The estimated impact is equivalent to nearly 5% of worldwide output based on a range of scenarios, but the lender said losses from "the worst pandemic in a century" could be higher.
"The estimated impact could be an underestimate, as additional channels such as...possible social and financial crises, and long-term effects on health care and education are excluded from the analysis," the ADB said.
The Manila-based bank said a shorter containment period could pare the losses to $2 trillion.
"No one can say how widely the COVID-19 pandemic may spread, and containment may take longer than currently projected," ADB chief economist Yasuyuki Sawada said.
"The possibility of severe financial turmoil and financial crises cannot be discounted," he added.
The forecasts assume the coronavirus outbreak will be contained this year and a return to normality in 2021.
However, there is still the potential for additional outbreaks and the severity of the pandemic remains uncertain.
"Outcomes can be worse than forecast and growth may not recover as quickly," the bank said.
Growth in China, the region's largest economy, could slow to 2.3% this year from 6.1% in 2019, before bouncing back in 2021.
"The outbreak became a demand shock as people stayed home. It became a supply shock as companies suffered shortages of labor...and of materials as supply chains faltered," the ADB said.
Close to 5%, or $628 billion, of China's gross domestic product could be lost. – Rappler.com