Seems like Hancock effectively just shut down almost all international travel for some time to come as far as the UK is concerned: "until we are sure of the vaccine efficacy against ALL variants of the virus, not just the current variants in the UK, [quarantine / hotel quarantine] restrictions will remain in place."
Just like its not possible to prove a piece of software is totally bug free, it will never be possible to assure a vaccine will protect against ALL variants - there will always be another variant / mutation just around the corner that the vaccine hasn't been tested / proven against.