Researchers found that after the third shot, protection against a symptomatic infection is good, and two weeks after the shot, the boosters cut the risk by nearly 70%. Though, after three months, the booster reduces the risk of a symptomatic infection only by 50%.
U.K. researchers found in a second analysis that the protection will decline even further to about 40% nearly four months after the third shot.
On the other hand, the study reported that the booster offers more robust protection against severe disease than against infection, meaning protection against hospitalizations starts out above 95% and remains around 80% even after four months.
The study raises the question of how sustainable vaccines and booster shots are for the future, immunologist Deepta Bhattacharya at the University of Arizona told NPR.
"Could we get to the point where public health officials recommend a shot once a year? I think that's fairly likely. Now, whether everyone will absolutely need that shot to prevent severe disease each year, that's a different question, and we'll have to wait for the data. I think it's possible that yearly shots won't be absolutely essential for everyone," said Bhattacharya.