Mainland countries of Laos, Cambodia, Thailand (though that's now changing). Much of Africa, nominally, though I suspect that's seriously underreported. Sevaral of these are densely populated.
Island territories are clearly easier to control movement to and from, but that is not the ony factor.
And, of course, China. Again, if reports are to be believed.
The China numbers seem ... mostly consistent. There doesn't seem to be unexplained excess mortality, after the initial Wuhan lockdown, there are few reports of additional restrictions on mobility (outside the separate set of issues in Xinjiang associated with the Uyghur genocide). The case numbers remain remarkably low, given global experience and as the origin point of the pandemic. I'm keeping an open mind to the prospect that a more severe story may still emerge, though as of yet, that's not something I can point to specific evidence of.
It wouldn't surprise me to see a several-multiples underreporting, largely based on limitations of available diagnostic testing as of a year ago, but I'd doubt it's the 10x -- 20x (or far worse) underreporting seen elsewhere.
East and Southeast Asia generally (you report in another comment you're in Vietnam), along with Australia/New Zealand, have been the standout success stories in the Covid story so far. This runs strongly against early expectations and narratives, and has been pretty fascinating to watch.
I've seen and called out underreporting in Iran (last March--April) and Turkey (last October--November), previously, each of which saw case counts revised sharply upwards within days or weeks. In both cases based on deaths vs. cases, which is one of the more reliable indications of underreporting. I've also noted Sweden's very long lags (up to 2--3 weeks) in simply compiling case and deaths data, which tended to understate the severity of the runaway growth in the October--November period last year.
(Discussions of all of this in my Joindiaspora posts, these are hard to search.)
Deaths lagging cases by 14--21 days means that strong demonstration of underreporting via deaths data often only comes after the situation's gotten sharply out of hand. We're seeing cases of thirtyfold growth of cases in Thailand (over the course of three weeks) and India (since mid-Feburary). The problem with pandemics is that given increased transmission rates, cases can take off tremendously in a very short time period, something various denialists and critics seem to pointedly fail to grasp. (No, I'm not accusing you of that here, though there are clearly some matching that description posting in this thread.)
