But how will the Pfizer and J&J shots affect you - can you, for instance, expect any side-effects? The Sisonke trial, which uses the J&J jab, was, for example, recently paused for two weeks to investigate reports about rare blood clots linked to the vaccine. So how do you know if a side-effect is something to be concerned about and how often are such symptoms likely to occur? We break it down. 1. What is a side-effect? To answer that, let’s take a quick refresher on how vaccines work. (A great place to start is to watch the first session of the Vaccine Science 101 course Bhekisisa and the Centre for Evidence-based Healthcare hosted in March.)