A man in his 80s receives a covid-19 vaccination on 11 January at the International Centre for Life in Newcastle upon Tyne. The location is one of several mass vaccination centres in England to open to the public this week

The UK’s race to vaccinate 13.9 million people in high-priority groups against covid-19 by 15 February is a Herculean undertaking. “Unprecedented” may have become an overused word in the pandemic, but the size and speed of the vaccine roll-out warrants it, though it may still be months before many people receive a covid-19 vaccine.

The numbers tell the story. Figures released today show that more than 2.2 million people in the UK have been given a first dose of one of the three vaccines approved by the UK regulator. Today, the UK government also revealed it aims to be vaccinating at least 2 million people a week in England by the end of January.

To reach the mid-February target that prime minister Boris Johnson announced on 4 January, 300,000 doses need to be administered every day across the UK. That is roughly the rate at which doses were being given each week at the end of December and start of January.

“This is the biggest vaccine programme ever that the UK has had to roll out. It’s definitely new territory,” says Doug Brown at the British Society for Immunology. On 11 January, the UK government began publishing daily updates on the number of people given their first of two vaccine doses.

There is no historical precedent for vaccinating people at this rate in the UK. The biggest annual programme, the flu vaccine, normally sees around 9 million people a year vaccinated. But that happens over a five-month period starting in September, averaging 60,000 doses a day and peaking at about 150,000 in late October.

And there are key differences between flu vaccines and covid-19 ones, says Nilay Shah at Imperial College London. The big one is that flu vaccine manufacturers, and the regulators who have to undertake quality control on the vaccines, have about five months to build up huge stocks before they are administered.

By contrast, covid-19 manufacturers are still very much in the start-up phase. Although the first step, mass production of the active ingredients inside the vaccines, has been under way for months, companies wouldn’t have started the next “fill and finish” step of putting doses into vials until closer to regulatory approval.

The other inherent hold-up during the start-up phase is the quality control process. This involves finished vaccines going to a lab at Potters Bar in the UK to see if they have enough of the active ingredient, to make sure there are no impurities, to test for sterility, and more. The sterility test alone takes at least 10 to 14 days, building in a lag as production is scaled up.

UK health secretary Matt Hancock has saidthe supply of the doses from manufacturers is the “rate-limiting step”. Brown agrees. “The biggest bottleneck does seem to be the vaccine supply issue,” he says. Reports suggest a few million doses are ready for the UK healthcare system to use on priority groups of the general population, with another 15 million working their way through the necessary quality checks.

Sandy Douglas at the University of Oxford, who led work to make the vaccine the institution developed with AstraZeneca at scale, says flu vaccines are made with an existing manufacturing process with slight tweaks depending on the flu virus. By comparison, the processes for making covid-19 vaccines are new. “People need to understand how difficult it is to get it going for the first time,” he says.

The army has been called in to help with distribution logistics once doses are made and checked. Yesterday, around 780 centres run by GP surgeries and 210 hospitals were the main two places people are being vaccinated in England. However, seven mass vaccination centres around England have now opened. Pharmacies have offered to help give shots, but not all are being utilised, says Shah.

The Pfizer and BioNTech vaccine made in Belgium and the AstraZeneca and University of Oxford vaccine made in the UK were the first two approved for use by the UK regulator, followed on 8 January by the vaccine created by US firm Moderna.

The European Union approved Moderna’s vaccine slightly earlier, on 6 January. Nonetheless, some European countries – in particular Belgium and France – have come in for criticism for bureaucratic delays to their vaccine roll-outs after earlier approval of other vaccines.

Israel, with its relatively small population of 9 million, is the world leader for number of doses given, at 20.93 per 100 people – although this doesn’t include the 5 million Palestinians living in Gaza and the West Bank. The UK comes sixth, at 1.94 per 100 people, according to figures collated by Our World in Data.

Shah says anecdotally he hears Israel is getting six doses out of each vial of the Pfizer/BioNTech vaccine rather than the five each is meant to contain, while the UK is only getting five.

The UK’s target will be a tough challenge and opinions differ on whether it can be met. “I think we will struggle with the February target because I think we will take at least another three or four weeks to get [to peak, steady production of vaccines]”, says Shah. “I think this is an achievable target,” says Douglas. “But it still requires everything to go smoothly.”