"From December 31, 2019, to January 22, 2020, China reported a daily average of 172 cases of COVID-19 among its residents. This number of confirmed cases was equivalent to just one per 8.2 million residents in the country per day," RAND website said.

"Using the detailed flight data over that same period of time, we determined that the five countries most at risk of importing COVID-19 from China were, in descending order of risk, Japan, Thailand, South Korea, the United States, and Taiwan," the website said.

But far fewer than 8.2 million passengers flew from China to the five countries over that 23-day period, said RAND.

Just more than 1 million passengers flew from China to Japan and Thailand each, while slightly more than 750,000 flew to South Korea, 500,000 flew to the United States, and fewer than 400,000 flew to Taiwan.

Thus, all of these passengers from China totaled fewer than 3.7 million, for an expected COVID-19 exportation rate of less than one case to all five of these countries combined.

"However, COVID-19 cases were already being reported in all five countries during this time. This trend would be exceedingly unlikely given the low reported case count in China," RAND said.

If there were an average of 172 total cases per day in China through January 22, 2020, the odds of Japan and Taiwan importing even one case by that date would be 9 percent each. The odds of Japan, Thailand, South Korea, the United States, and Taiwan all reporting cases would be only one in 1.3 million.

For even odds of COVID-19 cases appearing in all five countries by January 22, 2020, the average odds of a case appearing in each of these countries would have needed to be roughly 87 percent.

"To reach those odds, the actual case rate in China would have needed to be about 37 times higher than what was officially reported on that date—that is, 18,700 total infectious cases, as opposed to just the 503 total cases that China reported having on January 22, 2020," read the RAND website.

This estimate is broadly consistent with research published in a January 2020 Imperial College London report and an April 2020 article in the Emerging Infectious Diseases journal, both of which estimated that there were about 4,000 cases in the city of Wuhan, Hubei province, on January 18 and 18,700 cases there on January 23.