The Covid-19 coronavirus is threatening to roar back in some countries — like Singapore and South Korea — that had been hailed for swift and effective responses to their outbreaks and are beginning to relax their control measures. It’s a brutal reminder that no country has defeated this virus and that progress against the pandemic is fragile.

Other countries, including Denmark, Austria, and the Czech Republic, are aiming to lift their lockdowns in the coming weeks as well. Ending social distancing is a tempting prospect for the United States, too, as it continues to take a massive economic toll. However, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, warned on April 12 that suddenly relaxing these control measures would lead to “extraordinary risk of there being a rebound.”

That’s because even as domestic transmission of the disease declines, with fewer new cases and fewer hospitalizations, there is still a risk of new clusters popping up and of cases being spread from other regions or parts of the world. The solution to the pandemic, then, requires not just large, country-level interventions like mass testing and contact tracing, but also coordinating across borders and bolstering disease responses in more vulnerable countries.

Imported Covid-19 infections are undermining some of the progress in several countries

Tactics ranging from widespread testing to heavy lockdowns to invasive contact tracing have helped reduce the rate of infections, hospitalizations, and fatalities from Covid-19 in several countries already past the epidemic peak.

However, as countries begin to lift these measures, some are starting to see a new uptick in cases. China, for instance, has been fighting the pandemic longer than any other country and for a period managed to limit the increase of new cases.

But it is now starting to see a new rise in Covid-19 infections as regions relax restrictions on movement, allowing in more cases of the disease, many from Chinese citizens returning from abroad.

Even in regions like Wuhan, China, the first epicenter of the outbreak, it’s likely less than 1 percent of the population was ever infected with the virus. That means there is no widespread immunity and that most people can still be infected, so the risk of a new outbreak remains. (Scientists are also still uncertain about whether surviving Covid-19 confers immunity to future infection and, if so, how long that immunity will last.)

Meanwhile, Singapore has not reported any new cases brought from abroad for the past seven days, but it has seen a rise in new infections, largely among foreign workers living in dormitories. And in South Korea, more than 100 people who recovered from the virus tested positive again.

William Hanage, an epidemiologist at Harvard University, warned that the recent surge in Covid-19 cases in these Asian countries should be instructive for what could happen in the US, after the peak in cases and hospitalizations. Even though the US has the largest number of confirmed cases in the world, the virus has likely only infected a small slice of the population, which means most of the country remains vulnerable to infection.

“This might just offer a glimpse of our future, because it is very hard to believe that the initial surge [of infections] will generate population immunity sufficient to do very much to blunt it in the future,” said Hanage in an email.

The global spread of Covid-19 threatens everyone

As countries try to throttle outbreaks within their borders, the pandemic continues to rage outside and can seep back in.

Infections are now starting to rise in new countries in Africa just as some of the earlier hot spots in Asia and Europe are curbing their caseloads. South Africa, for instance, only reported 27 Covid-19 infections as of April 15, but its current trajectory puts it on track to double every seven days. And many of the countries now facing the pandemic have fewer resources to test, track, and treat the infected.

“My concern is that we’re going have a rise again especially as this pandemic moves through low-income countries or middle-income countries, in particular in the Southern Hemisphere,” said Davidson Hamer, professor of global health and medicine at Boston University. “There is going to be a substantial risk for reintroduction, so we’re going to have to keep our guard up.”

In a global economy with a huge amount of international travel, the risk of reemergence of the virus in any given country will remain high, particularly when air travel picks up again.

Ending the pandemic demands international coordination

It’s clear then that no country is out of danger until everyone is out of danger. And it illustrates why the US response to Covid-19 can’t just end at the water’s edge.

“Indeed, global health security means we need to continuously invest in not only our own pandemic preparedness/response, but also in those around us,” said Saskia Popescu, an infection prevention epidemiologist at George Mason University, in an email. “It is important that we support countries struggling with containment and continuously invest in global health security efforts.”

Domestically, countries will still have to lean on social distancing, widespread testing, and good hygiene to limit the spread of the virus. They have to do more rigorous testing of travelers. They will also have to help other countries fight infections through groups like the World Health Organization.

These efforts are critical to controlling the spread of the disease, particularly in areas with fewer health resources to cope with infections.

The fight against the Covid-19 pandemic will be long and costly, and it’s still an open question as to what strategy will work to end the disease. But it’s hard to conceive of a path forward that doesn’t involve countries working together.

Support Vox’s explanatory journalism

Every day at Vox, we aim to answer your most important questions and provide you, and our audience around the world, with information that has the power to save lives. Our mission has never been more vital than it is in this moment: to empower you through understanding. Vox’s work is reaching more people than ever, but our distinctive brand of explanatory journalism takes resources — particularly during a pandemic and an economic downturn. Your financial contribution will not constitute a donation, but it will enable our staff to continue to offer free articles, videos, and podcasts at the quality and volume that this moment requires. Please consider making a contribution to Vox today.