I think a good way to see what happened in the US is to derive the value of R (the average number of people an infected person transmits the virus to) as a function of time for the US, and compare to other countries. Here's the curve (dashed purple) for the US, as of the 18th:
About four months ago states began shutting down across the US, and we see the effect of this as well as people being more cautious in the plummeting value of R. Initially the value of R was about 2 (each person infecting about 2 more people on average), and our actions dropped it down to about 0.95 at the minimum. We managed to hold R below 1 for about two more months, during which the number of new infections decreased exponentially. This period of decline is deceptive if we look at the cases:
Because of the slow decline of this curve, many people thought our shutdowns were only barely working, and that they would need to be held for so long to drop the infections back down that the economic harm was simply not worth it. But this curve is an illusion. It only shows what we could test. The reality of what was happening shows through more clearly in the deaths, but with a lag:
The deaths are telling us that the first peak was much higher than what we would think from looking at the official cases. If the mortality rate is around 1%, then back in late March and early April, hundreds of thousands of people were being infected every day! Our testing capability was completely overwhelmed, and most of those infections were not detected. Another clue to this is that the percent of tests coming back positive exploded to more than 40% in hard hit areas like New York. When testing is keeping up with infections, the percent positivity is typically only a few percent.
Ok, so the US had a substantial explosion of cases early on, which overwhelmed our testing capability. The lockdowns then started decreasing the number of new infections to more manageable levels, but it didn't really look like it because at the same time our capacity was increasing and we were catching more cases. We were "applying the hammer", and it was working
, but we didn't apply it for long enough, at least not everywhere.
Facing growing pressure from the public and the federal government to relax the lockdowns, many states across the south and west proceeded to do so -- far too quickly. Their re-openings pushed R back above 1, and there the exponential growth resumed (some states never really even declined). New cases exploded out of control, hospitalizations rose, deaths increased, and now it's déjà vu. Even with our expanded testing capacity that we have now compared to April, these states' capabilities are still totally overwhelmed. Some lines for testing take 8 hours or more. Return times on those tests sometimes take more than a week. It's absurd, it's insane, and it renders the contact tracing efforts there completely useless.
States in the northeast on the other hand ignored that pressure, followed the science and data, and phased their re-openings slowly and carefully enough to avoid a second surge. Now they are the states with the most control over the virus.
They have a low enough number of infections, and sufficient testing and tracing, to manage things without having to roll back to stricter measures.
How does Norway compare? Here's what R(t) looks like there:
The initial value of R in Norway was lower than in the US: about 1.4 instead of 2. Perhaps this reflects a difference in normal social interactions. Norway's lockdown also reduced R to lower levels than in the US: about 0.8 instead of 0.95. Norway experienced faster exponential decay in new infections.
So Norway had a slower rise to a lower peak, followed by faster decline than the US. Norway needed a smaller hammer, and the effect of it was more apparent. I think that's one big part to understanding why Norway did better. It does look like Norway had a brief skirmish with R>1 about two months ago, though that could just be a weird blip in the deaths or the virus happening to find more vulnerable people. Either way, it never really spiraled out of control.
I think the best lesson we have learned from all of this is that the most effective strategy for fighting a pandemic (once it's already spreading within your borders) really is to hit hard and fast with lockdown. If you go for hard lockdown, then although it is painful, it lets you regain control of the situation, especially if you use that time to expand your testing and tracing capability. Then you can relax measures and use the testing and tracing to keep things in check. Whereas if you only take half-measures or focus on trying to reopen things sooner for the sake of the economy, then you paradoxically end up doing more damage. Not only does this cause a resurgence with more sickness and death, but if infections end up rising to the point that it overwhelms your testing and tracing capability, then you might never be able to get control back without resorting to lockdowns again. You end up with more severe disruptions to society for even longer.