While official figures show nearly 56 million people have been confirmed infected with COVID-19, a new study says the true figure is likely above 300 million.
Since emerging at the end of last year, the deadly virus has killed at least 1.34 million people, and left an unknown number suffering long-term effects such as fatigue and shortness of breath.
While early data suggested the virus - known as SARS-CoV-2 - could kill between 3 and 4 percent of those infected, that's since been revised downwards - but with limited testing in parts of the world, particularly early in the pandemic, the mortality rate has been difficult to devise.
"I couldn't find out whether it was COVID-19 or not... the level of testing is just appalling," Newshub Europe correspondent Lloyd Burr, based in London, said in April after spending nearly three weeks in bed sick.
Others have tested positive despite not even showing symptoms, after possibly having spread the disease without even realising they had it.
Australian researchers have now tried to put a figure on just how many people have had the virus. They looked at data from 15 countries with a combined population of more than 800 million people, and using some complex mathematics estimated the number confirmed infected is 6.2 times lower than the true figure.
"We found COVID-19 infections are much higher than confirmed cases across many countries, and this has important implications for both control and the probability of infection," said co-author Quentin Grafton of the Australian National University.
They used a process called 'backcasting' to calculate the true spread of the disease in 11 European countries, Australia, Canada, South Korea and the US.
"We analysed statistics on how many people had died from COVID-I9 in a given country and then worked backwards to see how many people would have to have been infected to arrive at that number of deaths," said Ikigai Research's Steven Phipps.
In-depth studies have found the infection fatality rate - how many people die after being infected - is somewhere around 0.68 percent (this can vary between regions, depending on demographics). This is lower than the case fatality rate - the number who die after being confirmed infected - of about 3 percent, because not all infections are detected.
"Our approach is particularly advantageous in locations where there is little testing or limited capacity to forecast rates of infection but where there is a need, for the purposes of public health planning, for a population measure of COVID-19 infection," added Prof Grafton.
In contrast, in countries like New Zealand that managed to avoid large outbreaks and have comprehensive testing and contact tracing systems, one expert told Newshub it's likely most infections have been detected.
On the other hand, "in some countries... such as Belgium, France, Italy and the United Kingdom, our estimates indicated that the reported number of cases, as at August 31, 2020, were likely to represent less than 10 percent of the true number of cases", the study reads.
Contact tracers in the UK estimate they're only reaching about 60 percent of close contacts, while their New Zealand equivalents are reaching more than 80 percent.
The researchers behind the latest study excluded New Zealand from their analysis as the database they were using had gaps in the data. Singapore was also looked at, but excluded as the vast majority of its cases were amongst young migrant workers, whose age distribution wasn't representative of the rest of the country.
The new research was published on Wednesday in journal Royal Society Open Science.