Antibodies that people make to fight the new coronavirus last for at least four months after diagnosis and do not fade quickly, as some earlier reports suggested, scientists have found.

Tuesday’s report, from tests on more than 30,000 people in Iceland, is the most extensive work yet on the immune system’s response to the virus and is good news for efforts to develop vaccines.

If a vaccine can spur production of long-lasting antibodies like natural infection does, it gives hope that “immunity to this unpredictable and highly contagious virus may not be fleeting,” independent experts from Harvard University and the U.S. National Institutes of Health wrote in a commentary published with the study in the New England Journal of Medicine.

One of the big mysteries of the pandemic is whether having had the coronavirus helps protect against future infection and for how long. Some smaller studies previously suggested that antibodies disappear quickly and that some people with few or no symptoms may not make many at all.

The new study was done by Reykjavik-based deCODE Genetics, a subsidiary of the U.S. biotech company Amgen, with several hospitals, universities and health officials in Iceland.