Pfizer claims its Covid-19 vaccine is 90% effective

One week ago today, the Presidential election was held in America, and the result is still reverberating around the world, with President Trump still refusing to concede defeat. Yesterday, the American pharmaceutical giant Pfizer and the German biotech firm BioNTech announced in a press release that their joint Covid-19 vaccine candidate, BNT162b2, is more than 90 percent effective at preventing infection. It brings the US and the world a step closer to a viable Covid-19, which we urgently need to help contain and end the pandemic.

What if the press release had been released one week earlier ? It could well have made a difference to the Election result, with President Trump legitimately retaining power for the next four years. Let me be clear. Thank goodness the the first interim efficacy analysis was conducted “on November 8, 2020 by an external, independent Data Monitoring Committee (DMC) from the Phase 3 clinical study,” rather than a week before.

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.

Having said that, I have read the press release in detail together with the Vox analysis of the data, and I am none the wiser in understanding results and the claim that the vaccine is 90% effective. Pfizer did not publish any trial data, let alone any peer-reviewed findings. What I don’t understand and what is not explained in the press release is how 90% efficacy was calculated from this data. If 94 people became infected, most of whom came from the placebo group, how many came from the vaccine group ? Apparently, 94 cases is not enough to conclude the trial, the endpoint of their trial is when they can confirm 164 Covid-19 infections in their pool from the original 43,538 volunteers in the full clinical trial. click for full VOX article.

Update from Unherd – I have found a succinct explanation of the trial study results here which I understand. I think, though, we can be pretty confident that the vaccine actually works. It would be understandable to be sceptical because there has, as yet, been no scientific paper released, or even very much data, beyond one simple number: 94. 

That figure is the total number of cases in both arms of the trial. There were 43,538 people on the trial; according to the study protocol (137 pages), half of them were given the real vaccine, and half a placebo. Three weeks later, all but 4,000 or so received a booster shot, of the same jab (either the vaccine or the placebo) they had before. According to the Pfizer press release, there were 94 cases in total, and “the case split between vaccinated individuals and those who received the placebo indicates a vaccine efficacy rate above 90%”.

What that means is that the vaccine prevented at least 90% of cases, so there were 10 times as many in the placebo group as the real vaccine group. Say there were 100 cases in the placebo group and 50 in the other: that would mean that it was 50% effective; if there were 100 in the placebo and ten in the real vaccine group, that would mean it was 90% effective.

In this case, if there are 94 cases in total, there can’t have been more than eight cases in the real-vaccine group, and 86 in the placebo group, for it to be “above 90% effective”. And we also know that the vaccine is astonishingly safe, given no major adverse events in the 20,000 people who were given it.

Again, we have no further data, and it’s always sensible to be wary when ‘pharma company puts out press release saying pharma company’s product is great’. But the results weren’t released by Pfizer — they were put out by an independent monitor, the Data and Safety Monitoring Board (DSMB), so it would be pretty astonishing if they have just made this up.

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