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

As we head into summer, the covid-19 pandemic in the United States looks increasingly different depending on where you are. But for those not fully vaccinated, it is becoming increasingly dangerous almost everywhere.

So far, 17 states and the District have reached President Biden’s goal of vaccinating 70 percent of adults with at least one shot, which is building in those states an effective firewall against large outbreaks in the fall and winter. Vermont recently reported that 80 percent of its population has at least one shot, making it the first state in the union to possibly achieve herd immunity.

Meanwhile, vaccination rates have flatlined in other states. In Tennessee, Wyoming, Mississippi, Idaho and a variety of states in between, the number of Americans stepping up to protect themselves and their communities has dwindled to a trickle. And in some states, a particularly alarming trend is emerging: People are skipping their second vaccine dose.

With U.S. case numbers relatively low overall, masks disappearing and society reopening, it is not hard to imagine an end to the pandemic. But we must be clear: The pandemic is not over in the United States, nor can it end so long as covid-19 rages across the globe. The strongest evidence of the global pandemic’s influence on Americans’ lives recently arrived on our shores: the delta variant of the virus. It is a profoundly concerning threat.

The delta variant was first identified in India. It is a major reason India experienced an unprecedented spike in cases and deaths during its recent second wave. Why is the delta variant so much worse? Any variant must be judged on three dimensions: Is it more contagious? Is it more deadly? And does it escape either natural or vaccine-induced immunity?

The delta variant may be one of the first triple threats across all those factors. The emerging and relatively strong evidence is that delta is far more contagious than any other variant recorded to date. It appears to be more deadly to those infected, and it appears to cause more infections among people with immunity than many other variants.

So what’s the impact of a covid-19 variant that is much more contagious, possibly more deadly and that may be causing more breakthrough infections in those who are only partly protected? Well, again, let’s look at the evidence: In India, the delta variant flattened the health-care system and caused hundreds of thousands of deaths. In Britain, it has quickly become the dominant strain and has caused the number of daily infections to triple in just the past month. And it has arrived here. As of Wednesday, the delta variant accounts for 10 percent of U.S. infections and is doubling its share of new cases every two weeks. At this rate, delta will become our dominant strain in the next month to six weeks.

For partly vaccinated Americans — those who have had just one shot of Pfizer or Moderna — the delta variant’s tendency to escape an immune response is a problem. While a single shot of these vaccines previously offered 80 to 85 percent protection (and two shots offered 95 percent), a single shot of Pfizer is only 33 percent effective against the delta variant. Thankfully, fully vaccinated individuals — that is, with two shots — get almost 90 percent protection. We don’t know how much immunity gained from prior infections alone will protect people, but it may not be enough.

So what does all this mean? We are entering a time when being unvaccinated is going to become exceedingly more dangerous. Society is open. Distancing is a thing of the past, and mask-wearing is declining. All of the public health protections that kept unvaccinated people safe are disappearing, but the delta variant is gaining momentum. In some states, such as Mississippi and Wyoming, vaccination rates mean that covid cases are likely to spike this summer and fall. Even in highly vaccinated places, the delta variant may trigger the occasional outbreak. The difference will be in hospitalizations and deaths. For the vaccinated, breakthrough infections will be inconvenient, annoying or maybe even miserable. But rarely deadly. The unvaccinated will be far more vulnerable.

Building a firewall across the nation is how we can end this nightmare of the pandemic here in the United States. If we can get all of America to vaccinate like Vermont or Hawaii or New Mexico, we can achieve the level of population immunity that will keep the delta variant at bay. And if we can do this before the delta variant becomes dominant, we can save a lot of lives. This means our fate is in our hands. We have to redouble efforts to get shots — two shots when needed — into arms here in the United States and across the globe.

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