The vaccines aren’t perfect. They do a good job of preventing severe disease, hospitalization and death, and have proven very safe, with only rare serious side effects. But protection against milder infection begins to wane after a few months.

Like flu vaccines, COVID-19 shots must be updated regularly to match the ever-evolving virus — contributing to public frustration at the need for repeated vaccinations. Efforts to develop next-generation vaccines are underway, such as nasal vaccines that researchers hope might do a better job of blocking infection.

The good news for the future pandemics is that same mRNA platform that produced the COVID-19 vaccines can and is being adapted for other viruses, Sweat said. For instance, a mRNA vaccine against bird flu — the H5N1 virus currently infecting birds and dairy herds in the U.S. that has sickened some farmworkers and killed a Louisiana man recently — has already shown success in animal testing.

Which variant is dominating now?

Genetic changes called mutations happen as viruses make copies of themselves. And this virus has proven to be no different.

Scientists named these variants after Greek letters: alpha, beta, gamma, delta and omicron. Delta, which became dominant in the U.S. in June 2021, raised a lot of concerns because it was twice as likely to lead to hospitalization as the first version of the virus.

Then in late November 2021, a new variant came on the scene: omicron.

“It spread very rapidly,” dominating within weeks, said Dr. Wesley Long, a pathologist at Houston Methodist in Texas. “It drove a huge spike in cases compared to anything we had seen previously.”

But on average, the WHO said, it caused less severe disease than delta. Scientists believe that may be partly because immunity had been building due to vaccination and infections.

“Ever since then, we just sort of keep seeing these different subvariants of omicron accumulating more different mutations,” Long said. “Right now, everything seems to be locked on this omicron branch of the tree.”