But the virus has continued to evolve into new, concerning variants, each with a slightly different genetic code. Although current vaccines still work well against new variants, they are no longer a perfect match.
Vaccine makers like Pfizer and Moderna are now exploring tweaked booster shots to match the now-dominant omicron variant, but the U.S. government is aggressively pursuing a different approach: a pan-coronavirus vaccine that would work equally well against any COVID-19 variant.
"Since September of 2020 there have been five SARS-CoV-2 variants of concern -- alpha, beta, gamma, delta and now, the current, omicron," Dr. Anthony Fauci said at a White House task force briefing Wednesday. "So, obviously, innovative approaches are needed."
Fauci, who heads up the National Institute of Allergy and Infectious Diseases, has issued $43 million in research grants across several academic institutions to support the development of a pan-coronavirus vaccine, sometimes called a "universal" coronavirus vaccine.
The idea, scientists say, is to create a vaccine that works as a generalist rather than a specialist. A pan-coronavirus vaccine will be designed using features of the virus's genetic code that are shared universally across all different versions of the virus -- and hopefully, any new versions that will emerge.
"We're chasing the virus, and we're always behind," said Dr. Jeff Taubenberger, Ph.D., deputy chief of the Laboratory of Infectious Diseases at the NIAID.
"I think that a better approach could be ... if we could be clever enough to come up with a strategy to keep us ahead of the virus, so that we wouldn't have to know what the variant is," he added.
Several research groups are already working on a pan-coronavirus vaccine, including scientists at the California Institute of Technology, Duke University, University of Washington, Brigham and Women's Hospital in Boston and the University of Wisconsin, Madison.
Stéphane Bancel, Moderna's CEO, said in a statement Wednesday that the company may incorporate the omicron-specific booster into its "multivalent" booster program, which seeks to create a single shot that would work against at least two major variants.
But scientists at the Walter Reed Army Institute of Research are arguably the furthest along. The Army vaccine appears to work well in monkeys, and is now being tested for safety in a phase 1 study in human volunteers.
In a rare look inside the Walter Reed laboratories last year, ABC News' Bob Woodruff spoke to a team of Army scientists hopeful that their vaccine candidate would work not only against COVID-19 variants, but also against related coronaviruses, like those that caused the SARS-1 and MERS outbreaks in 2003 and 2012, respectively.
But designing a pan-coronavirus vaccine is no easy feat. Scientists say it could take months, even years, to find a vaccine that works equally well against multiple coronavirus strains.
"I don't want anyone to think that pan-coronavirus vaccines are literally around the corner in a month or two," Fauci said. Current vaccines dramatically reduce the risk of hospitalization and severe illness, even against new variants like omicron. And crucially, they are available today.
"Do not wait to receive your primary vaccine regimen," Fauci said. "If you are vaccinated, please get your booster if you are eligible."
ABC News' Matthew Seyler contributed to this report.