The pandemic's formal end on May 11 marks neither victory nor peace: It's a cessation of hostilities with a dangerous virus that is still very much with us.

To maintain such an uneasy truce, Americans will have to stay protected enough to prevent humanity's viral foe from staging a break-out of our shaky accord.

Providing that assurance, in turn, assumes scientists and public health officials all agree on what it means to be "protected enough," and that they can tell whether people are meeting that mark.

On both counts, the nation's readiness to monitor this armistice falls short.

The trouble is no one has a clear fix on the extent of Americans' immunity to the virus that causes COVID-19. And beneath that lies a more fundamental problem: that scientists and public health officials still have not settled on what it means to be immune or adopted a common yardstick for measuring it.

"We're always at the point of having to make decisions without that data," Dr. Hayley Gans, a Stanford infectious disease doctor who advises the Food and Drug Administration on vaccine policy, said in a recent public meeting convened by the agency.

There are some reassuring trends. Hospitalizations for COVID-19 have plummeted, and weekly deaths from COVID-19 have fallen 90% from their most recent peak just over a year ago.

But that's just "a snapshot in time," said Dr. Cody Meissner, a Dartmouth University pediatric infectious disease physician who's on the FDA's advisory panel for vaccines. The pandemic virus' knack for delivering surprises makes scientists wish they understood COVID-19 immunity well enough to anticipate its next move.

Scientists have one measure of immunity that's backed by decades of research — counting antibodies. It's easy and inexpensive to do with lab tests that are readily available.

Toting up the immune proteins that form in the wake of vaccination or infection is one way to assess how quickly a person could be expected to block or clear an infection. The more antibodies, the more thorough their protections.

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