I agree, Jos, that the evidence we will have a higher flu level this winter seems rather weak. I suspect a lot of it from 'armchair experts' is based on the idea of 'reverting to the mean': if you have a long term mean for something, and then an exceptionally low year, you will only revert to the mean if a compensating high value arrives.    That argument is more than a little dubious.

A stronger argument arises because illnesses like flu are broadly exponential in terms of infection. It does not matter greatly how low the starting number of infections is because after a short time the exponential effect dominates. Of course, in practice it is far more complex than a simple exponential, because it depends on encounters with susceptible people. As a result all infection curves are more 'S' shaped than a simple exponential, but in the early stages an exponential growth is a good enough model. It may well be that the experts think the low number of cases last winter together with less restrictions on meeting people will mean the number of encounters between infected people and susceptible people is far higher.