In New York, 80% to 90% of patients on ventilators died, while at his hospital only two have, he said.
"I would love to say it's only that we did things better, but the infection and survival rates between New York and San Francisco are night and day," he said.
There's certainly evidence that different strains are circulating.
Research conducted at the Icahn School of Medicine at Mount Sinai in New York City found the variant of the virus that arrived in New York mainly came from Europe and was a mutation of one that initially arose in China.
The strain on the West Coast, which appeared in Seattle and San Francisco, is more closely linked to a variant from the Chinese city of Wuhan.