South Australia has confirmed its third coronavirus death, bringing the nation's death toll to 51. SA Health said the 76-year-old man contracted COVID-19 in the Barossa Valley.

The vast majority of COVID-19 cases around the world are going undetected, new research published by the Lancet shows.

Only about 6 per cent of cases have been identified worldwide, the study estimates.

That means “several tens of millions of people” are likely infected without them, or their governments, knowing.

Researchers from the University of Gottingen in Germany compared estimates of coronavirus infection fatality rates in a previous study with the confirmed number of cases and United Nations population data, the South China Morning Post reported.

The number of COVID-19 cases officially issued by countries “dramatically understates the true number of infections,” a statement on the German Ministry of Education and Research stated.

“Insufficient and delayed testing may explain why some European countries, such as Italy and Spain, are experiencing much higher casualty numbers (relative to reported confirmed cases) than Germany, which has detected an estimated 15.6% of infections compared to only 3.5% in Italy or 1.7% in Spain.

“Detection rates are even lower in the United States (1.6%) and the United Kingdom (1.2%) – two countries that have received widespread criticism from public health experts for their delayed response to the pandemic,” the ministry said.

The authors estimate that on March 31, 2020, Germany had 460,000 infections.

Based on the same method, they calculate the United States had more than 10 million, Spain more than five million, Italy around three million and the United Kingdom around two million infections.

On the same day the Johns Hopkins University reported that globally there were less than 900,000 confirmed cases, meaning the vast majority of infections were undetected.

The huge number of estimated cases has policy implications for governments around the world.

Sebastian Vollmer, professor of development economics at the University of Göttingen, said: “These results mean that governments and policymakers need to exercise extreme caution when interpreting case numbers for planning purposes.

“Such extreme differences in the amount and quality of testing carried out in different countries mean that official case records are largely uninformative and do not provide helpful information.”

Christian Bommer added: “Major improvements in the ability of countries to detect new infections and contain the virus are urgently needed.”

The new research reinforces comments made by Australia’s chief medical officer, Professor Brendan Murphy, last week.

He said the true number of coronavirus cases around the world might be five or even 10 times higher than the current official estimate.

“Worldwide, we’ve now passed a million reported cases of COVID-19,” Prof Murphy said at a press conference with Prime Minister Scott Morrison in Canberra.

“We believe that the true number is probably five or 10 times as much as that because we know that judging by the death rate, the testing rate, in many countries they’re not detecting all the cases. There are some countries that don’t have the capacity to test.”