For 2006, their average gross domestic product (GDP) growth is projected to stay at 6.2 percent but a pandemic would set off shocks in regional economies, the Washington-based lender said in its twice-yearly regional outlook.
With East Asia already affected by a cyclical slowdown in the technology sector and higher oil prices, the spectre of a bird flu pandemic looms as a major threat to the region, it said.
"One large shadow looms over the generally positive economic outlook we have sketched out ... and that is avian flu," Homi Kharas, the bank's chief regional economist, said in a video conference from Washington.
A pandemic will mean "enormous global costs" for the world economy," he said, making it paramount that East Asian governments place fighting the virus at the top of their priorities.
If the H5N1 virus mutates into a form that makes it easily transmissible among humans, the immediate impact could be similar to that of the Severe Acute Respiratory Syndrome (SARS) outbreak two years ago, the World Bank said.
"There are great uncertainties about the timing, virulence and general scope of a future human flu pandemic but all agree it could lead to at least several million human deaths.
"The most immediate economic impacts of a pandemic might arise, as in Sars, not from actual death or sickness but from the uncoordinated efforts of private individuals to avoid becoming infected, as well as public policy actions like quarantines and travel restrictions."
The World Bank said these could lead to "severe demand shocks for service sectors such as tourism, mass transport, retail, hotels and restaurants etc as well as supply shocks due to workplace disruption."
The World Bank gave no estimates of how much East Asian economies could suffer from a pandemic but said several rural areas in the region were already feeling the impact from outbreaks, particularly the poultry sector.
In Vietnam for example, which has accounted for more than two-thirds of the some 60 people killed by the virus in Asia since 2003, the outbreak has already had an impact due to lost economic activity, the World Bank said.
"At the overall macroeconomic level, costs so far have been fairly limited ... but could rise significantly going forward, and have already been high for specific sectors and communities," it said.
The World Bank urged the international community to avert a possible catastrophe.
"There is clearly a priority on curbing avian flu 'at source' in the agricultural sector, through implementation of strong animal and human health surveillance, disease control and mitigation measures, thereby reducing the probability of a far more costly human epidemic," the World Bank said.
"Policy makers everywhere need to give the influenza threat top political attention and priority to avert, delay or mitigate what could be a global disaster," it said.
In its report, the World Bank noted that the region's slower growth in 2005 compared with 2004 was due to reduced export orders, higher energy prices and a cyclical downswing in the technology sector.
For economies like Indonesia, the impact of higher oil prices will still be felt in 2006 after the government announced moves to slash fuel subsidies.
Barring a bird flu pandemic, the World Bank predicted 2006 growth of 8.7 percent for China, 7.5 percent for Vietnam, 6.0 percent for Indonesia, 5.3 percent for Malaysia, 5.0 percent for the Philippines, 5.0 percent for Thailand and 6.1 percent for Cambodia.
For the more advanced East Asian economies, the bank predicted 2006 GDP growth of 1.8 percent for Japan, 4.4 percent for Hong Kong, 4.6 percent for South Korea, 4.7 percent for Singapore and 4.1 percent for Taiwan.