How do you vaccinate more than 7 billion people against COVID-19? That's the challenge currently facing public health officials around the world.
The UK is the only country in Europe currently administering a vaccine that is internationally-recognised for public use.
But other nations are also working on strategies for when their own authorities give the green light — not least deciding who will be first in line.
UK administers first vaccines to the over 80s and care home staff
The UK is the first country in the world to administer a vaccine against COVID-19 and it was 90-year-old Margaret Keenan who made history with that dose.
The grandmother, originally from Enniskillen in Northern Ireland, was applauded by staff at Coventry Hospital after receiving the Pfizer/BioNTech formula on December 8.
She marked the start of a global programme that has gained momentum in the weeks since.
As part of the UK's strategy, the first 800,000 vaccines have been prioritised for the over-80s, who are either in hospital already or have an outpatient appointment booked.
Care home staff are also at the front of the queue.
Russia prioritises high-risk workers for Sputnik V
Moscow began administering its vaccine to citizens three days before Margaret Keenan received her jab, although the Russian version has not yet been internationally approved.
Naming it Sputnik V, the Russians raised eyebrows when they claimed it was ready back in August. This is due to concerns among scientists about the brief testing time of the vaccine and the small number of volunteers who received it.
As a result, Sputnik V is still in the middle of being trialled and is only being given to people under 60 years old who have no chronic conditions.
Priority is also being given to those in careers with a high risk of contracting the disease such as health, education and social work.
Germany looking for vaccine authorisation in time for Christmas
Germany is hoping to start vaccinating citizens before the end of 2020, which means an authorisation will be needed before Christmas.
Health Minister Jens Spahn said this at a press conference on Tuesday, which marked the government's increasing impatience regarding the amount of time it is taking for European health authorities to issue an approval.
Similar agitation has been heard from Poland and Hungary.
At present, it's not clear whether the European Medicines Agency (EMA) will reach the Christmas deadline hoped-for by Germany. However, the latest date for approval has been moved forward from December 29 to December 21.
EU Commission fast tracks procedure to see approval 'within days'
The green light is expected to be given by the Commission shortly after approval is issued from the EMA. According to the Commission, this could happen "within days" after a fast-tracked procedure.
On Twitter, Commission President Ursula von der Leyen praised the EMA for bringing forward its panel for assessing the vaccine to December 21, saying this meant programmes may be rolled out before the end of the year.
Several European countries have also pledged to coordinate their vaccination strategy when authorisation is given.
A statement released by Italy, and signed by health ministers in Germany, France, Belgium, Luxembourg, the Netherlands, Spain and EU neighbour Switzerland, promised coordination of launch schedules, along with information sharing about progress.
France's five-phase plan
France plans to start its initial phase in January, with nursing home residents and high-risk care workers at the front of the queue for the first batch.
The second phase, beginning in February, will expand to the over 65s, and any healthcare workers over the age of 50 who are considered to be vulnerable.
By spring, the third phase will see people under 65 receive the shot, along with all other healthcare workers. Teachers may also be included in this band as essential workers.
Phase four will prioritise people who are most exposed to the virus in their careers, particularly those who work in enclosed spaces and are in regular contact with the general public. Under this phase, prisoners and the homeless will also be included.
The final phase, therefore, will expand the programme to all citizens above the age of 18. This is expected to be between April and June of 2021.
Portugal prioritising 400,000 most vulnerable citizens
Portugal is also planning to follow a similar route to France by prioritising 400,000 of its most vulnerable citizens for the vaccine from January. The country plans to have vaccinated 1 million people by April.
Prime Minister Antonio Costa said the plan would be guided by four fundamental principles: That it is "universal for everyone", that it is optional, free, and "distributed to the whole population according to the technical and scientifically defined priority criteria".
Spain's 'live, agile and flexible' strategy
Similar to other European countries, Spain has devised a strategy to put the most vulnerable first. The government has also stressed that the plan will need to be kept under constant review to keep it "live, agile and flexible".
Laying out four stages, care home residents and staff will be the first in line when the European Medicines Agency authorises a vaccine.
Second in line will be front-line health workers, followed by other health and social personnel.
In the final stage of priority vaccinations, people with disabilities who need intensive support measures will receive the jab.
'Let's not fool ourselves' that a vaccine is enough
Despite this European optimism, the United Nations has warned a vaccine will not be enough to undo the damage already done by the pandemic.
"Vaccines may become available within the next weeks and months," UN Secretary-General Antonio Guterres said, adding: "But let's not fool ourselves. A vaccine cannot undo the damage that will stretch across years, even decades, to come."
The UN chief then pointed to rising poverty, the threat of famine and the "biggest global recession in eight decades", something he stressed is not due to coronavirus alone.
"Nearly a year into the pandemic, we face a human tragedy and a public health, humanitarian and development emergency," he added.
Guterres has since been urging the world's richest nations to back the World Health Organisation's ACT-Accelerator to help distribute vaccines to the world's poorest populations.
He said there was still a $28bn (€23bn) financial gap, which includes $4.3bn (€3.5bn) "urgently needed" in the next few months.
Both French President Emmanuel Macron and German Chancellor Angela Merkel have backed the programme, encouraging further funds from the international community.
Macron, who said $10bn (€8.2bn) had so far been raised, also reiterated a proposal for "a donation mechanism" to ensure vaccinations are given to priority groups in developing countries.
EU chief Charles Michel, meanwhile, has floated the idea of an international treaty to help the WHO deal with any future pandemics.
This would be used to monitor infectious diseases in animals, to improve access to healthcare, and to address financing issues.
"The principle of universal access to the new COVID-19 vaccines is fundamental. Our aim must be to guarantee access to vaccines, treatment and tests for future pandemics. This should be laid down in a treaty," he said.