US health officials closely tracking possible side effects of the first authorized COVID-19 vaccine said on Saturday they have seen at least six cases of severe allergic reactions out of more than a quarter million shots given - prompting one Illinois hospital to pause vaccinations. Medical experts said that a chemical called polyethylene glycol (PEG), a compound most commonly used in laxatives, 'could be the culprit' causing the reactions. PEG is an ingredient in the Pfizer vaccine as well as the Moderna Inc vaccine authorized on Friday.