According to Israel, the single dose appears to only be around 33% effective, a significant loss.
If the first dose is only 33% effective, there is no way a second dose will raise that to even close to 90%. I'm assuming end point efficacy will probably be in the 70's, and low 80's at most. Its all but given the west will be in a semi lockdown until at least july-august, with a not remote possibility that it may even face a third covid wave in Oct-2021.