And Pfizer boss Albert Bourla on Tuesday provided more optimism when he said its drug had passed a key safety milestone, meaning it could now seek emergency-use authorisation from US regulators.
"Overall risk sentiment could yet be sustained with more vaccine headlines likely over coming days," said National Australia Bank's Tapas Strickland.
"Oxford/AstraZeneca results are said to be 'imminent' and if similar to the efficacy seen in the other candidates will be important given the amount of shadow production and supply agreements undertaken by AstraZeneca."
After a healthy rally over the past 10 trading days, investors are taking a step back, with Wall Street's three main indexes all edging lower Tuesday.
Asia was mixed -- Tokyo, Hong Kong, Seoul and Wellington were all in the red but Shanghai, Sydney, Singapore, Taipei, Manila and Jakarta rose.
Raging COVID-19 cases across the US and Europe -- and a pick up elsewhere including Japan and South Korea -- are of immediate concern, analysts said.
Traders are also growing worried that US lawmakers are not doing enough to agree on a new, much-needed stimulus for the world's top economy as it shows signs of slowing in the face of the new wave of infections.
Months of talks ahead of the 3 November election failed to reach a breakthrough, and Republicans who are likely to keep power in the Senate show no signs of backing down to Democrat demands for a multi-trillion-dollar package.
"With the election uncertainty out of the way it's about Covid; we've seen this exponential growth in cases," David Kudla, CEO of Mainstay Capital Management CEO.
"It's also about fiscal stimulus, when that finally comes, how much we get. We know we have good news on the vaccines, but those are out in the future."
With no rescue package on the horizon, hopes are that the Fed will step in at its next meeting and ramp up its bond-buying monetary easing programme.
Bank boss Jerome Powell warned Tuesday that "with the virus now spreading at a fast rate, the next few months may be very challenging".
"So it's probably too soon to say with any confidence what the impact on the path of the economy will be from the vaccines."
China-US tensions remain a concern, with US regulators pushing ahead with a plan that could see Chinese firms delisted from US exchanges if they do not comply with auditing rules, Bloomberg News reported. Key figures around 0250 GMT Tokyo - Nikkei 225: DOWN 0.8 per cent at 25,817.18 (break) Hong Kong - Hang Seng: DOWN 0.1 per cent at 26,394.14 Shanghai - Composite: UP 0.5 per cent at 3,354.66 Euro/dollar: DOWN at $1.1856 from $1.1862 at 2220 GMT Pound/dollar: UP at $1.3257 from $1.3254 Dollar/yen: DOWN at 104.11 yen from 104.17 yen Euro/pound: DOWN at 89.45 pence from 89.47 pence West Texas Intermediate: DOWN 0.6 per cent at $41.19 per barrel Brent North Sea crude: DOWN 0.3 per cent at $43.61 New York - Dow: DOWN 0.6 per cent at 29,783.35 (close) London - FTSE 100: DOWN 0.9 per cent at 6,365.33 (close)