Well, if you read the article, it's one of the several vaccines being worked on is showing promise, and the head of that study is 80% confident it will work.
Also the 'by September' needs to be looked at sceptically:
"""It's not just a hunch and as every week goes by we have more data to look at. I would go for 80%, that's my personal view.""
"She added that having something ready by the autumn is "just about possible if everything goes perfectly", but warned that "nobody can promise it's going to work"."