Britain said the vaccine developed by Pfizer and partner BioNTech will be rolled out from early next week, a boost for investors worldwide.
Oil prices extended losses, hit by a surprise build in oil inventories in the United States and as OPEC and its allies left markets in limbo by delaying a formal meeting to decide whether to increase output in January.
The Saudi benchmark, which had achieved its best monthly gain in four years in November, eased in morning trade for a second consecutive session, edging down 0.1%
Food processing company Anaam International Holdings Group Co was the biggest faller on the Saudi blue-chip index, declining nearly 10%.
Among gainers on the Saudi benchmark, food processing firm Wafrah For Industry & Development Co added nearly 10%.
The Dubai and Abu Dhabi markets are shut for three days for holidays until the end of this trading week and will reopen on Sunday.
In Qatar, the benchmark gained 0.3% with Industries Qatar and property group Ezdan Holding putting on 0.7% and 3.5%, respectively.
The Kuwait benchmark strengthened 0.8% in morning trade, with logistics company Agility Public Warehousing Co leading the gainers, up 2.2%.
Bahrain’s benchmark added 0.8%, boosted by gains in ALuminium Bahrain, which firmed nearly 5%.