Problems measuring vaccinated population
PHE and now its successor UKHSA keep a database of the health records of the population to record details of vaccination. 850,000 under 60 year olds were added into the database from April to October. In total, 1 in 20 people who have been vaccinated since April 2021 had to be added to the total population rather than removed from the unvaccinated numbers. These were largely people who had not been registered with a GP, so were not recorded as unvaccinated, but then became registered in order to receive their first vaccination. This problem highlights the difficulty with measuring a population.
HART has published a detailed analysis of the changes to this database and the findings are summarised here.
Early on in the vaccination campaign the measurement was representative because it was an accurate assessment of the proportion of the registered population who were or were not vaccinated. Now that the vaccination campaign has reached many people who were never registered it has become biased. People who have never been registered and remain unvaccinated are not counted but those who are vaccinated are. Because NIMS (National Immunisation Management Service) is a measurement not an estimate, it is not possible to acknowledge that there is a problem with undercounting of the unvaccinated community to rectify this bias.
An astounding 270,000 over 60 year olds were apparently vaccinated for the first time in October. Perhaps the latest campaigns were more motivating than the five months of campaigns that preceded them. Perhaps someone adjusted the settings so numerous people were simultaneously moved up to the over 60s cohort having had a birthday. Or perhaps people attending pop up vaccination centres without ID or NHS registration would still have received a vaccine and may only lately have been identified within the system as people have returned for boosters.
Finding 270,000 over 60 year olds to vaccinate who weren’t previously registered is a significant finding. The whole population of unvaccinated over 60s is estimated at under 1 million. The implication is, therefore, that prior to registering these people the unvaccinated population was significantly underestimated.
An over measurement of the total population size of over 60 year olds by 3%, e.g. from duplicate records, would give a case rate identical to the double-dosed. There will be uncertainty within the NIMS figures and it is not unreasonable to think that may account for a few percent of the totals. Changing the total population estimate results in large swings in the estimate of ‘case’ rates in the unvaccinated. Given the inability to be accurate within a few percentage points, and the uncertainty about whether the inaccuracies are overestimating or underestimating the total, the differences between populations is not as informative as the trend in rates within each of the populations.
