But there are questions over the speed at which those most at risk across the country will receive their jabs on concerns over the government's decision to delay offering the public second doses off.

Both the Oxford on Pfizer vaccines is our science editor David Shook.

Just going to be speaking.

It began with a blaze of publicity, and now the vaccination program moves up a gear to try to protect tens of millions of people.

Now that a second type of vaccine is about to be deployed.

I hear from particularly my general practice colleagues up and down the country that they are raring to go there really, really busy.

But they also know that this is the best thing that they can do to protect their patients.

Eso, as supply allows, more sites will be able to offer the vaccine, and that's exactly what we need to aim for.

The big hope is the Oxford AstraZeneca vaccine.

It's easy toe handle.

It doesn't need to be stored in extreme cold on the first doses air due to be given tomorrow so in the coming months.

How will this mass program take shape?

Well, the government's ambition is that by Easter that's April the fourth to have reached something like 30 million people.

They're the most a risk over fifties health and care workers on adults with vulnerabilities that involves around two million injections every week.

And for many people, that will be their first dose Only.

All of this depends on two key factors.

Production.

Can it be ramped up sufficiently and also delivery?

Will it be smooth in this massive operation?

This comes as government advisers say it's better to delay giving second doses so that more people can quickly get a first days and at least have some protection.

But Pfizer says this idea hasn't been tested for its vaccine, and some leading scientists want more research.

I think it's so important that will this decision has been made, that there are actually scientific studies that go on to look at the effect of delaying the second does you know, I personally think that it may well produce as good or better in the main response, but we really need to study it.

Sites like this off a hope of a way out of the pandemic.

The challenge now is making sure the vaccines get used as effectively as possible.