UK hospitals have started giving people the first doses of the Pfizer/BioNtech coronavirus vaccine.
But while many people want an injection as soon as possible, others are worried about putting something unknown into their bodies.
How do we know a vaccine is safe?
Safety trials begin in the lab, with tests and research on cells and animals, before moving on to human studies.
The principle is to start small and only move to the next stage of testing if there are no outstanding safety concerns.
What role do trials have?
As long as the safety data from the labs is good, scientists can check that the vaccine or treatment is effective too.
That means tests on large numbers of volunteers - around 40,000 individuals in the case of Pfizer/BioNTech.
Half the volunteers are given the vaccine and the other half a dummy or placebo jab. The researchers and participants are not told which group is which, until after the results have been analysed, to avoid bias.
All of the work and findings are checked and verified independently.
The Covid vaccine trials have happened at breakneck speed, but they haven't skipped any of these steps.
The Oxford/AstraZeneca Covid vaccine trial was voluntarily put on hold at one stage to investigate why one participant - out of many thousands - had died. It restarted once it was clear it was not related to the vaccine.
Who approves vaccines or treatments?
Approval will only be given for a vaccine in the UK if the government regulator, the Medicines & Healthcare products Regulatory Agency (MHRA), is happy that it's both safe and effective.
Checks on the vaccine will continue to make sure there are no further side effects or long-term risks.
Independent experts on the Joint Committee on Vaccination and Immunisation (JCVI) decide how best to use a vaccine and who should get it first.
What's in the Covid vaccines?
Pfizer/BioNTech's vaccine (and Moderna's) uses bits of genetic code to cause an immune response, and is called an mRNA vaccine.
It does not alter human cells, but merely presents the body with instructions to build immunity to Covid.
The Oxford AstraZeneca vaccine uses a harmless virus altered to look a lot more like Sars-CoV-2 - the virus that causes Covid-19.
Vaccines sometimes contain other ingredients, like aluminium, that make the vaccine stable or more effective.
Will a vaccine make me ill?
There is no evidence that any of these ingredients cause harm when used in such small amounts.
Vaccines do not give you a disease. Instead, they teach your body's immune system to recognise and fight the infection they have been designed to protect against.
Some people do suffer mild symptoms after being vaccinated, such as muscle aches or a raised temperature.
This is not the disease itself, but the body's response to the vaccine.
Allergic reactions to vaccines are rare. For any approved vaccine, the ingredients will be listed.
The MHRA says it hasn't identified any "serious adverse reactions" during the trial of the Pfizer/BioNTech vaccine.
Be aware that anti-vaccine stories are spread online through social media. These posts are not based on scientific advice (or blend facts with misinformation).
Is it safe for someone who had Covid to have the vaccine?
People will still be offered the jab even if they have had Covid-19 in the past.
That's because natural immunity may not be long-lived and immunisation could offer more protection.
Guidance from Public Health England says there are no safety concerns about giving jabs to people with "long" Covid either. But people who are currently unwell with Covid-19 should not receive the vaccine until they have recovered.
How animal-friendly are vaccines?
Some vaccines, such as the shingles vaccine and the children's nasal flu vaccine, can contain pork gelatine.
And some vaccines are grown on hen's eggs, or cells from chick embryos.
There are hundreds of Covid vaccines in development. We don't have details on every ingredient yet, but many of the Covid vaccines are expected to be vegetarian or vegan-friendly. Source: BBC Health
