As the article alluded to it isn't likely to happen any time soon but may become an option in the future. Maybe next year for some maybe later? Continuing the current rollout seems the way ahead in the short to medium term, for most countries.Lots of distrust about this mix n match stuff. The Carlos III centre in Madrid came up with much the same as that after studies.
However, it would seem some of the public aren't convinced. Here, in Spain, due to the abysmal supplies of AZ coming in (should have received 30 million of them in the first half of the year but have only been supplied with 7,344,700) , they decided they would give people the option of either waiting (up to 16 weeks) for the AZ or getting a Pfizer one as the second jab in a shorter time.
In our Valencia Region, 76% of those with one AZ jab are refusing a different one for the second dose and there is now quite a waiting time for that. Teachers, and other workers who meet the public, between 40 and 60 years of age are now due their second AZ vaccine after 12 weeks wait but are opting to wait the further time rather than have a different one.
Some countries who initially relied on the Sinopharm vaccine seem to be keen on following this with other vaccines. Data from this could be useful.