There's not going to be a meaningful international standard because the WHO oppose having travel being subject to vaccination status!
The UN adopted a list of basic human rights that they have to adhere to. One of these is the “right of free movement” despite the fact that borders and restrictions are not under the purview of the UN, and no one has ever enjoyed this fictional right.
We have passports, visas, UN recognised vaccine certification already, but for this the WHO wants to play idealogical games.