Will the coronavirus disease 2019 (Covid-19), which has already killed more than 2 million people worldwide, eventually be eliminated by a global vaccination campaign, like smallpox? Will dangerous new variants evade vaccines? Or will the virus stick around for a long time, transforming into a mild annoyance, like the common cold?

Eventually, the virus known as SARS-CoV-2 will become yet “another animal in the zoo,” joining the many other infectious diseases that humanity has learned to live with, predicted Dr. T. Jacob John, who studies viruses and was at the helm of India’s efforts to tackle polio and HIV/AIDS.

For now, scientists agree on the immediate priority: Vaccinate as many people as quickly as possible. The next step is less certain and depends largely on the strength of the immunity offered by vaccines and natural infections and how long it lasts.

“Are people going to be frequently subject to repeat infections? We don’t have enough data yet to know,” said Jeffrey Shaman, who studies viruses at Columbia University. Like many researchers, he believes chances are slim that vaccines will confer lifelong immunity.

If humans must learn to live with Covid-19, the nature of that coexistence depends not just on how long immunity lasts, but also how the virus evolves. Will it mutate significantly each year, requiring annual shots, like the flu? Or will it pop up every few years?

While immunity acquired from other coronaviruses — like those that cause the common cold or SARS or MERS — wanes over time, symptoms upon reinfection tend to be milder than the first illness, said Ottar Bjornstad, a co-author of the Science paper who studies viruses at Pennsylvania State University.

“Adults tend not to get very bad symptoms if they’ve already been exposed,” he said.

The prediction in the Science paper is based on an analysis of how other coronaviruses have behaved over time and assumes that SAR-CoV-2 continues to evolve, but not quickly or radically.

TAKING CUES FROM 1918 PANDEMIC

The 1918 flu pandemic could offer clues about the course of Covid-19. That pathogen was an H1N1 virus with genes that originated in birds, not a coronavirus. At the time, no vaccines were available. The US Centers for Disease Control and Prevention estimates that a third of the world’s population became infected. Eventually, after infected people either died or developed immunity, the virus stopped spreading quickly. It later mutated into a less virulent form, which experts say continues to circulate seasonally.

“Very commonly the descendants of flu pandemics become the milder seasonal flu viruses we experience for many years,” said Stephen Morse, who studies viruses at Columbia University.

It’s not clear yet how future mutations in SARS-CoV-2 will shape the trajectory of the current disease.

As new variants emerge — some more contagious, some more virulent and some possibly less responsive to vaccines — scientists are reminded how much they don’t yet know about the future of the virus, said Mark Jit, who studies viruses at the London School of Hygiene and Tropical Medicine.

“We’ve only known about this virus for about a year, so we don’t yet have data to show its behavior over five years or 10 years,” he said.