The search for a vaccine against AIDS has been long and fruitless — mostly because the virus mutates so fast.

As is well known, flu vaccines have to be reformulated every year because influenza viruses mutate so steadily. But the human immunodeficiency virus, which causes AIDS, mutates as much in a single day as flu virus does in a year, presenting scientists with an almost insurmountable challenge.

This month, South African researchers announced that they had found a vulnerable spot on the virus’s outer shell that might present a good vaccine target, and that they had also learned, for the first time, at what stage of an infection it develops. They found only two women whose virus had the vulnerability — and it wasn’t the same virus that first infected them, but a mutant that developed a few months later.

The research, published by Nature Medicine on Oct. 21, was praised as “very interesting” by several AIDS experts.