(New throughout, updates prices, yields, market activity and comments, adds auction results) By Ross Kerber Dec 8 (Reuters) - Longer-term U.S. Treasury yields were lower on Tuesday as investors tried to judge how quickly vaccines could be distributed to fight the COVID-19 pandemic. The benchmark 10-year yield was down 1.3 basis points in afternoon trading at 0.9146%. It traded as low as 0.893% in the morning, its first time below the 0.9% threshold since Dec. 1, before rising in step with higher equity markets. Investors seemed to be trying to judge how widely and quickly vaccines would be available, which will determine the pace of reopening plans and economic recovery, said Eric Jussaume, director of fixed income for Cambridge Trust. With a lack of clarity on the schedule, "Enthusiasm could be postponed a little bit," he said. Britain on Tuesday became the first Western nation to begin a mass inoculation campaign. In the United States, the Trump administration sought to shore up the vaccine supply, after a report that Pfizer Inc may not be able to provide more of its vaccine there until next June because of commitments to other countries. After opening lower on Tuesday, the S&P 500 and the Dow reversed course while the Nasdaq hit a record high. An afternoon U.S. Treasury auction of $56 billion of 3-year notes produced average results according to a research note from BMO Capital Markets analyst Ben Jeffery, with a high yield of 0.211% and dealers accounting for 34.9% of accepted bids. A closely watched part of the U.S. Treasury yield curve measuring the gap between yields on two- and 10-year Treasury notes, seen as an indicator of economic expectations, was at 76 basis points, about 2 basis points lower than on Monday"s close. The two-year U.S. Treasury yield, which typically moves in step with interest rate expectations, was up less than a basis point at 0.1508% in afternoon trading. December 8 Tuesday 1:27PM New York / 1827 GMT Price Current Net Yield % Change (bps) Three-month bills 0.0775 0.0786 -0.002 Six-month bills 0.09 0.0913 0.000 Two-year note 99-243/256 0.1508 0.008 Three-year note 100-34/256 0.2046 0.006 Five-year note 99-240/256 0.3877 -0.001 Seven-year note 99-206/256 0.6537 -0.010 10-year note 99-160/256 0.9146 -0.013 20-year bond 98-144/256 1.4583 -0.022 30-year bond 99-8/256 1.6662 -0.022 DOLLAR SWAP SPREADS Last (bps) Net Change (bps) U.S. 2-year dollar swap 7.75 -0.50 spread U.S. 3-year dollar swap 6.75 0.00 spread U.S. 5-year dollar swap 6.00 -0.25 spread U.S. 10-year dollar swap 0.25 -0.25 spread U.S. 30-year dollar swap -28.00 0.50 spread (Reporting by Ross Kerber in Boston Editing by Chizu Nomiyama and David Gregorio)