90% effective Covid-19 vaccine

The headlines are jubilant with “Covid-19” vaccine 90% effective. As one of those skeptical scientists, I’m in a difficult position because I don’t want to dampen any hope, but at the same time it’s worth questioning the headline to see what’s really behind it.

The announcement was from Pfizer and BioNTech, and the media are pretty much echoing the contents of their press statement. I’ve blogged previously on science by press statement, rather than relying on peer reviewed literature and so this should should be the first warning bell. 

Many might assume that 90% effective means that on average in a population of 100 people, 10 will get full blown Covid-19 and 90 will be symptomless and not be carriers. This is however, a simplistic interpretation because effectiveness can be calculated based upon (1) ability to prevent infection (2) ability to prevent the disease, although individuals are still infected, and (3) ability to prevent serious disease. There is also the issue of how effective a vaccine reduces the infection rate to others, generally known as herd immunity. (This is, incidentally, the genuine type of herd immunity associated with vaccines, not the idea that we go out and get the disease, which has pretty much now been debunked).

The Pfizer-BioNTech vaccine is of the messenger-RNA type, which if effective, will be the first of its type. Although the science is sound, this class of vaccine does not have a track record, as yet. Looking any deeper at present is difficult without the full peer reviewed publication and all we have to go on is a press release. I’ll nevertheless, end by saying a vaccine even with a low rate of effectiveness could make a huge difference, and so we should very optimistic. Nevertheless, let’s temper the optimism with a little realism and wait and see how things develop. 

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