors that could be behind the differences in how the pandemic is playing out in Canada versus the U.S.

They pointed to differences in pandemic policy and response co-ordination, as well health care and factors such as population density.

Potter's comments were shared widely on the social media platform, though they attracted criticism as well.

Some took aim at Potter for his work in the industry."Apology not accepted," one commented wrote.

There was also dispute over the claim that there aren't long wait-times for procedures and emergency care in Canadian hospitals.

A 2019 report from the Canadian Institute for Health Information found that most Canadians are getting timely access to urgent procedures, but that about 30 per cent of those needing cataract surgery or hip or knee replacements did not receive those procedures within the recommended timeframes.

Research shows, however, that the U.S. spends far more on health care yet has the lowest life expectancy and highest infant mortality rates out of 11 high-income countries including Canada.

Read more: Coronavirus ‘travel bubble’ idea gains steam but wouldn’t work for Canada, U.S.: expert

Potter worked as a journalist prior to switching to communications and started at Cigna in 1993.

What he called a crisis of conscience over his work — and "vilifying the Canadian system" — prompted Potter to leave the industry more than a decade ago.

"I was very ashamed of what I was doing. I became very ashamed," he said.

Amid the corona virus crisis, Potter said he believes the U.S. is at a turning point when it comes to support for health-care reform, and U.S. Democratic presidential candidate Joe Biden will face public pressure on that issue.

"I think it's just a matter of time before we ultimately have a system that's more like yours," he said.