Originally published 2020-11-09.
On 9 November 2020, Pfizer and BioNTech announced that their COVID-19 vaccine candidate, BNT162b2, had been more than 90% effective at preventing symptomatic illness in the first interim analysis of a large Phase 3 trial. It was the first time any developer had reported efficacy data from a late-stage COVID-19 vaccine study, and the news moved stock markets, governments and ordinary people within hours.
The headline number came from 94 confirmed COVID-19 cases among 43,538 volunteers, and the independent monitors reported no serious safety concerns. A final analysis nine days later put efficacy at 95%. Within a month the vaccine was being given to the public. This page explains what the November announcement actually said, what "90% effective" means, and how the story unfolded.
What the companies announced on 9 November
According to Pfizer's press release, an independent Data Monitoring Committee looked at the trial data once 94 participants had developed confirmed COVID-19. Comparing how many of those cases occurred in the vaccine group versus the placebo group gave an efficacy estimate above 90%, measured from seven days after the second dose.
Key points from the release:
- 43,538 people had enrolled, and 38,955 had received their second dose by 8 November 2020.
- The vaccine was given as two injections, three weeks apart.
- The committee found no serious safety concerns and recommended the study continue.
- The trial would run until 164 cases had accrued for a final analysis, and participants would be followed for two years.
- The companies expected to make up to 50 million doses in 2020 and up to 1.3 billion in 2021.
Pfizer chairman and CEO Albert Bourla summed up the mood in one line: "Today is a great day for science and humanity." BioNTech's co-founder and CEO, Uğur Şahin, described it as a victory for innovation, science and global collaboration.
What "90% effective" really means
A common misunderstanding at the time was that 90% efficacy meant 90% of vaccinated people would become immune and 10% would not. That is not how efficacy is calculated.
In a placebo-controlled trial, efficacy compares the rate of disease in the two groups. If 100 cases occur among unvaccinated participants over a period and only 10 occur among an equal number of vaccinated participants, efficacy is 90%. In other words, vaccinated people had roughly a ninefold or greater reduction in their risk of symptomatic COVID-19 during the study period. It says nothing on its own about how long protection lasts, how well the vaccine stops transmission, or how it performs against future variants.
For comparison, seasonal flu vaccines usually reduce the risk of illness by somewhere between 40% and 60% in good years, so a figure above 90% was remarkable. Many scientists had said publicly they would have been pleased with 50% to 70%.
How the trial worked
Volunteers in several countries, including the United States, Germany, Turkey, Argentina, Brazil and South Africa, were randomly assigned to receive either the vaccine or a saltwater placebo. Neither they nor the staff knew who got which. Researchers then waited for natural infections to occur in the community and counted confirmed cases in each group.
About 42% of participants worldwide came from racially and ethnically diverse backgrounds, which was important for showing the vaccine worked across different populations. Early reports sometimes claimed no one in the trial had any side effects; that was not correct. Like most vaccines, BNT162b2 commonly caused sore arms, tiredness, headache and sometimes fever for a day or two. What the monitors reported was the absence of serious safety concerns at that stage.
How experts reacted
Scientists were openly emotional. Sir John Bell, regius professor of medicine at Oxford and a member of the UK government's vaccine taskforce, was asked by the BBC whether life could get back to normal by spring. His answer: "Yes, yes, yes, yes. I am probably the first guy to say that but I will say that with some confidence."
Peter Horby, professor of emerging infectious diseases at Oxford, was more cautious about timing but just as relieved, telling reporters that there was still a long way to go but that "this feels to me like a watershed moment."
Bell's spring prediction turned out to be optimistic. Vaccines did transform the outlook in 2021, but new variants, especially Delta and later Omicron, meant restrictions came and went for much longer in many countries.
What happened next
On 18 November 2020 the companies announced the final efficacy analysis: 95%, based on 170 cases, 162 in the placebo group and 8 in the vaccine group. The result was later published in the New England Journal of Medicine.
The United Kingdom became the first country to authorise the vaccine for emergency use, in early December 2020, and Margaret Keenan, aged 90, became the first person in the world to receive it outside a trial on 8 December 2020. The US Food and Drug Administration granted emergency use authorisation on 11 December, and on 23 August 2021 the FDA fully approved it under the brand name Comirnaty, making it the first fully approved COVID-19 vaccine in the US.
Supply was a real challenge. The vaccine had to be shipped and stored at between -80°C and -60°C, which required special freezers and dry-ice logistics. Most countries prioritised older people, care-home residents and health workers first, then moved down the age groups. Pfizer and BioNTech eventually produced far more than the 1.3 billion doses originally projected for 2021, after BioNTech raised its target to more than 2 billion.
The science behind the speed
BNT162b2 was an mRNA vaccine. Instead of using a weakened virus or a protein grown in cells, it delivers a set of genetic instructions, wrapped in tiny fat particles, that tell the body's own cells to make a harmless copy of the coronavirus spike protein. The immune system then learns to recognise that protein.
The approach seemed sudden to the public, but it rested on decades of research. BioNTech had been developing mRNA medicines, mainly for cancer, for years before the pandemic, and its founders had the technology ready to adapt as soon as the virus's genetic sequence was published in January 2020. Pfizer contributed large-scale trial management and manufacturing. That combination, plus massive funding and parallel rather than sequential testing steps, is why the timeline was so much shorter than for earlier vaccines.
Separately, the University of Oxford and AstraZeneca were developing a different type of vaccine, which had shown a strong immune response in earlier trials. Moderna's mRNA vaccine reported its own interim results a week after Pfizer's. Having several successful vaccines was what eventually made global supply possible.
If you have questions about COVID-19 vaccination for yourself or your family today, including boosters and updated formulations, talk to a doctor or pharmacist, as recommendations change over time.
Sources: Pfizer press release, 9 November 2020; Wikipedia: Pfizer–BioNTech COVID-19 vaccine.
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