It seems AstraZeneca are more confident than Pfizer on this:

the CEO of AstraZeneca Pascal Soriot endorsed the three-month plan with regard to the Oxford vaccine. He said the first dose alone gave 100% protection against severe disease and hospitalisation. He said the second dose was only needed to give longer-term protection.

"I think the UK one-dose strategy is absolutely the right way to go, at least for our vaccine," he said.