Britain is about to begin the mass vaccination of its citizens against the coronavirus.

But there’s confusion about how much of the two vaccines that are available – those made by drug companies AstraZeneca and Pfizer – are ready to be injected into people’s arms now.

Here’s what we know and what we don’t know.

What was the original plan?

In May, the UK government said AstraZeneca would make up to 30 million doses available by September for people in the UK.

The rest of the 100 million doses pre-ordered by the government was apparently destined for developing countries.

Neither of those things has happened: it’s now January 2021 and we have nowhere near 30 million doses of AstraZeneca vaccine, and ministers now say all the 100 million shots which are eventually delivered will stay in Britain.

In November, the government said 10 million doses of the Pfizer vaccine were due to arrive by the end of 2020. We have also fallen well short of this target.

How much is ready now?

The government says 530,000 doses of AstraZeneca vaccine is available for use from this Monday (January 4), with “tens of millions” of doses expected by the end of March.

The Telegraph has been reporting than an extra 407,000 doses were added at the last minute, making the real total closer to 1 million, but government spokespeople have told FactCheck they do not recognise this number and the 530,000 figure is accurate.

An unknown number of Pfizer doses are in play too.

The company scaled down its production estimates in early December, but the Health Secretary, Matt Hancock, said “several million” doses of Pfizer vaccine were still expected by the end of 2020.

The government has not confirmed or denied whether even this revised target has been reached.

A spokesman said: “We have sufficient doses to maintain our vaccination programme as it continues to accelerate, and are working closely with Pfizer to ensure vaccines keep arriving into the UK.

“As of 25 December, we have received 22 deliveries of the Pfizer/BioNTech vaccine to the UK. We have plans in place with the company to ensure sufficient supply throughout 2021.”

The latest official figures show that just under 950,000 first doses of the Pfizer vaccine have been given out, more than the initial shipment of 800,000 doses announced in early December, although the supply available in the UK for use today is unknown.

Why is there less AstraZeneca vaccine available than expected?

People involved in the vaccine procurement project have stated that there are already several million doses of AstraZeneca vaccine in the UK ready to be deployed.

Kate Bingham, outgoing head of the UK government’s Vaccines Taskforce, told the science select committee on 4 November: “As of now, we have low numbers of million doses in bulk drug substance, not vialled, and the third batch of 1,000-litre manufacturing is now under way.

“That should probably get us up to about 4 million doses at the end of the year.”

She added: “We have not yet put it into vials because, as soon as you do that, you start the clock for its shelf life and how quickly you have to use the vaccine.”

Media reports today have speculated that this process of putting the vaccine into vials is the reason why only 530,000 doses are available from Monday.

But the government has not mentioned vials in its explanation.

Instead, a spokesman said that the MHRA, the regulator which recently approved the AstraZeneca vaccine, “set out conditions with their authorisation on 30 December and batch testing is taking place to ensure the vaccines consistently meet these strict requirements”.

“This could not be done before the conditions were outlined by the MHRA.”

FactCheck verdict

The UK government says just over half a million doses of the AstraZeneca vaccine will be ready on Monday, with “tens of millions” deployed by the end of March this year. An unknown number of Pfizer vaccine doses are being administered too.

All the evidence suggests the number falls well short of the 40 million doses of both vaccines that we were originally supposed to have by this stage.

The government says the supply of AstraZeneca vaccine is smaller than expected because batches are being tested according to criteria set out by the MHRA regulator just two days ago.