A delay in deliveries to the UK from India and the need to retest a batch of 1.7 million doses is behind an expected shortfall in coronavirus vaccine supply in April, Britain's Health Secretary Matt Hancock has said.

Mr Hancock told MPs a partnership with the Serum Institute of India is one the UK "can be proud of", despite a delay in deliveries of the AstraZeneca vaccine from its plant.

He said: "We have a delay in the scheduled arrival from the Serum Institute of India.

"Now, I want to put on the record my gratitude to the Serum Institute of India for the incredible work that they're doing producing vaccine not just for us in the UK, but for the whole world.

"Their technology and their capability, which has been approved by the MHRA (Medicines and Healthcare products Regulatory Agency), is remarkable.

"The Serum Institute of India producing a billion doses of the Oxford/AstraZeneca vaccine this year, it truly is partnership that we can be proud of."

Mr Hancock also told the House of Commons a batch of 1.7 million doses of vaccine had been delayed in the last week due to the need to retest its stability.

"Events like this are to be expected in a manufacturing endeavour of this complexity and this shows the rigour of our safety checks," he said.

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As a result the health service in Britain is to suspend vaccination of people aged 49 and under, due to the significant shortfall in vaccine supplies. 

In a letter to vaccination project leaders, NHS England instructed them to prioritise giving existing recipients their second dose, rather than extending coverage to new cohorts.

It said the supply shortage is expected to last around four weeks, as a result of which "volumes for first doses will be significantly constrained".

The UK alone has followed a 12-week dosing interval for both the Pfizer and the AstraZeneca vaccines authorised in the country.

The ramping up of the vaccination programme in January and February means the number of people requiring a second dose will double from the start of April, and the NHS has directed that this group be prioritised for the available vaccine supply.

It means no new bookings will be taken for people aged 49 and younger unless there are exceptional circumstances, such as clinical vulnerability or employment in the health service or as caregivers.

The health service has instructed vaccination centres to prioritise "short-life stock (vaccines close to their use-by date) to avoid wastage; put in place reserve lists of people who could be vaccinate; and share longer-life stock between regional units so that supply and demand are better met.

The letter states "from today, the supply constraint means vaccination centres and community pharmacy-led local vaccination services should close unfilled bookings from the week commencing 29 March and ensure no further appointments are uploaded to the National Booking system or local booking systems from 1 to 30 April".

Health Secretary Matt Hancock told a Downing Street press conference that the nation was "on course" to meet the target of offering a first dose to all over-50s by 15 April.

Ministers have pledged that all adults will be offered a vaccine by the end of July.

Asked about the issue at the Downing Street coronavirus briefing yesterday, Mr Hancock said: "Supply is always lumpy and we are on course to deliver the offer that everybody who is aged 50 and above will be able to get vaccinated by the 15th of April. I recommit to that today.

"We are committed to all adults being able to get the jab by the end of July and we are on track to deliver on that commitment."

He added: "We regularly send out technical letters to the NHS to explain the ups and downs of the supply of the future weeks and what you are referring to is a standard one of those letters."

He said there would be a focus on vaccinating the most vulnerable before moving on to the over-40s.

Separately, over 300 Northern Ireland pharmacies are to assist with the Covid-19 vaccination programme.

Northern Ireland's Department of Health has set 30 March for the official launch of the initiative but it expects many pharmacies will begin providing the service before then.

Since the beginning of this week, those over 50 have been able to book a vaccine appointment. 

Health Minister Robin Swann said the pharmacies' involvement and the opening of a mass vaccination at Belfast's SSE Arena on 29 March will help build momentum.

Additional reporting Sean Whelan, Tommie Gorman