Covid vaccine update: When will others be ready?

A third Covid vaccine developed by Moderna has been approved for use in the UK.

But how does it compare with the other vaccines?

The vast majority of people are still vulnerable to coronavirus. It's only the current restrictions that are preventing more people from dying.

Vaccines teach our bodies to fight the infection by stopping us from catching coronavirus, or at least making Covid less deadly.

Having a vaccine, alongside better treatments, is "the" exit strategy.

The Moderna vaccine is a new type called an RNA vaccine, and uses a tiny fragment of the virus's genetic code.

This starts making part of the virus inside the body, which the immune system recognises as foreign and starts to attack.

The Moderna vaccine uses the same approach as the Pfizer vaccine but it is easier to store, because it stays stable at -20C for up to six months.

The roll-out of the Oxford vaccine began on 5 January. It was approved late in 2020 after trials showed that it stopped 70% of people developing Covid symptoms.

The data also showed a strong immune response in older people.

This may be one of the easiest vaccines to distribute, because it does not need to be stored at very cold temperatures.

It is made from a weakened version of a common cold virus from chimpanzees, that has been modified to not grow in humans.

The big breakthrough came when Pfizer-BioNTech published its first results in November.

The vaccine must be stored at a temperature of around -70C. It will be transported in a special box, packed in dry ice and installed with GPS trackers.

On 2 December, the UK became the first country in the world to approve the Pfizer/BioNTech coronavirus vaccine for widespread use.

Six days later 90-year-old Margaret Keenan became the first patient to receive the vaccine at University Hospital in Coventry. Since then, more than a million people in the UK have been vaccinated.

Like Moderna, Pfizer's is an RNA vaccine - the first ever to be approved for use in humans.

Other trial results are also expected in the coming weeks.

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

This depends on where Covid is spreading when the vaccine becomes available and in which groups each is most effective.

Older care home residents and staff top the UK's preliminary priority list, followed by health workers like hospital staff, and the over-80s.

Age is, by far, Covid's biggest risk factor.

It was thought that 60-70% of the global population must be immune to stop the virus spreading easily (herd immunity) - billions of people, even if the vaccine works perfectly.

However, those figures will rise considerable if the new, more transmissible, variants spread widely.

People respond differently to immunisation.

History suggests any vaccine could be less successful in old people because an aged immune system does not respond as well, as happens with the annual flu jab. But data so far suggests this may not be a problem with some of Covid vaccines

Multiple doses may overcome any problems, as could giving it alongside a chemical (called an adjuvant) that boosts the immune system.

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