CNN  — 

The UK became the first Western country to authorize a Covid-19 vaccine on Wednesday, marking a pivotal moment in the global fight against coronavirus.

The Pfizer/BioNTech vaccine has been granted emergency authorization by British regulators, and the first doses are expected to be rolled out from early next week.

Britain has been one of the countries hardest hit by the pandemic, with the highest death toll in Europe, and its government has been heavily criticized for its handling of the crisis.

But it has now leapfrogged both the European Union and the United States with this announcement.

Here’s what you need to know about how it authorized the vaccine:

Why was the UK first?

The vaccine was granted emergency authorization in the UK by its independent regulator, the Medicines and Healthcare products Regulatory Agency (MHRA), which has played a crucial part in the process.

The MHRA began a rolling review of Pfizer and BioNTech data from October, with each “package” of data reviewed as soon as it became available. This allowed regulators to examine the data in detail before a final authorization application was submitted.

According to the MHRA, a rolling review “can be used to complete the assessment of a promising medicine or vaccine during a public health emergency in the shortest time possible.”

This approach helped accelerate the authorization process and a formal review of all the necessary information began in the UK on November 23, leading to Wednesday’s announcement.

“I think the advantage is that the MHRA has been undertaking a rolling review, which means that as Pfizer accumulated data on how they manufactured the vaccine … MHRA could keep pace with that,” David Salisbury, associate fellow in Chatham House’s Global Health Programme, told CNN. “That has allowed the MHRA to be nimble and keep pace.”

A similar rolling review approach is being used by the European Medicines Agency (EMA). The EMA began its review process of the Pfizer data on October 6 and BioNTech and Pfizer submitted an application to complete the review process on December 1. The EMA has said it will conclude its review by December 29 at the latest.

EU member states cannot distribute a Covid-19 vaccine until it has been authorized by the EMA and signed off by the European Commission, according to EMA rules.

The European Commission (EC) would then need a few days to prepare legal paperwork and discuss the authorization decision with member states, according to an EC spokesperson.

“The fact that the MHRA has been able to do this quickly will be a reflection at the pace of which Pfizer was interacting with them,” Salisbury added.

As well as the UK and the EU, Pfizer has also applied to the US Food and Drug Administration (FDA) for emergency use authorization for its vaccine candidate. The request was submitted on November 20.

The FDA’s Vaccines and Related Biological Products Advisory Committee, a panel of independent experts, is due to meet on December 10 to discuss Pfizer’s application.

According to a US Operation Warp Speed document obtained by CNN on Tuesday, the first shipments of Pfizer’s coronavirus vaccine will be delivered on December 15, if the vaccine is granted emergency authorization.

BioNTech’s Chief Medical Officer Özlem Türeci said the company expected EMA and FDA responses by mid-December.

Türeci said Wednesday that the rolling review process played “an important role” in the UK’s authorization. She said the process allowed authorities to “start right away to go through the dossiers, review the data, come back with questions which we can respond to immediately. And this massively accelerates the process of assessing in depth the data we have provided.”

When can I get the vaccine in the UK?

The UK will begin rolling out the vaccine next week, according to Health Secretary Matt Hancock. But emergency authorization is only the first stage of that process – doses will be assigned according to clinical priority.

Each recipient of the Pfizer/Biontech vaccine will need two doses.

An independent panel of experts, the Joint Committee on Vaccination and Immunization (JCVI), has recommended that care home residents and staff are vaccinated first.

The panel recommends that people should then be vaccinated according to age, starting with people older than 80 as well as frontline health workers.

Age will then continue to be the deciding factor, with older adults vaccinated down to those older than 50.

JCVI experts have also advised that workers in the UK’s National Health Service (NHS) and those considered clinically extremely vulnerable to coronavirus should be prioritized under the initial phase of vaccination.

Those considered vulnerable include patients with cancer, those who are on drugs that weaken the immune system and those who have severe lung disease, severe kidney disease and other health conditions.

Hancock said Wednesday that the timing of how many people can be vaccinated “will be determined by how rapidly (doses) can be manufactured.”

“We haven’t put a figure on the numbers before Christmas,” he said. “But what we do know is we can get started next week with that first load, and several millions will be coming throughout December. People will be contacted by the NHS when it’s their turn.”

“I urge you very strongly to come forward, because obviously being vaccinated is good for you,” he added. “It’s approved as clinically safe by the regulator and it’s good for your community as well to help get this virus finally under control once and for all.”

There are logistical challenges facing the rollout as the vaccine needs to be kept at temperatures of minus 70 degrees Celsius (minus 94 Fahrenheit) prior to use. Once defrosted, Pfizer says the vaccine can be stored for up to five days at 2 to 8 degrees Celsius in refrigeration units that are commonly available in hospitals.

Speaking to Sky News on Tuesday, Hancock said there would be “a combination of three modes of delivery.”

The vaccine will first go to hospitals – 50 of which are on standby to receive doses. This will be followed by vaccination centers, which Hancock said were being set up now, before a “community rollout” including doctors’ offices and pharmacists.

How does the mRNA vaccine work?

The Pfizer/BioNTech vaccine uses a new approach to making vaccines that uses messenger RNA, or mRNA.

mRNA is a single strand of the genetic code that cells can “read” and use to make a protein.

For this vaccine, the mRNA instructs cells in the body to make a particular piece of the virus’s spike protein. The immune system sees it, recognizes it as foreign, and is prepared to attack when actual infection occurs.

What are the side effects of the Pfizer vaccine?

An independent group has been keeping an eye on trial results and side effects from the vaccine.

Pfizer and BioNTech say there were no serious side effects during the large-scale trials.

To date, the Data Monitoring Committee for the study “has not reported any serious safety concerns related to the vaccine,” the companies said.

The only notable side effect was fatigue in some trial participants.

“The only Grade 3 (severe) solicited adverse event greater than or equal to 2% in frequency after the first or second dose was fatigue at 3.7% following dose 2,” the companies said.

Paul Offit, professor of pediatrics at the Children’s Hospital of Philadelphia, told CNN on Wednesday that some side effects are normal for vaccines.

“That means your immune response is working for you. You should feel good about that,” he said, while discussing side effects.

He added that should people experience side effects from the first dose, then there shouldn’t be “any difficulty coming back for that second shot, knowing that you’re now in a much better position to fight off this awful virus.”

Is it safe to take more than one vaccine?

Other drugmakers, Moderna and AstraZeneca, also have promising vaccine candidates.

The UK government says studies are underway to determine whether the AstraZeneca and Pfizer/BioNTech vaccines can be given interchangeably across the two doses, since “both … are based on the spike protein of the virus,” but so far it’s unclear whether this is effective.

“There is no evidence as to the interchangeability of the different COVID-19 vaccines although studies are underway. Therefore, every effort should be made to determine which vaccine the individual received and to complete with the same vaccine,” the UK government says, though exceptions are permitted.

The guidance adds that for people who have had one dose and “attend for vaccination at a site where the same vaccine is not available, or where the first product received is unknown, it is reasonable to offer a single dose of the locally available product,” though it adds that this option is preferred if the individual “is likely to be at immediate high risk or is considered unlikely to attend again.”

Correction: A previous version of this story misidentified Özlem Türeci. She is female.

CNN’s Amy Cassidy, James Frater and Lindsay Isaac contributed to this report.