But causation describes reasons. Correlation doesn't. It just says that these two things are more likely to happen together. Thats it.It can't though because there are plenty of reasons why B may occur (possibly more probable reasons) that have nothing to do with A. Those that aren't puking may have still drank spoilt milk and those that are puking may not have. There are too many variables to pick one and decide they have a bi-directional relationship.
I agree with this on a far more fundamental level than even you would be willing to take it.It's irrelevant though, tbh. This is all based on speculation since no one has any hard data for the relationship in question.