Margaret Keenan’s injection marks the start of the world’s first national vaccination program, after the United Kingdom became the first country to approve a COVID-19 jab last week. The program will be closely watched not just by Britons anxious to return to normal but also by public health officials around the world who have to tackle a scale and speed issue without precedent in modern medicine.

Keenan, who received the first dose of the Pfizer and BioNTech vaccine Tuesday morning from her National Health Service nurse May Parsons at her local hospital, said receiving the vaccine ahead of her 91st birthday was an enormous “privilege” that would allow her to see friends and family in the new year, after isolating on her own for much of the year. She will receive the second dose in 21 days.

British Prime Minister Boris Johnson called it “a shot in the arm for the entire nation,” before warning the public they still cannot let their guard down. As is the case across much of Europe, Britain has exacted tough rules restricting movement and limiting the operating hours of pubs and restaurants as the economy faces recession and the labor market is in tatters.

The rollout news failed to move the markets, however, on Tuesday. The benchmark FTSE and pound sterling are trading lower in morning trade as investors remained spooked about post-Brexit talks, the country’s other existential crisis.

Who will receive the first jabs?

The U.K., a country of over 66 million people, has secured 80,000 doses of the Pfizer vaccine in its first round, which is being transported from a manufacturing hub in Belgium. Because the vaccine requires two doses, that will mean only 40,000 people will receive the jab.

The U.K. has ordered 40 million doses of the Pfizer vaccine in total. On Tuesday, the NHS said those doses will go first to patients over 80 who are either in a hospital as an outpatient or being discharged after a hospital stay.

Care home staff and residents are next on the list for the vaccine, and some care home staff have been invited to book appointments to receive the vaccine.

There have already been some changes to the vaccine rollout plan. Although the U.K. government had said patients in care homes would receive the vaccine first, logistical hurdles involving splitting up doses of the vaccine have already complicated how it will be administered outside hospitals. As of Tuesday, nearly 68,000 people in Britain have died from COVID-19, one of the highest mortality rates in the developed world.

This vaccine is also particularly complicated to distribute, because it must be kept at deep-freeze temperatures. Although Pfizer now says it can be kept for a number of days in normal refrigeration, there is a hard limit on how long a dose will last under such conditions. This means that any transport of the vaccine, especially to local distribution points, must be carefully planned.

Who will receive it next?

The U.K. will take a tiered approach to rolling out the vaccine—starting with the older people, health care workers, care home workers, and those who are at a greater risk.

After this first stage, the vaccine will be given to people over 80, plus frontline medical and health care workers. It will then go to 75s and over, followed by 70s and over, and those who are deemed medically vulnerable. By the time the vaccine has been administered to everyone 50 and over—the ninth stage—about a quarter of the population will have been covered, and 90% to 99% of those most vulnerable to the virus, according to the U.K. government’s joint committee on vaccination and immunization.

The hubs for administering the vaccine will also expand as more people are vaccinated, the government says. As of now, hospital “hubs” across the U.K. will distribute the vaccine. That will expand to local care homes and general care practices, and what are expected to be the opening of huge vaccination centers at local football stadiums and conference centers.

The vaccine will also be administered solely through the country’s National Health Service. It won’t be possible to buy doses privately, so nobody, for love or money, can push ahead in line.

Will there be enough?

The scale of the vaccine rollout will be unlike anything seen before. Every person in the U.K., 16 and over, is expected to be vaccinated—a process that is expected to take at least most of 2021.

That equates to about 50 million people, equal—if two doses are needed—to 100 million doses. In addition to the 40 million doses of the Pfizer-BioNTech vaccine, the U.K. government has secured doses for seven different vaccines, including 100 million doses of the homegrown AstraZeneca/University of Oxford vaccine alone.

But access to the vaccine globally is undoubtedly weighted heavily toward some countries. In November, Nature reported that the member states of the EU, plus five other rich countries—the U.K., the U.S., Canada, Australia, and Japan—had secured over half the potential doses of vaccines. Canada alone has secured about nine doses per person.

Will the vaccine be mandatory?

No—the vaccine will be optional, just like all other vaccines in the country. However, the NHS seems to have already begun a public health campaign to assure people that the vaccine is safe and that they should take it.

Last month, it was reported that the NHS was looking to get “very sensible” celebrities and influencers to promote the vaccine’s safety, as well as public facing doctors and health professionals. (Politicians are not a possibility—but the Royal Family reportedly is.)

And Keenan herself, suddenly an international name after receiving the jab this morning, also did her part to encourage people to trust the vaccine, while thanking her doctors and her nurse.

“I can’t thank May and the NHS staff enough who have looked after me tremendously, and my advice to anyone offered the vaccine is to take it—if I can have it at 90 then you can have it too!”