Explainer: What do the UK allergic reaction cases mean for Pfizer's COVID-19 vaccine

Vials with a sticker reading, "COVID-19 / Coronavirus vaccine / Injection only" and a medical syringe are seen in front of a displayed Pfizer logo. [Reuters]

Britain’s medicines regulator has advised people with a history of significant allergies not to get the Pfizer-BioNTech COVID-19 vaccine after two people reported adverse reactions on the first day of its rollout in the UK.

Here are some questions and answers about the cases and what they might mean.