Will we need booster shots against Covid-19 in the future?

The world's leading Covid-19 vaccines may offer lasting protection that diminishes the need for frequent booster shots.

That's according to scientists, who are finding clues in how the body remembers viruses. But they say more research is needed and that virus mutations are still a wild card.

Critical studies are underway, and evidence is mounting that immunity from the mRNA vaccines made by Pfizer and Moderna doesn't depend exclusively on antibodies that dwindle over time. The body has overlapping layers of protection that offer backup.

Scientists do not yet know what's called the correlate of protection, the level below which antibodies cannot fend off the coronavirus without additional help. Dr. Anthony Fauci, the US government's leading infectious disease expert, told a Senate subcommittee last week that vaccine protection would not be infinite.

Pfizer and Moderna officials have said people might need yearly shots, just like with flu vaccinations. The companies plan to have some candidates ready this fall. But companies won't decide when boosters get used. That's up to health authorities in each country. Some experts say boosters may be needed only every few years.