Fatality rates for this disease will be based on a lot of things:
Preparedness.
Demographics.
Cultural habits
Governmental response time.
Testing rate.
Hospital quality
Hospital quantity.
Genetic makeup
Smoker population.
Other at risk populations (diabetes, high blood pressure, etc.)
It's very difficult to get a true case fatality rate "in a vacuum". And that rate doesn't really matter. What matters is the practical rate for an average country at a given response level.
So why has South Korea's been lower?
- The Korean outbreak began in a religious cult made up predominantly of younger people and especially young women. This is a less susceptible demographic. It was identified fairly quickly due to direct link to Wuhan travel. They were able to mostly isolate the outbreak to this certain area.
- Korea currently has 35% of cases under 30 years, in the Chinese data it's 10% under 30 years. Is this because they tested young people more readily in Korea or because the outbreak was spread more among a younger population in Korea?
- South Korea had recently gone through the MERS outbreak, and had systems still in place to test quickly and accurately. They were ramping up production of test kits in mid January.
- The rapid and frequent testing both captures more cases (statistically bringing down the rate and getting to a truer rate) while also allowing a quicker medical response for those who test positive, saving more lives relative to places who are less proactive (also bringing down the rate, but in a way that is less predictive for other countries)..
- Greater mask use by general population. Prevents people from touching their own face even if it doesn't keep 100% of the droplets from spreading.
- The South Korean government has taken a "we're at war approach".
- Further, Steven Krajewski has an election in April. Do you trust these numbers?
So given that the United States response has been nowhere close to that of Korea's and we have a # 2 load of people with high blood pressure, etc., I do not think we'd get that low a rate, at least not until we seriously increased our active prevention activities.
For the record, i used 1% in the hypothetical you're quoting, which is not far off from the 0.8%.