The team analyzed 241,204 visits to emergency departments or urgent care facilities and 93,408 hospitalizations among adults older than 18. The researchers did not include milder coronavirus infections.

Protection against emergency department and urgent care visits declined from 69 percent within two months of the second dose to 37 percent after five months or more. Booster shots restored those levels to 87 percent.

The effectiveness of boosters also waned. Protection against emergency department and urgent care visits dropped to 66 percent within four or five months, and to just 31 percent after five or more months of receiving the third shot, the researchers found. The latter estimate may not be reliable because few people received boosters more than five months ago, and so the data is limited, the researchers said.