Vaccine coverage and denominators

What proportion of people had the Covid-19 vaccine?

What share of people have had a Covid-19 vaccine dose? For the United Kingdom on 25th May, different sources give different statistics:

There is a conceptual difference. PHE uses an Office for National Statistics estimate of the UK population aged 18 or over (~52.6m). Our World in Data uses the UN World Population Prospects 2019 estimate for 2020 (~67.9m).

Both are valid statistics. It is important to see the vaccine coverage of those eligible, and of the total population. Vaccine eligibility may change.

What about vaccine coverage by age groups?

This is more complicated.

The latest NHS England vaccination report has vaccinations up to 23rd May. It states over 100% of 75–79 year-old people received at least one dose.

Vaccine coverage by age using the 2019 population estimates is likely to be too high. The question is: why?

People age. As people get older, they move into more senior five-year age groups. Some of the people who were 73 or 74 in June 2019 are now in the 75–79 age group.

People age, and so the number in each age band changes. (Image: Carl Baker)

Given there were more people aged 73–74 than 78–79 in June 2019, our age group estimate for March 2021 is now too low. Mortality and migration also affect these population estimates by age. Statisticians are still processing the 2021 census. The 2019 estimate flows from the census a decade earlier.

Another denominator comes from the National Immunisation Management Service (NIMS). The NIMS data-set contains a list of NHS-registered people living in England. The NIMS denominators change in each publication, to exclude those who died.

Across England, people may register with doctors in more than one place. The NIMS total cohort has 50.6m registrants aged 16 or older. The ONS population estimate for English adults (16+) is around 45.5m.

What about coverage by place?

The two sets of denominators could lead to confusion. The BBC’s ongoing report on the vaccination programme uses the ONS estimates for England. In the latest report, the estimated coverage with first doses was 73% in England.

Wales leads for vaccine coverage among the four nations. (Image: BBC)

For English regions, the next graph uses the NIMS denominators. That means every region has first dose coverage of 70% or under. That mimics what the Public Health England dashboard does.

There may be large disparities between ONS estimates and NIMS counts in some areas. Knowing how many people could get a vaccine in each place is challenging.

This blog looks at the use of statistics in Britain and beyond. It is written by RSS Statistical Ambassador and Chartered Statistician @anthonybmasters.

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