1234Littleham where have you read that there are no side effects and that it's working?
Below is the position statement from the JCVI when they reversed the earlier decision, and decided it was just about worth it for under 1s.
www.gov.uk/government/publications/meningococcal-b-vaccine-jcvi-position-statement
On p5 it says "There was however a lack of evidence on vaccine efficacy, since the vaccine had not yet been evaluated in an efficacy trial, and was not being used routinely in any country worldwide." In other words there is information on antibody production short term from lab tests, but no information on whether this will translate to a reduction in Men B cases in reality.
As for safety, they note that: "Data were too limited to identify rare adverse reactions to the vaccine, however the Committee agreed that the infrastructure and expertise available in the UK would allow the acceptability and safety of the vaccine to be assessed." In other words, before the vaccine was rolled out to under 1s the only safety data available were from clinical trials (around 6000 people total), but in order to assess whether there were adverse reactions more rare than that (say 1:10,000 for instance) they would need to monitor the reactions in the under 1s receiving the vaccine.
Before the UK rolled out Bexsero to under 1s no-one else had used this vaccine at a population scale. The world is watching what happens here to see whether a) it works to reduce Men B and b) there are less frequent adverse effects that are note-worthy. The UK populus is in a sense the first mass trial of this vaccine, and at this point no-one knows the answers to the two questions above.
In layman's terms the facts of the situation are laid out quite nicely here:
www.bbc.co.uk/news/health-35706020