The start of a Covid-19 vaccination programme in the UK is welcome news, but the public has been warned that the pandemic is not over yet.

Coronavirus restrictions could still be in place for weeks and months to come as the NHS, regulators, scientists and manufacturers take on the challenge of rolling out safe vaccines for the country’s population.

Here the PA news agency looks at some of the reasons why life will still not be returning to normal for everyone soon:

Millions of vaccine doses are yet to be delivered

Health Secretary Matt Hancock said he expects “millions” of doses of the regulator approved Pfizer/BioNTech vaccine to be delivered this year, but has not provided a specific figure.

Some 800,000 doses of the vaccine have been delivered to UK hospital hubs – enough for 400,000 people – but it is unclear when more doses will arrive from the manufacturer.

In total, the UK has ordered 40 million doses of the Pfizer jab, enough to vaccinate 20 million people as patients need to receive two doses.

Mr Hancock said the NHS will “vaccinate as quickly as the manufacturers can produce the vaccine”.

The only UK regulator approved vaccine needs two doses

Recipients of the Pfizer jab are only protected when they have received two doses, administered 21 days apart, with another week needed for it to provide immunity.

Mr Hancock highlighted that, despite the start of vaccinations on Tuesday, “we’re still 28 days off the first person being inoculated and protected from Covid-19”.

The time delay is one reason why people should continue to observe social-distancing rules, the Health Secretary emphasised.

The Pfizer jab comes with logistical challenges

The Pfizer/BioNTech vaccine needs storage temperatures of minus 70C to minus 80C before being thawed out and it can only be moved four times within that cold chain before being used.

Public Health England has secured 58 specialised ultra-low temperature freezers to store approximately five million doses of vaccines that require ultra-low temperatures.

These are said to be fully operational at national storage facilities in the UK, but are not portable.

These storage requirements mean most early vaccinations will take place in hospital hubs while experts work on safely deploying doses to other settings.

Vaccine boxes contain 975 doses and will need to be split so they can be taken to care homes.

Professor Stephen Powis, of NHS England, said vaccinations for care home residents will start before Christmas.

Armed forces personnel have been drafted to help construct Covid-19 vaccination centres.

Uncertainty remains over other vaccines’ potential

The UK’s medicines regulator, the Medicines and Healthcare products Regulatory Agency (MHRA), has only approved one of dozens of Covid vaccine candidates in development.

It is currently examining data from the University of Oxford and AstraZeneca jab, which health officials hope could be approved by Christmas.

Data from two arms of the phase-three Oxford trial gave a combined efficacy of 70%.

A half dose followed by a full dose was found to be 90% effective in protecting against Covid-19, according to a subset of data, but the figure was 62% for people given two full doses. A further global trial is to be carried out.

Some scientists questioned the validity of the data, while others have defended the clinical trial results.

Professor David Salisbury, a former Department of Health director of immunisation, said ministers would need to think “very carefully” about their handling of the vaccine if its efficacy was lower than for other jabs.

The UK has secured access to some 100 million doses of the Oxford jab, which is said to present fewer logistical challenges if approved for deployment.

Lack of certainty over whether vaccines stop spread of virus

Although data from Covid-19 vaccine candidates indicates they can prevent disease, it is not yet known how well they halt the spread of the virus.

Mr Hancock warned it was unknown to what degree the Pfizer vaccine reduces the chances of a recipient passing on the virus asymptomatically, with the Government continuing to monitor its rollout’s impact on Covid-19 cases, admissions to hospital and deaths.

Christine Tait-Burkard, assistant professor of immunology at the University of Edinburgh, said that while some vaccine candidates appeared good at preventing disease, trials involving monkeys showed they were still “shedding virus” in their noses, a risk that would mean more people would need to be vaccinated.

She highlighted that the German government has said 55% to 65% of people will need to be vaccinated for life to return to normal.

We are still in the middle of winter

The winter months typically put pressure on the health service even without it having to cope with a global pandemic.

Prof Powis said: “We’re not out of this yet. It’s winter, it’s cold. We’ve still got January, February, the rest of December ahead of us – tough times for the NHS.

“It’s really important that we keep socially distancing, we keep sticking by those rules that we’ve all been sticking with, because we need to get through the next few months to get to the point where sufficient people have been vaccinated that we can start to relax those rules.”