In a worrying sign that coronavirus may not be done sweeping through nations that are beginning to emerge from lockdown, recent studies in Spain, France and England indicate that only a small fraction of these countries' populations had been infected with the virus.
Meanwhile in China, where the outbreak began late last year, health officials said they would intensify the detection and investigation of COVID-19 to prevent any rebound of cases.
In France, where 16,642 people have died from coronavirus so far, according to an NBC News tally, a study led by the Pasteur Institute found only 4.4 percent of the population — or 2.8 million people — had been infected by the virus. This rose to between 9 and 10 percent in hard-hit regions such as Paris, according to the study released Wednesday.
The figure is far lower than the 65 percent of the population many experts believe is needed to achieve so-called "herd immunity" and control the pandemic, the report found.
Herd immunity is when enough people in a population have immunity to an infection to be able to effectively stop that disease from spreading.
"Our results show that without a vaccine the herd immunity alone will not be enough to avoid a second wave at the end of the lockdown. Efficient control measures must thus be upheld after May 11," French researchers said.
A Spanish nationwide antibody study, also published on Wednesday, showed similar results, finding only about 5 percent of the country's population had contracted the virus. Again, indicating there was no herd immunity as the country progressively lifted its lockdown.
Such antibody or "seroprevalence" tests could help governments gauge the true extent of the epidemic in their nations, taking into account all those who may be immune or resistant to the coronavirus.
In China, health officials also said they would step up testing in the city of Wuhan, where the outbreak began late in 2019. State run CCTV broadcast images of citizens being tested on Thursday and said officials would screen residents across the city for 10 days.
New clusters have also been reported in Jilin and Liaoning, raising fresh concerns for Beijing.
National Health Commission spokeswoman Song Shuli told reporters on Thursday that the country had to prevent a rebounding of the infection, that has so far killed 4,633 people, by stepping up tracing and testing.
China has maintained a streak of no new coronavirus deaths for some 17 days but is keeping close watch on asymptomatic cases, where those infected show no symptoms but nonetheless carry and can spread the deadly virus.
Song said that over the past 10 days, the number of asymptomatic infections under medical observation had decreased by 22 percent.
But the World Health Organization warned on Wednesday that COVID-19 may never go away and could become endemic like HIV.
"It is important to put this on the table: this virus may become just another endemic virus in our communities, and this virus may never go away," WHO emergencies expert Mike Ryan told an online briefing.
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Meanwhile, in England, just 148,000 people or 0.27 percent had the infection, according to national statistics published on Thursday. But health officials raised hopes after saying they were in talks with Swiss drug maker Roche over the possible roll out of a "game-changer" antibody test that could get the country up-and-running again.
A Public Health England laboratory concluded that the antibody test had 100 percent specificity, which means it can detect antibodies to the exact disease rather than similar ones.
"This has the potential to be a game-changer," said Edward Argar, Britain's junior health minister.
"We are now moving as fast as we can to discuss with Roche purchasing of those, but I can't give you an exact date when we'll be able to start rolling them out."
The Roche test also received support from the European Union on April 28 and Emergency Use Authorization from the U.S. Food and Drug Administration on May 2.
Although antibody tests show who has been infected, scientists around the world warn it is not yet clear whether that equates to permanent immunity.
Reuters contributed to this report.