TREASURIES-Yields fall to three-week lows as vaccine worries spook investors

* Bonds rally after Moderna CEO's vaccine warning

* Yields set to log monthly declines at most tenors

* Powell testimony to begin at 1500 GMT (Updates in London trading)

Nov 30 (Reuters) - Bonds rallied to multi-week highs on Tuesday after the head of drugmaker Moderna raised concerns about the efficacy of vaccines against the Omicron coronavirus variant and the demand for safe assets put Treasuries on course to end the month firmer.

The yield on benchmark 10-year U.S. government bonds fell 10 points (bps) to 1.42% by 0949 GMT in London trading, the lowest in three weeks.

Yields fall when prices rise and the rally extends gains made since scientists announced the detection of the Omicron variant late last week.

U.S. money markets pushed back their expectation of a first, full 25 basis-point rate hike to September 2022, versus July last week.

Moderna's CEO told the Financial Times https://www.ft.com/content/27def1b9-b9c8-47a5-8e06-72e432e0838f that present vaccines would likely be less effective against the new variant and that it would be a risk to completely shift production to an Omicron-targeted dose while other variants remained in circulation.

The market moves have 10-year Treasuries heading toward snapping three months of selling to post a monthly gain, with yields down 14 bps in November as investors reel back wagers on higher rates.

Shorter and longer-dated bonds also rallied on Tuesday.

Two-year yields were down over 4 bps to 0.47%. Five-year yields were down about nearly 10 bps to 1.09% and 30-year yields dipped 6 bps to 1.82%.

"The head of Moderna saying that the vaccines will be less good is what's driving the move," said Lyn Graham-Taylor, senior rates strategist at Rabobank in London. "More good news was priced into the States curve so that is coming off."

"There's virtually been a pricing out of a rate hike in the U.S. whereas there's not as much to price in the euro are," Graham-Taylor said.

Ahead on Tuesday, Federal Reserve Chair Jerome Powell and Treasury Secretary Janet Yellen are due to testify before U.S. lawmakers from 1500 GMT.

In prepared remarks already released, Powell noted Omicron posed risks to growth and said inflation could persist longer than first thought.

(Reporting by Tom Westbrook, additional reporting by Dhara Ranasinghe and Yoruk Bahceli; Editing by Kim Coghill)