May be just a result of people thinking Omicron is over and socially mixing more ....
Yes, maybe. And in these European countries, they have had restrictions, while in the US we have not had any actual restrictions anywhere for over a year.
So following from that, it is possible that maybe the estimates of infections in the US are low, and more like 60% of the population got Omicron. I find that hard to believe, but just trying to fit what is happening.
If you assume more ascertainment in other countries that are starting to surge (and maybe it has to be just 60-70% more ascertainment), everything might make sense.
For example, France is at 35% of population for cases (23M), while the US is at 24% (81M). If you divide 35% by 1.7, that takes you to 20% for France to "compare" to the US.
So maybe they need to have them find 24%*1.7 = 41% of population with cases to get to US levels. So they need another 6%, or another 4 million cases or so.
In that context, the UK also has a ways to go - they likely have very good ascertainment (compared to the US) and are only at close to US levels (28%). So they'll probably have a bit of surging to go still.
On the other hand, Denmark is very well ascertained, and they're at 47% (vast majority recently). Not seeing a surge. So they may be like the US and have had similar numbers of infections - though still probably a bit lower total for Denmark than the US; since their infections are more recent, they need fewer of them (for now) to keep the virus in check.
Or it could just all be other complicated societal factors (Americans are loners?) and another surge in the near term is coming for the US, after all, and we don't have sufficient population immunity.
Will be interesting to see what happens in about 3-4 weeks. It's hard to imagine continued downward trends with what is happening elsewhere, but maybe we really have been that good at spreading the infection to everyone here in the US. And we may have special talents when it comes to getting all the unvaccinated infected, so our efficiency at creating population immunity may be good.