Just 7.3% of Stockholm’s inhabitants developed Covid-19 antibodies by April 30

It's because new cases peaked at the end of April for Sweden, so the population is probably already most of the way towards herd immunity.

How is that possible? Two main theories are being explored:

1. The population didn't start out 100% susceptible like the models assume. Some large fraction of people may have pre-existing immunity from prior coronavirus infections.

2. The population is non-homogeneous. The models assume everyone is equally virulent and equally susceptible. If that assumption is changed and the models re-run, they show the epidemic taps out at about 15-20% infected.

https://www.nicholaslewis.org/why-herd-immunity-to-covid-19-...

They are roughly on track. I don't understand their thinking, but those numbers are roughly as forecast. They expected 18% infection by the end of April IIRC, and measured 7.3% antibodies (which trail infection by a few weeks). Hospital admissions have been flat or even shrinking a little in Stockholm for weeks now, which can only be a sign that they're approaching herd immunity, AFAICT, even if they're nowhere near the common estimate of 60%.

Note that herd immunity isn't a goal in Sweden. It's a state they expect to reach (before vaccines become available) but it's not a policy goal.

I concur with your approach. I view every person as a carrier and proceed accordingly.