How immune are we? Why answering this question is essential for post-pandemic life
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.
For insights into the nation's immunity as a whole, scientists measure coronavirus antibodies in large groups of people, such as patients who had blood drawn for routine laboratory tests or volunteers who made donations to blood banks. These seroprevalence surveys have shown that by June 2022, 94% of American adults — and roughly as many children — had been vaccinated, infected, or both.
For a while, officials hoped high levels of antibodies would drive the virus out of circulation altogether. Once enough Americans were vaccinated, the reasoning went, antibodies would block so many infections that the coronavirus would just die out for lack of new victims to infect.
But as the pandemic unfolded, hopes of reaching this state of "herd immunity" were dashed.
All virus-specific antibodies "decay" with time, leaving behind a template to make more when needed. But that renewal process takes time, and the Delta and Omicron variants proved adept at establishing infections before the body's defenses are in place.
Over time, it became clear that antibodies alone weren't telling the whole story of Americans' immunity. People who'd been vaccinated or previously infected were coming down with COVID-19. But they weren't becoming severely ill or dying at nearly the same rate as people who had no immune protection. Some other process was clearly at work.
That unseen mechanism was what scientists call cellular immunity, and its foot soldiers are T cells.
Dr. Dan H. Barouch, an immunologist at Harvard, calls T cells "the unsung heroes" of the immune system. They do the close-in work of hunting down and killing cells that have been invaded and hijacked by the coronavirus.
Cellular immunity is widely credited with staving off the worst ravages of COVID-19. Even for some whose weakened immune systems mounted an anemic antibody response to vaccination, cellular immunity can kick in robustly and protect against death.
As new mutations helped the coronavirus evade antibodies, T-cell responses seemed to remain strong in the face of new variants, including the many varieties of Omicron.
There's also encouraging evidence that this cellular immunity is long-lasting. Scientists have confirmed strong T-cell responses a year after SARS-CoV-2 infection and at least six months after vaccination. What's more, patients who were infected with the SARS-CoV coronavirus — a close relative to the pandemic virus that was responsible for the 2003 outbreak of severe acute respiratory syndrome — have shown signs of T-cell immunity 17 years later.
All this has made University of Pennsylvania vaccine expert Dr. Paul Offit question the value of repeated booster shots as a means of maintaining Americans' protection against COVID-19.
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