Fig 2. Actual vs. predicted pandemic status in the UK and US during the vaccination campaign.

As the fraction of the (first-dose) vaccinated (green line) increases, the extrapolated R (dotted blue line) declines. This extrapolation assumes that factors such as the social distancing restrictions, variant composition, weather, etc. remain unchanged from the start of the vaccination campaign. The empirically measured R (solid blue line) has remained in the vicinity of unity (with a temporary jump to 1.5 just after the introduction of the more infectious delta variant). Furthermore, the extrapolated number of infections (dotted red line) declines much faster than the actual number (solid red line). These trends indeed confirm that adaptive social behavior leads to a relaxation in prophylactic measures, in response to the accumulation of population immunity.