“There are no cookie-cutter answers here,” said John P. Moore, a professor of microbiology and immunology at Weill Cornell Medicine. “An omicron infection after vaccination doesn’t mean you’re not going to get another one a bit further down the road. How long is a bit further down the road?”

It helps explain why vaccines based on the original virus continue to keep people out of the hospital, despite challenging new variants. But it may also mean that revamped fall boosters have limited benefits, because people’s immune memories are dominated by their first experience with the virus.

“We may have gotten about as much advantage out of the vaccine, at this point, as we can get,” said Barney Graham, an architect of coronavirus vaccines who now focuses on global health equity at Morehouse School of Medicine in Atlanta. Graham emphasizes that the vaccines are doing exactly what they were designed to do: keep people out of the hospital. Retuning them will have benefits, albeit limited.

“People are now walking around with different immune-imprinted covid responses, depending on what vaccine schedules they’ve had — one, two or three doses — and what infections they have had in the past,” said Rosemary Boyton, a professor of immunology and respiratory medicine at Imperial College London. “Imprinting is different according to where you live in the world, what vaccines you received — and that’s determining the subsequent immune response.”

There’s not a consensus on how original antigenic sin plays out with the coronavirus — and it’s a touchy subject among immunologists. Many quarrel about whether “sin” is the appropriate word for a phenomenon that undergirds our immune system’s ability to provide partial protection against changing viruses.

“Maybe 10 to 15 years from now, we live in a world where the vaccine is birth-year specific or make strain selection decisions that take into account different immune histories in the population,” said Katelyn Gostic, a researcher at the University of Chicago. “I think we need and are actively developing better technologies and better techniques to try to work at the science fiction frontier here, of figuring out these imprinting questions.”

“What our immune system likes to do best is recognize things it already has seen. It responds very quickly to these parts of the virus that haven’t changed,” said Matthew S. Miller, a viral immunologist at McMaster University. “The vaccines are still doing an exceptionally good job in preventing us from getting severe illness. The reason is that is, essentially, original antigenic sin.”

This hair-trigger immune response isn’t fine-tuned to block the new virus; people can still get infected. But a suboptimal response that’s ready to go, many scientists think, is better than waiting for the body to create one from scratch.

“Essentially, original antigenic sin is often a very good thing,” said Laura Walker, chief scientific officer of Adagio Therapeutics, a biotechnology company focused on developing monoclonal antibody drugs. Walker recently published a paper showing that vaccinated people who came down with an omicron infection had an initial immune response driven by the immune cells created by their original vaccination.

One possibility: The immune system creates a new memory of the new variant. The next time a descendant of omicron comes along, the body can draw from an expanded memory bank to mount its next defense.

Companies showed preliminary data that vaccines containing those versions of the virus can trigger stronger immune responses in the weeks after vaccination. But the advantage of a switch was modest, and long-term effects of those vaccines will depend in part on whether they help create new memories. If they simply provide a short-term boost of the existing memory response, many scientists are debating a change in vaccine strategy.

But not all “sin” is created equal. For a virus like dengue, original antigenic sin can be harmful. For flu, it may help in some scenarios and hinder immunity in others. The limited data has left experts in a familiar place during this pandemic: watching what happens next.

“I’m struggling to say: Is this a good thing or a bad thing?” said Christian Gaebler, an assistant professor of clinical investigation at the Rockefeller University. “If someone says they fully understood this, they would be lying.”