But he cautioned that "extreme vigilance is required."
More than 280,000 people have died out of the more than four million known COVID-19 infections worldwide.
And while the drastic measures implemented by many countries have allowed them to get a tentative handle on the virus, there are widespread fears that there could be fresh waves of intense transmission.
Mr Ryan urged countries to boost their public health responses, ensuring they can identify fresh cases, and trace and isolate all contacts, which he said could help "avoid a major second wave".
But he warned that while "many countries have made very systematic investments in building up their public health capacities during the lockdowns, others have not."
No 'herd immunity'
"If disease persists in countries at a low level without the capacity to investigate clusters, identify clusters, there is always the risk that the disease will take off again," he said.
Without naming names, Mr Ryan decried that some countries were choosing to "drive through this blind" by not dramatically ramping up their capacity to test and trace cases while they have the chance.
The WHO warned against the notion in some countries that even if they do not take the measures needed to halt the spread of the virus, their populations will quickly build so-called "herd immunity".
"Early serological studies reflect that a relatively low percentage of the population has antibodies to COVID-19," Tedros said, pointing out that this means "most of the population is still susceptible to the virus".
More than 90 so-called serological studies, which reveal the presence of antibodies in the blood to determine whether a person has had a past infection, were being conducted in several countries.
The WHO's COVID-19 technical lead Maria Van Kerkhove said that while the UN agency has not yet been able to critically evaluate the studies, initial data released showed that between one and 10 percent of people had antibodies.
"There seems to be a consistent pattern so far that a low proportion of people have these antibodies," she said.
Mr Ryan agreed, saying the early results belied the widely-held assumption that most cases of the virus were mild and going undetected.
Preliminary results were "showing the opposite... that the proportion of people with significant clinical illness is actually a higher proportion" than previously thought, he said, stressing that "this is a serious disease".
"This idea that maybe countries that have lax measures... will all of a sudden magically reach some herd immunity, and so what if we lose a few old people along the way... is a really dangerous, dangerous calculation," he said.
People in Australia must stay at least 1.5 metres away from others. Check your state’s restrictions on gathering limits.
Testing for coronavirus is now widely available across Australia. If you are experiencing cold or flu symptoms, arrange a test by calling your doctor or contact the Coronavirus Health Information Hotline on 1800 020 080.
The federal government's coronavirus tracing app COVIDSafe is available for download from your phone's app store.
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