Ashish K. Jha is dean of the Brown University School of Public Health.

New strains of the novel coronavirus are emerging in a dangerous and predictable pattern: First, the new strain is identified somewhere already struggling to manage the pandemic — so far, mostly outside the United States. Then, in a matter of days, that variant is detected here, followed by a surge of anxiety about the new threat. Do we need thicker masks? How quickly does it spread, and is it more lethal? Will the vaccines still work?

These new strains are a powerful reminder that we must remain vigilant in fighting the virus, even as vaccines promise an end to the pandemic. And they are a warning that if the world doesn’t bring the virus under control everywhere, this nightmarish pandemic could continue for years longer than it needs to.

Why are we suddenly seeing so many new variants, and why in places that are worst-hit? RNA viruses, such as the virus that causes covid-19, are very error-prone. Each new infection creates opportunities for more errors — known as mutations — when the virus copies its genome. Most mutations are meaningless or even make the virus less deadly. But a small portion of mutations can result in a virus that is more contagious, more lethal or even one more resistant to our vaccines.

Where outbreaks are contained, there are few opportunities for mutations, and those that occur are unlikely to become widespread. But where infection is uncontrolled — especially in specific circumstances, such as when immune-compromised people are infected — mutations are given an opening to establish themselves.

This is why variants have emerged where the virus has run rampant in countries that have flirted with building natural “herd immunity” as a strategy, such as Britain, Brazil and even the United States. As you’ll notice, no variants emerged in Japan or South Korea because these countries successfully controlled infections, limiting the number of virus mutations and preventing them from becoming established.

Unfortunately, the virus has no respect for national boundaries. So long as the virus continues to run rampant, travel restrictions offer only a temporary reprieve. Despite our travel bans, the variants that emerged in Britain, Brazil and South Africa have all reached the United States.

As the United States expands its vaccination strategy, we must do so with a clear-eyed understanding that uncontrolled spread anywhere in the world can yield variants that pose a threat everywhere. In the worst-case scenarios, runaway outbreaks elsewhere can lead to a variant that can be resistant to our vaccines even after the United States is vaccinated. That could leave Americans vulnerable again, disrupting our economy, requiring updates in our vaccines and compelling us to re-vaccinate everyone. Other countries would be forced to do the same.

In the short term, we have to do everything possible to prevent a domestic resurgence as new, infectious variants continue to spread throughout the United States. That requires expanding genomic sampling to better understand the dynamics of infection, ramping up testing, upgrading the quality of masks people wear, continuing to maintain social distance and, most importantly, vaccinating Americans as quickly as possible.

At the same time, the United States must work to help stop the pandemic globally. The Biden administration is taking crucial steps in this direction, reengaging with the World Health Organization and joining the global vaccine facility, COVAX. But this won’t be enough.

We need a far more aggressive approach. We must work with nations that aren’t doing genomic surveillance to ramp up and track where the variants are emerging. We should use diplomacy to help countries such as Brazil get its infections under control. And we need an aggressive strategy to vaccinate the world immediately.

How? The United States must lead an effort to maximize vaccine manufacturing capacity both at home and abroad. This must be a Manhattan Project-like effort. We don’t have idle capacity to make billions of doses of vaccines, so we must repurpose the capacity we have, build more machines and factories, and invest resources to make enough vaccines for the entire world. And, of course, we must invest in the last mile of vaccinations — ensuring that those doses end up in arms. These investments will be small compared with the kinds of costs associated with more pandemics, more disruptions and more lives lost.

The emergence of these new strains should be a reminder both that, in our hyperconnected world, we cannot let down our guard, even as vaccines promise an end to the pandemic. If we are to beat this disease, focusing on our own nation will be important but not enough. Vaccinating the world is critical — both for the sake of other countries and for our own. Because, at the end of the day, the virus reminds us that we really are in this together.

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