- What side effects? - Results from final-stage clinical trials of two of the frontrunner vaccines were published this week and both are considered safe.Data for the Pfizer/BioNTech vaccine -- already authorised in several countries -- was released in the peer-reviewed scientific journal New England Journal of Medicine.The drug is based on experimental technology that uses a synthetic version of a molecule called "messenger RNA" to hack into human cells and effectively turn them into vaccine-making factories.Its trial involved 40,000 volunteers and suggested the vaccine provokes only mild side effects.Some 80 percent of those vaccinated felt pain at the injection site.