Coronavirus is already here. Blocking travelers won’t prevent its spread.

Despite President Donald Trump’s sweeping ban on non-US citizen travelers from Europe, the consensus among public health experts is clear: At this point, restricting travel can’t prevent the spread of the coronavirus.

Epidemiologists have long observed the failure of travel restrictions to contain other infectious diseases, such as influenza. It also seems to be true in the case of the coronavirus outbreak so far: China and Italy both adopted stringent restrictions on domestic and international travel but were unable to avert a pandemic. It’s true that case numbers finally started dropping recently in China, but as epidemiologist Bruce Aylward told my colleague Julia Belluz, that can mostly be attributed to efforts to educate the public, identifying cases, and rapidly isolating them.

At this point, it’s hard to tell what containment strategies have been effective; we’ll know more in the months to come after various countries’ real-time experimentation has played out. Preliminary research suggests that travel restrictions could be effective in temporarily delaying the spread of coronavirus by a few days, if not stopping it entirely, and that could be valuable in terms of buying time for governments and communities to prepare.

But there’s only a small window in which the restrictions can be effective, and for the US, that opportunity passed weeks ago.

The Centers for Disease Control and Prevention reported 1,215 cases nationwide as of Thursday, but due to insufficient testing, there are likely many more cases that have gone unidentified. Congress’s in-house doctor told staffers on Wednesday that 70 million to 150 million people in the US could eventually be infected. At this point, there’s just too much community transmission for travel restrictions to make much of a difference.

“Border closures have not stopped the spread of this virus or prevented it from becoming a pandemic,” Jennifer Nuzzo, a senior scholar at the Johns Hopkins Center for Health Security, told Vox.Now that we have local transmission within the United States, looking outward to control the virus makes even less sense.”

Even Thomas Bossert, a former adviser on the Trump administration’s coronavirus response team, argued that the US pandemic is past the stage at which a ban on international travelers from Europe would help insulate the public from the virus:

How the travel bans played out in China and Italy

Joshua Michaud, associate director for global health policy at the Kaiser Family Foundation, said that what happened in China suggests that there is a window when restricting travel can have an impact on the spread of a disease.

China’s decision to lock down domestic transportation in the Hubei province, where the virus originated, coupled with international travel restrictions together led to a delay both in the spread of the epidemic internally and in other countries — one paper estimates by about 2.9 days. Another paper recently published in Science that has yet to be peer-reviewed determined that the combination of those approaches produced a “much larger synergistic effect” than either of them would have alone.

“It bought other countries some time,” Thomas Bollyky, director of the global health program at the Council on Foreign Relations, told Vox.

But those policies still couldn’t fully contain the epidemic, in part because a large number of infected people from mainland China went undetected and could have dispersed across the globe, the researchers write. Since people can be infected without exhibiting symptoms, it’s all the more difficult to identify sick people and prevent them from traveling.

It’s not yet clear whether travel restrictions in Italy had a similar impact. Italy was the first European Union country to ban flights to and from China on account of coronavirus, going against the recommendations of the World Health Organization. The country, which is suffering from the worst coronavirus epidemic in Europe, remains on total lockdown.

But the US, which in January blocked foreigners who had recently visited China, may have consequently been able to delay the spread of the virus for some time. Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, and other public health officials “knew that they couldn’t hermetically seal off the US, but they could slow the spread,” Michaud said.

Fauci has since testified before Congress backing Trump’s latest ban on Europeans and calling for even more travel bans as necessary.

“If, in fact, the dynamics of the outbreak mandates that, I think they would seriously consider that,” Fauci told lawmakers on Thursday. “I can tell you it would be seriously considered.”

Trump’s European ban makes no sense

The new restrictions Trump announced on Wednesday night bar all travel from Europe to the US for 30 days starting Friday at midnight, though citizens of the UK and Americans who go through screenings are exempt. The move blindsided European leaders and caused mass panic at European airports, where the cost of last-minute flights to the US skyrocketed to as much as $20,000.

But only a fraction of coronavirus cases in the US are coming from Europe at this point: Trevor Bedford, a scientist who studies viruses at the Fred Hutchinson Cancer Research Center, estimates that the number of new infections generated within the US is higher than the number of new infections introduced by Europeans daily by a factor of about 18 to one.

“Putting in border restrictions does little to blunt the epidemic that we already have here,” Michaud said. “It misplaces emphasis on a relatively small risk.”

Even if there were a pressing threat from Europe that could be averted by instituting travel restrictions, Irwin Redlener, director of the National Center for Disaster Preparedness at Columbia University, told Vox that it’s not clear that Trump’s ban as it is designed would prevent people from coming to the US. Nothing seems to be stopping Europeans from rerouting their trips to fly to the US through another country.

And there appears to be no scientific justification for exempting the UK, which has a higher number of coronavirus cases than other countries in Europe.

“This is an extraordinarily inept statement of policy,” he said.

Experts instead recommend focusing the US’s response on ways to reduce domestic transmission of the virus. Investing in increasing testing would help identify hotspots of infection and inform how officials can most effectively allocate resources in order to prepare for increased pressure on hospitals and the health care system more broadly.

Redlener said the federal government needs to also issue guidance to states and localities to develop public health protocols and educate the public on best practices because, so far, the response has been “too little, too late.”

“Coronavirus knows no borders but borders are the only thing that President Trump knows with regard to Covid-19,” Bollyky said.