How do we know this vaccine is safe?
This is the first and most important question that scientists ask themselves when they start developing a new vaccine or new drug.
Its safety tests begin in the laboratory, with experiments and research done on cells and animals, before transferring those experiments to humans.
The principle is to start small and move on to the next step only when they see that there are no security concerns.
What is the significance of experiments?
When safe data from the laboratory is good, scientists can check that vaccines or treatments are effective as well.
This means that for the vast majority of people who volunteer to be vaccinated - about 40,000 took part in the Pfizer / BioNTech trials.
In trials, half of people are vaccinated and the other half is given doses to reduce the severity of the disease. Researchers and participants are not told which group they are in, until after the results have been evaluated, to avoid bias in certain results in the assessment.
All work and results are monitored and evaluated by an independent institution.
Covid vaccine trials have been conducted at a rapid pace, but have not exceeded these measures.
The Oxford / AstraZeneca Covid vaccine was deliberately delayed in one phase to investigate why one participant - out of thousands of people - died. Her research continued when it became clear that her death was not related to the vaccine.