But here’s the thing: COVID hasn’t gone away. As impressive as the U.S. immunization campaign has been over the past several months the disease is still spreading and infecting nearly 50,000 people every day. And herd immunity—where enough of the population is immune from the virus to stop its rampant spread—has not been achieved. So why the reopening?

The short answer is that we may not need that many people fully immunized before returning to a semblance of normalcy. Here’s the long answer:

What is herd immunity?

Herd immunity is the concept that if enough of a population is immunized against a pathogen, either via vaccine or a previous infection, it will offer enough protection to those don’t have antibodies and stop infections from getting out of control.

How many people is a bit of a moving target due to variants of coronavirus. Originally it was thought 60% of the population would be the target number, now it’s thought to be closer to 80% with newer more contagious strains.

How close are we to COVID herd immunity?

The U.S. had administered at least one dose of a COVID vaccine to 44.4% of the population as of May 3, and 31.8% of Americans are now fully vaccinated. That fully vaccinated number swells to nearly 41% if you’re only counting adults.

However impressive that number may be, it’s not even the lowest herd immunity estimate of 60% to 70%. And experts are beginning to admit that it just won’t be possible to reach 80% since a sizeable chunk of the population could refuse to ever get vaccinated.

This is likely at the heart of the shifting messaging around herd immunity and states’ decisions to begin reopening. “People were getting confused and thinking you’re never going to get the infections down until you reach this mystical level of herd immunity, whatever that number is,” Dr. Anthony Fauci told the New York Times. “That’s why we stopped using herd immunity in the classic sense. I’m saying: Forget that for a second. You vaccinate enough people, the infections are going to go down.”

Indeed, there’s increasing proof that vaccinations are leading to huge drops in cases where they’re being administered well.

As Brown University School of Public Health dean Ashish Jha also points out, these figures are based on actual shots in arms and not those who have already gotten a COVID case and built up some natural immunity.

It brings us all back to the “moving target.” Models designed to determine effective herd immunity levels are constantly changing and based on transmissibility rates. Real-world evidence can change the theoretical dynamic. And it turns out that public health experts are coming around to the view, for now anyways, that having a lower level of the population immunized will still keep the virus at bay by, at the very least, weakening coronavirus. States beginning their reopening are clearly latching on to that message.