However, the team said the majority of these “hidden” cases suffered from only mild symptoms and will not have been infectious after a couple of weeks.
The research team arrived at their estimate of missed infections after conducting blood tests on more than 4,000 people over the last year.
They found that six people below the age of 60 had developed antibodies specific to the Covid-19 virus. Five of the six had tested negative when they were quarantined as close contacts of confirmed cases.
None of the six had serious symptoms, with one completely asymptomatic, and researchers said they would not have spread the virus after 10 to 14 days.
The team calculated that between 4,439 and 19,659 adults and 1,756 minors in Hong Kong would have been similar undetected cases.
The city has confirmed 11,835 Covid-19 cases so far since the start of the pandemic.
Microbiology professor Paul Chan suspects the use of nasal or throat swab tests is the reason why so many infections have gone undetected.
“Maybe because the sample quality was not good, so it is false negative,” he said.
Chan suggested authorities do more tests using nasopharyngeal swabs – which collect samples from deeper in the nasal cavity – or use stool sampling as a supplement.
However, he said Hong Kong is outperforming other places, as overseas studies estimate that up to 90 percent of infections are not being picked up in the likes of the US, Pakistan, Spain and Geneva.
Chan said the key to reaching zero infections is widespread vaccination.
Another study his team has conducted found that fully vaccinated people, regardless of which jab they had taken, were equipped with enough antibodies to trigger “explosive protection” against the coronavirus.