What does the release say, and how have they changed advice?
The original said that there's no evidence that immunity is given after infection, as there isn't much data so far on level of immunity or length it would last. As a result, the immunity passport concept is not supported at the moment. (I don't think that's changed).
The amendment tweet says: "We expect that most people who are infected with
#COVID19 will develop an antibody response that will provide some level of protection."
Previously they've said that people would mostly have an antibody response, but it may be very low in some cases. They mention that it can take 1-2 weeks to create an immunity/antibodies upon exposure (presumably a fast recovery/minor symptoms are too fast for the body to create a reaction?)