A second wave of coronavirus has started in the U.S. — and people need to remain careful or risk stressing out the health-care system again, said William Schaffner, a professor at the Vanderbilt University School of Medicine.
"The second wave has begun," said the professor of medicine told CNBC's "Street Signs Asia" on Monday. "We're opening up across the country, but many, many people are not social distancing, many are not wearing their masks."
Even so, he said he "cannot imagine" a second shutdown due to the impact of the first one.
Several states in America have reported recent spikes in Covid-19 cases as measures are eased throughout the country. The U.S. has the highest number of cases in the world. Nearly 2.1 million people have been infected by the disease and more than 115,000 people have died, according to data from Johns Hopkins University.
Schaffner added that mass gatherings and religious services are also being held. "Many people are simply not being careful, they're being carefree," he said. "That, of course, will lead to more spread of the Covid virus."
Although Schaffner warned that the second wave may be underway, disease specialists said last week that the rise in Covid-19 cases across the U.S. was actually just a continuation of the first wave of the outbreak.
Ian Lipkin, a professor of epidemiology and director of the Center for Infection and Immunity at Columbia University, said that for the fresh outbreak to be defined as a second wave, the coronavirus would have needed to retreat and reappear, or a new strain of the virus would have had to emerge. He noted that the recent increase in U.S. cases "does not reflect either" scenario.
Despite claiming there were signs of a second wave, Schaffner said the option of another lockdown is "off the table."
Instead, governments, businesses and religious leaders have to work together to promote mask wearing and social distancing in order to flatten the curve.
He said wearing masks is "very, very important" and authorities should "persuade and educate" their residents to make this a "social norm."
"If we all do that in respect of each other, then I think we can make some progress," said Schaffner.
"If we do all the opposite — if we open up, do not have social distancing, don't wear masks and congregate in large numbers again, we are going to be very stressed in the medical care system," he warned.
"The complete shutdown was such a financial disaster, and had so many social and cultural implications that I cannot imagine we'll have a shutdown again," he said.
Other countries, however, have not shied away from reimposing measures when new clusters are detected in the community. In May, South Korea shut all night clubs and bars in Seoul after a new cluster surfaced there.
Last week, Beijing reportedly banned tourism and locked down 11 neighborhoods in response to infections related to a wholesale market.
— CNBC's Noah Higgins-Dunn and Berkeley Lovelace Jr. contributed to this report.