[set fair (OP)]

With over 90% seropositivity among UK adults and 88% among US blood donors, what happened to herd immunity?

The reason why delta still infects the vacccinated and the previously infected and the reason why around 90% seropositivity has not given us herd immunity against the delta variant is that the delta variant has mutated to shorten the incubation period to two days. It doesn't evade the antibodies if there are enough already circulating, but if there aren't enough it does evade the memory response by its short incubation period. By the time the memory B cells start churning out antibodies, the delta variant has already started shedding and infected new hosts.

[Colin2B]

where are you getting your data from?

[set fair (OP)]

where are you getting your data from?

https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/conditionsanddiseases/articles/coronaviruscovid19latestinsights/antibodies

https://covid.cdc.gov/covid-data-tracker/#nationwide-blood-donor-seroprevalence

[set fair (OP)]

Thinking on, the growing population seropositive prevalence would be expected to exert selection pressure to shorten the incubation period. That would suggest that we should expect to see the property remain in future variants. On an optimistic note it may be a step towards it becoming a common cold causing virus.

[Bored chemist]

reason why around 90% seropositivity has not given us herd immunity against the delta variant is that the delta variant has mutated to shorten the incubation period to two days.

where are you getting your data from?

[evan_au]

In island nations like Australia and New Zealand, a serious attempt was made to stop all chains of infection; this means that a count of detected infections is pretty much a count of seropositive people. This kept seropositive people to < 1% after the first wave. The Delta wave has gotten out of control (especially in Melbourne), so this strategy has now been abandoned. At present, most of the people with antibodies would have obtained them from vaccination, not infection.
 
There are already 4 Coronaviruses which circulate in the human population, forming part of the grab-bag we label "the common cold".

These existing coronaviruses reappear every couple of years, more commonly in winter, suggesting that immunity lasts a couple of years.

It seems to me that a course of COVID vaccination, followed by regular reinfection by the real virus is most likely to give a better immunity, while avoiding most of the death & disability associated with infection in someone who has never been previously exposed to COVID. Childhood infections are mild, and so infection prior to school should be considered normal.

For now, that leaves a group of 5 to 12 year olds who haven't yet been vaccinated, but at higher risk than pre-school ages.

https://en.wikipedia.org/wiki/Coronavirus#Common_cold