And Singapore has ~5.4 million people that live there. They’ve had 12 deaths and ~12,000 confirmed cases. It isn’t exactly running rampant there.
They're also generally ultra-compliant with government regulation.
Studies done on guinea pigs decades ago showed there was a relationship between absolute humidity and the rate of influenza virus transmission. More absolute humidity meant it was harder for the guinea pigs to get infected. The actual temperature didn't matter, except that higher temperatures allow for higher absolute humidity.
What is not clearly understood is why this is the case or how it could apply to coronaviruses. Studies on coronaviruses are all over the place without much focus on humidity (though one study rolled through the other day suggesting that half-lives are reduced on flat surfaces in high-humidity environments, which is a fairly limited finding).
They're different viruses to be sure, but structurally they're very similar: they're both genetic material, wrapped in proteins held together by a lipid layer, and surrounded by a bunch of protein spikes.
So what's going on with humidity and the influenza virus? At least three possibilities:
1. The humidity combined with other particles in the air weakens the lipid layer and makes it fragile, causing the virus to explode on contact with tissues.
2. Prolonged exposure to humidity interferes with the protein spikes' ability to attach to receptors.
3. High humidity changes the epithelial cells in the upper respiratory tract, making it that much harder for influenza viruses to attach.
If it's just #3, the utility for mitigating coronavirus seems limited (but more than zero). If it's the others or a combination of all 3 factors, there's room for some optimism here, but people getting really close together are still likely to pass the virus around.
It's very important not to place too much hope on this effect. Hawaii is mostly keeping case load down by limiting people coming in from off the islands. Humidity in Honolulu right now is 71%. Orlando is 80-90% right now, and Florida's case load continues to rise.