Pfizer tested its vaccine in roughly 44,000 participants across 150 trial sites in the US, Europe, Latin America and South Africa before it got the green light from UK regulator.

The UK is the first Western country to officially have a vaccine that prevents COVID-19.

The Medicines and Healthcare Products Regulatory Agency, the MHRA, gave a green light on Wednesday to a vaccine from Pfizer and BioNTech that protects against COVID-19, the disease caused by the coronavirus.

Most Brits will get the vaccine by spring, UK Secretary of State for Health and Social Care Matt Hancock said in a BBC interview on Monday. People most risk at of the disease will be first in line, getting the shot starting early next week, if logistical challenges are overcome, including transporting the vaccine from Belgium at ultra-cold temperatures. 

Read more: How the pharma giant Pfizer teamed up with a little-known biotech to develop an effective coronavirus vaccine in record time

It’s an incredible feat in the context of a global pandemic, achieved in record time. Other regulators, like the US Food and Drug Administration and the European Medicines Agency are expected to make decisions by the end of the year.

Despite the UK's quicker action, MHRA Chief Executive Dr. June Raine said in a televised press briefing on Monday that the "same stringent standards" have been met, and "no corners had been cut".

To come to the decision, the agency weighed up the vaccine's risks versus its benefits, known as a "risk-benefit analysis". During this process, the MHRA scrutinized the vaccine's safety, efficacy (how well it works), and the quality of the vaccine itself, including how it's manufactured. 

Raine said that the speedy decision-making stems from MHRA teams working in parallel, and analyzing data as it came in, known as a "rolling review". 

The detail of how and why the MHRA reached the decision remains unknown, making scrutiny and comparisons with other regulators challenging.

The MHRA told Business Insider that it plans to produce regulatory papers, but the timeline for this is unclear. And, Pfizer told reporters on Monday that it couldn't say when trial data will be published in a peer-reviewed journal. 

In the meantime, unanswered questions are starting to pile up. 

Here's what we still don't know about the vaccine.

Read more: How AstraZeneca and Oxford blew their big vaccine moment: A messy week that overshadowed what should have been a scientific victory

1. Is it safe in the long-run?

The short answer is we don't know. Pfizer's vaccine is new, and so it hasn't been tested for long enough for scientists to judge whether it causes side effects long-term. 

We do know more about potential short-term side effects. The side effects included in the official UK leaflet for health professionals are based on adverse effects reported by the volunteers. In total there were about 20,000 people that received the shot in Pfizer's key clinical trial. Side effects they reported include:

  • Pain at the injection site
  • Fatigue 
  • Headache 
  • Muscle ache 
  • Chills 
  • Joint pains 
  • Fever 

"Most of the side-effects are usually mild and last for a day or so," said chair of the Commission on Human Medicine Expert Working Group Professor Sir Munir Pirmohamed on Monday. The Human Medicine Expert Working Group has advised the MHRA in their decision making.

Pfizer said there were "no safety concerns" reported by an independent data monitoring committee.

However, decision-making was based on a "risk-benefit" analysis, and more details on how the UK regulator arrived at its decision won't be revealed until the regulator papers are made public. There's clearly a huge potential benefit of the vaccine because it protects people against a disease that's taken roughly 1.4 million lives globally. Does that mean they've accepted a big risk?

Pirmohamed said that the MHRA is "proactive" in monitoring the long-term safety of the shot. Part of this will include the yellow card system, a reporting system where anyone, including the public, can report adverse events. 

The MHRA also plans to launch a separate system where people can sign up and report side effects, Pirmohamed added.

However, it's unclear how many people will sign up to this voluntary system, and there's the potential to miss out on a lot of data if people choose not to.

Rain reiterated that public safety remains at the forefront of regulator's mind.

"Safety is our watchword and it will always continue to be so," she said.

2. How long does it protect against COVID-19?

Protection from the vaccine kicks in by seven days after the second dose, though people should have some protection 12 days after the first dose, Pirmohamed said, 

However, because scientists are learning about the disease at the same time they're developing vaccines, it's unclear how long immunity lasts for, and how often people will need to get vaccinated. People might need to get booster shots.

3. Does it stop people from spreading the disease?

Pfizer's large late-stage study wasn't designed to test whether the shot stops people from spreading coronavirus if they don't have symptoms. Instead, the drugmaker's priority was to test whether it prevents the disease, said Pfizer Vaccines' Vice President Ralf Rene Reinert.

Reinert said that "research is underway" to answer this question, and it involves testing people for COVID-19 every day.

4. Should it be given to pregnant people?

Pregnant people were excluded from Pfizer's trials. Therefore, there's no data regarding whether the shot is safe or effective in pregnant people right now. 

The UK says the shot shouldn't be given to pregnant people, or to people who are breastfeeding.

However, early stage studies investigating whether the vaccine's harmful to unborn babies are ongoing.

This means that if it's proven safe and works, Pfizer's COVID-19 vaccine could be used to protect pregnant people in the future.

5. Can children be given the shot?

At the moment, the MHRA has authorized the vaccine to be given to adults over 16 years old.  Pfizer tested the shot in its large late-stage trial in healthy individuals over 18 years old at first, but then expanded it to include those over 12 years old, according to the company.

However, Reinert said that Pfizer is planning additional clinical trials to make the vaccine available for children going forwards. 

"We are not at the end of development, we are in the middle." Reinert said.

Read more: Drugmakers behind 3 coronavirus vaccines say they work. Here's everything we know about the race for a vaccine and when you might be able to get a shot.

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