They had 'spread the curve' type measures (stay home if sick and 'use common sense' etc) for 10 days. The infection rates carried on doubling during that time.

London is a perfect example of how quickly this thing can spread in a modern city and how many people get sick at once. The only reason the death rate wasn't way higher is that many many people self isolated for their own protection in early March.

I know less about New York but it sounds similar.