Jab fears explained: a base rate fallacy
More or Less
BBC
4.6 • 3.7K Ratings
🗓️ 7 August 2021
⏱️ 9 minutes
🧾️ Download transcript
Summary
As some countries rapidly roll out vaccination programmes, there have been concerns that increases in infection rates amongst vaccinated groups mean vaccines are less effective than we hoped, especially in the face of the feared Delta variant.
Epidemiologist Dr Katelyn Jetelina from the University of Texas Health Science Centre School of Public Health explains why this isn’t what the numbers show – rather than decreasing vaccine effectiveness, increasing rates can be explained by a statistical phenomenon known as ‘base rate fallacy’.
Presenter: Charlotte McDonald
Producer: Nathan Gower
Transcript
Click on a timestamp to play from that location
| 0:00.0 | Hello and welcome to more or less on the BBC World Service with me, Charlotte McDonald, |
| 0:05.3 | with a programme that examines those naughty statistical problems that leave the world scratching |
| 0:10.3 | its head. Today we're looking at a mathematical mirage that has confused and worried people |
| 0:16.8 | in recent weeks. Some countries around the world have been rolling out Covid-19 vaccines very quickly, |
| 0:23.4 | but as this has happened, some people think they've spotted something strange. The number of |
| 0:28.8 | infections happening in vaccinated people has been creeping upwards, and this seems like the |
| 0:34.2 | opposite of what we'd want to happen. And most worrying of all, it makes it look like the vaccines |
| 0:39.8 | aren't working. Well, breathe easy, because more or less is here to explain why this isn't what |
| 0:46.4 | the numbers are telling us. It's all down to a concept in statistics known as base rate fallacy |
| 0:55.2 | or base rate bias. This is a problem of perception of looking at certain statistics and coming to |
| 1:01.9 | the wrong conclusion, or because we don't keep in mind the necessary background data. |
| 1:09.2 | Let's say your government brings in a law saying everyone has to wear a seatbelt, |
| 1:13.8 | and 99% of people follow the law. But at the end of the year, when you look at deaths in car |
| 1:19.7 | accidents, you find out that many more people have died who are wearing a seatbelt |
| 1:24.9 | than the people who didn't. Now, does this mean that seatbelts aren't working? |
| 1:29.6 | Well, no, we just need to keep an eye on the base rate the number of people now wearing seatbelts. |
| 1:36.2 | Given that this group is so huge, it's little surprise that there will be more deaths from |
| 1:39.7 | car accidents among those than among the tiny group of non-seatbelt wearers. |
| 1:44.5 | The same principle applies to COVID vaccinations. As a vaccination programme rolls out, |
| 1:50.0 | we might be alarmed at the fact that an increasing percentage of infections are happening in vaccinated |
| 1:55.6 | people as opposed to unvaccinated people. But it's only surprising if you're not keeping |
| 2:01.6 | an eye on the increasing percentage of people in a population who have been vaccinated, |
... |
Please login to see the full transcript.
Disclaimer: The podcast and artwork embedded on this page are from BBC, and are the property of its owner and not affiliated with or endorsed by Tapesearch.
Generated transcripts are the property of BBC and are distributed freely under the Fair Use doctrine. Transcripts generated by Tapesearch are not guaranteed to be accurate.
Copyright © Tapesearch 2026.