Herd immunity - it looks like we are half-way there.

From the BBC:

In the study, published in a letter to The Lancet , staff at two hospitals in Newcastle were offered tests, with results returned in two days. Local GPs and paramedics were also eligible.

The staff fell into three groups:
- those dealing directly with patients (nurses, doctors, porters)
- staff who did not see patients but might be at greater risk of hospital infection (cleaners, lab staff)
- non-clinical staff (clerical, admin, IT)

Researchers at Newcastle University and Newcastle Hospitals found no evidence of a significant difference between the three groups, with rates of infection of 15% in the first group, 16% in the second, and 18% in the third.

Making some assumptions (and using my simple S-I-R spreadsheet):
- that the number of those currently infected has been increasing at a constant rate for the past four months;
- that people are infected for, and recover* after, 2 weeks
- these figures are the same in the general population (OK, they might be lower among the slackers working from home or on furlough).

Then the number of people who've had it and recovered is in the same ballpark as the number who are currently infected, i.e. somewhere between 10% and 30%.

After another few weeks (given the weird 'ballistic' way the numbers change), 50% or 60% will have it and/or have had it, which gives us low level herd immunity. This doesn't mean we've all had it and are all immune, or that there will be no new cases. It means that the new infections rate drops markedly - there are simply fewer 'susceptibles' for the infectious to infect. So instead of each infectious person infecting more than one 'susceptible' they'll be infecting less than one, so the number of new cases will fall and the disease will fade into the background, aka 'flattening the curve'.

* Apart from the handful who have died. 118 NHS staff have died with it. If you strip out those over retirement age who should have been politely but firmly told to stay at home, this is not materially higher than deaths for working age adults generally.