* Stable U.S. crop rating, big Australia forecast also cool wheat

* Omicron worries revived by Moderna warning on vaccine response

* Corn, soybeans also slip on virus fears

CHICAGO, Nov 30 (Reuters) - Chicago wheat prices fell to a nearly three-week low on Tuesday, as concerns that the spread of the Omicron coronavirus variant could slow the global economy drove investors to reduce risk exposure.

The entire grains complex felt pressure early in the session, after Moderna Inc's chief executive cautioned that COVID-19 shots were unlikely to be as effective against the Omicron variant.

Corn followed wheat lower, while soybeans fell for a fifth straight session, as a sharp drop in the crude oil market and good weather for crop development in South America added to the pressure on prices.

"The overwhelming fundamental in the market right now is fear over Omicron and what it could do to demand," said Jack Scoville, market analyst at The Price Futures Group, who noted that much of the sell-off was caused by funds seeking cover.

"Hopefully, we'll be seeing some better (export) demand with these weaker prices," Scoville said. If that happens, "we should be getting to the end of this panic selling and things should stabilize."

The most-active wheat contract on the Chicago Board Of Trade (CBOT) settled down 35 cents at $7.87-1/5 a bushel. Earlier in the session, the contract fell to $7.82-1/2, its lowest since Nov. 10.

CBOT soybeans settled down 24-1/4 cents to $12.17-1/4 a bushel, while corn settled 14-3/4 cents lower to $5.67-1/2 a bushel.

Wheat markets rallied to nine-year highs this month in Chicago and record peaks on Euronext as the possibility of more Russian export restrictions and the risk of rain damage to Australia's crop fanned fears of tight milling wheat supplies.

Egypt, the world's biggest wheat importer, also made its largest single wheat purchase in years on Monday after under-buying this season, given a surge in prices. But it is not known if Egypt will continue its buying spree or was just taking advantage of a dip in prices.

Global supply worries were eased by signs of a stabilizing U.S. crop, and after Australia's chief commodity forecaster, ABARES, revised its official estimate for the 2021/22 crop to a record 34.4 million tonnes. (Reporting by P.J. Huffstutter; Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by Kirsten Donovan, Barbara Lewis and Peter Cooney)