Covid: Have we finally got a coronavirus vaccine?

The quest for an effective vaccine against Covid-19 has seen a significant step forward, with the announcement of "milestone" results.

Preliminary findings show a vaccine developed by Pfizer and BioNTech can prevent more than 90% of people from getting Covid-19.

They are the first to share data from the final stages of testing - known as a phase 3 trial.

This is a crucial point in vaccine development, where some experimental vaccines will fail.

The vaccine uses a completely experimental approach, which involves injecting part of the virus's genetic code into people, in order to train the immune system.

About 43,000 people have been given the vaccine, and no safety concerns have been raised.

Pfizer believes it will be able to supply 50 million doses by the end of this year, and around 1.3 billion by the end of 2021.

The UK should get 10 million doses by the end of 2020, with a further 30 million doses already ordered.

Exactly who will be immunised first will depend on where Covid is spreading when the vaccine becomes available and in which groups it is most effective.

The UK has not decided, for example, how to prioritise health and care workers who work with the most vulnerable people, relative to those most at risk if they catch the disease.

In broad terms, the over-80s, care home residents and health or care workers will be near the top of the list.

Age is, by far, the biggest risk factor for Covid, so the older you are, the sooner you are likely to be vaccinated.

Most experts think the vaccine will not be widely available until the middle of 2021.

More results, from other teams working on advanced trials of other vaccines, are expected in the coming weeks and months.

There are 10 more vaccines in the final stage of testing,

The leading contenders are:

It is worth noting that four coronaviruses already circulate in human beings. They cause common cold symptoms and we don't have vaccines for any of them.

The point of a vaccine is to harmlessly show parts of the virus to the immune system so it recognises it as an invader and learns how to fight it.

However, there are many ways to do this and researchers are using different approaches.

Both the Pfizer and Moderna vaccines inject pieces of the coronavirus's genetic code. Once inside the body, this starts making viral proteins to train the body. This is a completely new technique.

The Oxford and Russian vaccines take a harmless virus that infects chimpanzees, and genetically modify it to resemble coronavirus, in the hope of getting a response.

Two of the big China-made vaccines use the original virus but in a disabled form, so it cannot cause an infection.

Understanding which method produces the best results will be vital. Challenge trials, in which people are deliberately infected, could help answer these questions.

It is hard to know without knowing how effective the vaccine is going to be.

It is thought that 60-70% of the global population need to be immune to the virus in order to stop it spreading easily (known as herd immunity) - in other words, billions of people, even if the vaccine works perfectly.

If you want your life to get back to normal, then we need a vaccine.

Even now, the vast majority of people are still vulnerable to a coronavirus infection. It is only the restrictions on our lives that are preventing more people from dying.

But vaccines safely teach our bodies to fight the infection. This can either stop us catching coronavirus in the first place or at least make Covid less deadly.

The vaccine, alongside better treatments, is "the" exit strategy.

Not everyone responds the same to immunisation.

People will be watching the results in older people closely, because they are most at risk from the virus.

But history also suggests any vaccine could be less successful in old people because an aged immune system does not respond as well to immunisation. We see this every year with the annual flu jab.

It may be possible to overcome this by either giving multiple doses or giving it alongside a chemical (called an adjuvant) that gives the immune system a boost.

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