How the definition of success has changed in COVID fight

In the early days of the pandemic, experts kept a close watch on the number of new cases -- one of the key metrics signaling success or failure of public health measures. But with new variants leading to more frequent breakthrough infections, mild COVID-19 cases will likely still persist, even if every person in the country is fully vaccinated.

"Even before any vaccines were authorized ... we knew that this was going to be an issue," said Dr. Lynn Goldman, dean of the George Washington University's Milken Institute of Public Health.

"No vaccine is 100% effective at preventing infection," added Dr. Kimberly Fisher, professor of medicine at University of Massachusetts Chan Medical School.

Now, public health and infectious disease experts are shifting their metric of success.

With vaccines still highly protective against severe illness, experts said we should focus less on cases, and instead on how many people are being hospitalized or dying.

"I think in some ways, the strong data around vaccines out of the gate created this illusion of perfection, which never was the case," said John Brownstein, Ph.D., an epidemiologist and chief innovation officer at Boston Children's Hospital and a contributor to ABC News.

"The major goalposts should have always been the hospitalizations and deaths," he said.

Many experts point to countries like Singapore as an example of living "with" the virus, rather than eradicating it completely. With more than 80% of the population fully vaccinated, the island nation is still seeing more than 1,000 cases on average per day, but very few deaths due to COVID-19.

Although there may be "increases in cases," Brownstein said, "that is not resulting in real impact in hospitalizations and deaths. ... That sort of divergence is super important."

In the U.S., a peek into COVID-19 intensive care units around the country reveals an important and recurring theme: ICU cases and deaths are overwhelmingly among the unvaccinated. A study by the health department in North Carolina found unvaccinated individuals are 15 times more likely to die from COVID-19 than the vaccinated. Washington state's King County, which includes Seattle, tracks the numbers daily with unvaccinated people being 42 times more likely to die over the past 30 days.

Emerging data from the post-COVID-19 vaccine era underscored vaccine success will not necessarily be measured in prevention of COVID-19, but overwhelming success at keeping people alive if they do.

"It makes sense to focus on rates of hospitalization and death for COVID-19 -- both of which the vaccine is very effective at reducing," said Kathleen Mazor, a professor of medicine at University of Massachusetts Chan Medical School.

Public health experts hope that by focusing on this new metric for success, the nation can start prioritizing what's important and readjust to the "new normal."

Meanwhile, the overarching message to the vaccine-eligible population is clear: The vaccine is not simply intended to stop you from getting COVID, it's so you live to talk about it if you do.

Nancy A. Anoruo, M.D., M.P.H., a faculty physician at Harvard Medical School and public health scientist, is a contributor to the ABC News Medical Unit.