The UK's Covid immunisation programme is set for a massive expansion with the approval of two available vaccines - one from Oxford-AstraZeneca and another from Pfizer-BioNTech.
Stocks of another, from the company Moderna, should become available in the spring.
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 - which was the first vaccine to be approved in the UK.
Half are given the vaccine and the other half a 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 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 is only given in the UK if the regulator, the MHRA, is happy that a vaccine is both safe and effective.
Checks continue after approval to make sure there are no further side effects or long-term risks.
Independent experts on the Joint Committee on Vaccination and Immunisation decide how best to use a vaccine and who should get it.
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 the pandemic virus.
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.
What about allergies?
Allergic reactions to vaccines are rare. For any approved vaccine, the ingredients are listed.
The MHRA says it hasn't identified any "serious adverse reactions" during the trial of the Pfizer-BioNTech vaccine.
As a precaution, it says people with a history of significant allergic reactions should not currently have the Pfizer vaccine, however.
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).
What if I've already had Covid?
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 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 and do they contain alcohol?
Some vaccines, such as the shingles vaccine and the children's nasal flu vaccine, can contain pork gelatine.
The Covid vaccines from Pfizer, Moderna and AstraZeneca do not contain this though. Nor do they contain any other animal products.
The British Islamic Medical Association recommend people who are eligible have the vaccine and stress that there is negligible alcohol in it - no more than in bread, for example.
If everyone else gets vaccinated then surely I don't need to bother?
There is overwhelming scientific evidence that vaccination is the best defence against serious infections.
Covid vaccines appear to stop people getting very sick and could save lives.
It is not yet clear how much protection vaccines might give in terms of stopping people from spreading Covid.
If they can do this well, vaccinating enough people would stamp out the disease.