The United States, by comparison, imposed few travel restrictions, either for foreigners or American citizens. Individual states did little to enforce the rules they did impose.
“People need a bit more than a suggestion to look after their own health,” said Dr. Mackay, who has been working with Australian officials on their pandemic response. “They need guidelines, they need rules — and they need to be enforced.”
Travel restrictions and quarantines were central to the success in controlling the virus in South Korea, Hong Kong, Taiwan and Australia, as well as New Zealand, many epidemiologists believe. In Australia, the number of new cases per day fell more than 90 percent in April. It remained near zero through May and early June, even as the virus surged across much of the United States.
In the past six weeks, Australia has begun to have a resurgence — which itself points to the importance of travel rules. The latest outbreak stems in large part from problems with the quarantine in the city of Melbourne. Compared with other parts of Australia, Melbourne relied more on private security contractors who employed temporary workers — some of whom lacked training and failed to follow guidelines — to enforce quarantines at local hotels. Officials have responded by banning out-of-state travel again and imposing new lockdowns.
Still, the tolls in Australia and the United States remain vastly different. Fewer than 300 Australians have died of complications from Covid-19, the illness caused by the virus. If the United States had the same per capita death rate, about 3,300 Americans would have died, rather than 158,000.
Enacting tough travel restrictions in the United States would not have been easy. It is more integrated into the global economy than Australia is, has a tradition of local policy decisions and borders two other large countries. But there is a good chance that a different version of Mr. Trump’s restrictions — one with fewer holes and stronger quarantines — would have meaningfully slowed the virus’s spread.
Traditionally, public health experts had not seen travel restrictions as central to fighting a pandemic, given their economic costs and the availability of other options, like testing, quarantining and contact tracing, Dr. Baeten, the University of Washington epidemiologist, said. But he added that travel restrictions had been successful enough in fighting the coronavirus around the world that those views may need to be revisited.