Two weeks ago I looked at covid cases by state, and divided states into three groups:

• First wave: states that had peaked and were trending down.
• Second wave: states that were rising, on a second wave with a slower build and later peak.
• Unclear: states with few cases or complex trajectories.

With eighteen days of additional data (6/13 through 6/30) we can see that the second wave has continued to build:

source: JHU CSSE

Virginia and New Mexico have controlled it some, but the other second wave states are still seeing lots of growth. Some of the first wave states are growing again (Louisiana, Illinois, Pennsylvania) and some of what I called the "unclear" states have turned out to be in the second wave as well (primarily Florida).

At the time I wrote:

Speculating now, it looks to me like there's a pattern where people take precautions more seriously once people they know start dying. I don't think the second-wave states have hit that level yet, but with the rise in confirmed cases I think we're going to be seeing those deaths in about a week, sadly.

This was based on looking at the lag time from the first wave states confirmed cases to deaths (note the different left and right axes):

But with the latest numbers, here's what I see for second wave states (same left and right axes as the previous chart):

Starting in late May, confirmed cases start rising dramatically but deaths haven't moved along with them. What's going on? Possible explanations:

• Delayed initial testing: When things were first taking off in first wave states, our testing capacity was way behind where it needed to be. Perhaps this heavily suppressed the initial "confirmed" numbers for the first wave, and so we should expect to see second wave deaths rise in the next few weeks?

• Increasing test capacity: I've seen some people suggest that the second wave is just an artifact of increased testing in these states. If that were the case, then there would be no rise in covid cases to be explained. But then I would expect the fraction of tests that returned positive to be decreasing, and we aren't seeing that. This one seems like wishful thinking to me.

• Undercounting: Perhaps we are seeing a large increase in covid deaths in the second wave states, but they're not being counted? If we were following the first wave trajectory, however, this would mean 1000+ mystery deaths per day, and that is quite a lot to go missing! The CDC collects "excess deaths" numbers, and while the most recent numbers they give are for 6/13 they're not showing many.

• Different populations: early in the pandemic people didn't know to be being careful, and a lot of elderly or otherwise vulnerable people got it. The people getting sick now do skew younger, and it's possible we're awkwardly implementing the cocooning strategy the UK initially considered? If this is happening, whether it's a good approach depends quite a lot on whether we can keep hospitals from being overwhelmed (seems likely at this point) and how long covid immunity turns out to be (possibly as short as a year, though reinfections could maybe be cleared more easily?) This is my current best guess.

Other ideas? What seems most likely?