why the vaccine-only approach will backfire

After yesterday's announcement of betting everything on the booster programme to hold the omicron variant at bay I hear that the medical staff meant to deliver a million jabs per day from today were as surprised as the rest of us to find out about it from yesterday's speech.

Nothing wrong with protecting people with a third dose of vaccine, and I've had mine a week ago, but it's a bad idea to rely only on a vaccine in the face of a raging epidemic and do nothing else. Here's just one little number crunching to show why:

If the booster (or any other vaccine applied in an ongoing epidemic) is 90% effective in preventing undesirable outcome (eg hospitalisation, or deaths) and makes people feel safe so they engage in 20x more risky behaviour (eg take the tube to work and come into close contact with 60 people per day, rather 3 if they cycle to work; or they go out clubbing rather than staying safe at home), what happens?

The 90% efficient booster divides the likelyhood of the undesirable outcome by 10, but the behaviour change multiplies it by 20, so you get twice as many cases of the undesirable outcome. Which is why relying _only_ on a vaccination with less than 100% efficiency may produce the opposite of the desired effect. Behaviour/culture eats vaccination/policy for breakfast as the old saying goes.

I remember epidemiologists warning of this backlash effect since before we even had vaccines, and it's been the reason why we've had unsustainably high numbers since September inspite of good vaccine coverage, but the UK govt still does not understand this. So, starting from a high plateau we're going straight over the cliff with omicron again, and will need another screeching U-turn to sort this out. When will they ever learn?

Update 15.12.: Here's a good opinion piece from SAGE member Stephen Reicher on the mess we're in with omicron's arrival. As Reicher points out, the booster programme is a slow-acting measure, so the important bit is to have contact reduction to slow omicron down until the boosters take effect.