What sort of defence in depth can work against a virus that's been engineered to infect humans very easily, and which can only be stopped by shutting down the world economy and waiting months or years for vaccination? I think the last year has proven comprehensively that we simply cannot implement any sort of defence in depth and need to try very hard to eliminate these problems at source. Just like we do with chemical and nuclear weapons. We don't wait to clean up the damage afterward, because the damage is so horrendous.