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Pfizer claims its Covid-19 vaccine is 90 percent effective so far. Here’s what we actually know.

The results are promising, but they’re drawn from a very small number of infections.

A health care worker holds an injection syringe of the phase 3 vaccine trial, developed against the novel coronavirus pandemic by Pfizer and German company BioNTech.
A worker holds a syringe with Pfizer and BioNTech’s Covid-19 vaccine. The companies announced that their vaccine is more than 90 percent effective in an early analysis.
Dogukan Keskinkilic/Anadolu Agency/Getty Images
Umair Irfan is a correspondent at Vox writing about climate change, Covid-19, and energy policy. Irfan is also a regular contributor to the radio program Science Friday. Prior to Vox, he was a reporter for ClimateWire at E&E News.

The American pharmaceutical giant Pfizer and the German biotech firm BioNTech reported on Monday in a press release that their joint Covid-19 vaccine candidate, BNT162b2, is more than 90 percent effective at preventing infection.

That’s great news. It brings the US — and the world — a step closer to another viable Covid-19 vaccine, which we urgently need to help contain and end the pandemic.

“Today is a great day for science and humanity,” said Pfizer CEO Albert Bourla in a statement. “The first set of results from our Phase 3 Covid-19 vaccine trial provides the initial evidence of our vaccine’s ability to prevent Covid-19.”

The key words there are “initial evidence.” The announcement is by no means a final result, and the phase 3 clinical trials of the Pfizer-BioNTech vaccine still need to be completed. That will likely take a few more months, and the subsequent findings could undermine the early results.

On the other hand, the recent announcement does hint that Pfizer and BioNTech, as well as some other manufacturers using the mRNA-based approach, are on the right track. That means that by next year, several vaccine candidates may begin distribution.

What we know and don’t know about the Pfizer-BioNTech vaccine

1) The vaccine announcement comes from a press release, not validated data

The results were reported Monday on Pfizer’s website. The company did not publish trial data at all, let alone any peer-reviewed findings.

It doesn’t mean that the findings are wrong or misleading, but it does mean that we only have the company’s word to go on. For for-profit companies like Pfizer and BioNTech, there may be an ulterior motive. “Science by press release is not reliable science,” as Vox’s Julia Belluz has noted. “Drug companies are notorious for exaggerating and skewing their early findings in public announcements to grab attention and boost investor interest.”

2) The numbers driving the announcement are minuscule

Phase 3 clinical trials are where a vaccine is tested against the virus in the real world. Pfizer and BioNTech enrolled 43,538 participants around the world and randomly sorted them into two groups: one that received the vaccine and one that received the placebo. Then the company waited to see how many confirmed Covid-19 infections emerged.

The findings reported on Monday showed that 94 volunteers in this pool developed Covid-19, but the vast majority of these cases were in the placebo group, indicating that the vaccine seems to be highly effective against the virus. The companies, however, did not disclose the exact split of infections between the treatment and placebo groups.

The more effective a vaccine, the earlier a signal will emerge in a clinical trial. But 94 cases is not enough to conclude the trial. Pfizer and BioNTech said that the endpoint of their trial is when they can confirm 164 Covid-19 infections in their pool.

Therefore, the results are not enough for the Food and Drug Administration to grant full approval to the vaccine. The safety standard for a vaccine is much higher than it is for a treatment, because a vaccine is distributed to vastly more people than a treatment, including those who are healthy and those who may have underlying health concerns. So even rare complications become magnified, making it critical to have a high bar for safety.

3) With those caveats in mind, the vaccine has a high efficacy

The results do indicate that the vaccine will offer protection to most people who get it. The reported 90 percent efficacy means that if 10 people received the vaccine, at least nine of them will have protection against the virus compared to the placebo.

That bodes well for a vaccination campaign. A highly effective vaccine means that fewer people would need to be immunized to contain the virus and eventually get to herd immunity, the point where the virus cannot easily jump from person to person because it can’t find a susceptible host.

But it’s not yet clear how effective the vaccine is at controlling the virus in an individual. It could be that the vaccine prevents severe Covid-19 but not mild symptoms, or it could be that it keeps the virus completely at bay. People with mild symptoms could still spread the disease. It’s also not yet known who is the ideal candidate for this vaccine. It may be more effective in some age groups and less in others, or it may not work well with certain preexisting conditions. So this vaccine may not be a one-size-fits-all solution to the pandemic.

4) This particular approach to a vaccine (likely) works

One thing that’s been remarkable about the push for a Covid-19 vaccine is the rise of new vaccine technologies. Prior vaccines used whole viruses, or portions of them, to generate immunity. But several companies, including Pfizer and BioNTech, are using an mRNA-based approach that involves genetic material. Here, the instructions for making a piece of the virus are inserted into the body where human cells read the instructions. Cells then manufacture the virus fragment. The immune system then identifies it and builds up protection against it.

Such approaches have never been approved for widespread use in humans before, so the fact that Pfizer and BioNTech’s vaccine has revealed good results so quickly is an important signal that this technique is viable.

The results posted Monday also indicate that the vaccine is aiming at the right target. The BNT162b2 vaccine codes for the spike protein of SARS-CoV-2, the virus that causes Covid-19. The spike protein is what the virus uses to break into cells and hijack their machinery to make copies of itself. Researchers reasoned that if they could teach the body to target the spike protein and block it, the immune system could stop the infection. The Pfizer and BioNTech results show that this logic is sound.

This is not just good news for Pfizer and BioNTech, but also for other vaccine manufacturers, like Moderna, that are developing genetic vaccines that target the spike protein.

“We believe these interim results also increase the probability of success of other COVID-19 candidate vaccines which use a similar approach,” said Richard Hatchett, CEO of the Coalition for Epidemic Preparedness Innovations, or CEPI, an international vaccine development consortium, in a statement.

5) This would be a tricky vaccine to distribute

Vaccines can be very fragile, with stringent storage requirements. According to the World Health Organization, more than half of all vaccines in the world go to waste, mainly because they spoil due to flaws in temperature control.

Pfizer and BioNTech’s BNT162b2 vaccine requires storage as low as -80°C. That may be a logistical nightmare for distributing the vaccine, as thousands of costly, new ultra-cold freezers would be needed. Pfizer is also developing a custom box that can keep the vaccine cold in transit. The question is whether this can be scaled up to vaccinate hundreds of millions of people across the US, and possibly billions around the world.

Another complication is that this is a two-dose vaccine. Recipients will have to get two shots spaced about three weeks apart in order to get protection. That means the company would have to produce twice as many doses as they would need for a one-dose vaccine, and clinics would have to keep track of recipients and make sure they come back at the right time for their second jab.

6) Pfizer and BioNTech didn’t take US government money to develop the vaccine, but they may get money to manufacture it

Kathrin Jansen, a senior vice president and the head of vaccine research and development at Pfizer, boasted to the New York Times that the company didn’t receive any funding from Operation Warp Speed. That’s the $10 billion program from the Department of Health and Human Services aimed at delivering 300 million doses of a Covid-19 vaccine by January 2021.

“We were never part of the Warp Speed,” Jansen told the Times. “We have never taken any money from the U.S. government, or from anyone.”

But the US government did agree to buy at least 100 million doses of Pfizer and BioNTech’s vaccine if it clears clinical trials. And the FDA streamlined a lot of its regulations in order to speed up clinical trials, something that has clearly benefited these companies.

7) The timeline for when a vaccine might be rolled out is still up in the air

The fact that Pfizer and BioNTech detected 94 Covid-19 cases in their pool of volunteers so soon after enrolling their clinical trial shows that the research is progressing quickly. But the companies still have to finish the trial. Recall that the endpoint is 164 confirmed infections in the trial pool.

That said, the companies may be close to the threshold needed to file for an Emergency Use Authorization from the FDA, which would allow the vaccine to be made available to people in high-exposure roles for Covid-19. Pfizer and BioNTech said they expect to reach the necessary benchmark for an emergency authorization by the third week of November.

That means the first doses of this vaccine going out, if they clear their benchmarks, are likely several weeks, if not months, away.

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