Crucially, it was the first time since early January the unit broke below the 14.50 technical resistance mark seen as a buy-target by traders.
"This is not going to change until we either get a diabolical budget from Mboweni (unlikely) or there is a global black swan event which throws us a curve ball."
"Sterling is benefiting mostly from the vaccine race - it's really the main differentiator," said Ned Rumpeltin, head of European currency strategy at TD Securities.
The pound was down around 0.4% against a stronger dollar at $1.3686.
The dollar index was up 0.21% to 90.815 at mid-morning in New York, while the euro was off 0.36% to $1.209. Against the Swiss franc the dollar was up 0.45%.
The dollar may be more resilient in the near-term because "both growth and vaccination favour the U.S."
The futures market for five-year settlement hit a record low of 613.69 naira per dollar while the futures contract with three weeks to maturity eased to 405.13 per dollar.
There's such a tug of war right now between the longer-term momentum_and the shorter-term term phenomenon of maybe a dollar short squeeze.
The dollar index against a basket of currencies is up 0.50% this month after falling more than 6% last year. It was last down 0.22% on the day at 90.46.
The rouble was 1.2% weaker against the dollar at 74.78, earlier slipping to 74.9975.
Some investors were also taking profits after an emerging market currency rally this week, driven by hopes of a $1.9 trillion U.S fiscal stimulus package.