A study conducted by Health Data Research UK, a national institute for health data research, and the University of Edinburgh, characterised factors associated with under-vaccination and investigated the association with severe outcomes from COVID-19.
Although COVID-19 vaccine adherence and efficacy have been extensively studied in the UK and elsewhere, few studies have examined the particular association between under-vaccination and severe outcomes from COVID-19. This study provides precise estimates of the association between complete vaccination for the entire UK population and the incidence of severe outcomes from COVID-19. It also provides information on which population subgroups have the highest levels of under-vaccination, useful for targeting policy, public health and research agendas.
The COVID-19 vaccination programme in the UK started on 8 December 2020. As of 1 June 2022, the vaccination schedule recommended by the UK's Joint Committee on Vaccination and Immunisation (JCVI) was one dose for the 5-11 age group, two for the 12-15 age group, three for the 16-74 age group, and four for those aged 75 or older.
In the UK, there was high adherence to the first dose of vaccine. By January 2022, more than 90% of the population over the age of 12 had been vaccinated. Vaccination adherence then declined for subsequent doses.
Hesitancy towards the COVID-19 vaccine was found in groups at high risk of severe COVID-19 outcomes. This is despite the fact that clinical trials for approved COVID-19 vaccines demonstrated causal efficacy against infections, hospitalisations and deaths, and these were associated with a reduction in adverse outcomes from COVID-19 infections.
The study was conducted using electronic health registers with almost full national coverage of England, Northern Ireland, Scotland and Wales. These datasets contained linked data from primary and secondary care, mortality data, immunisations and virological tests.
The cohorts included 58.9 million individuals in England, 1.9 million in Northern Ireland, 5.0 million in Scotland and 2.4 million in Wales. Those under 5 years of age were excluded. The observation was made between 1 June 2022 and 30 September 2022.
People were grouped according to vaccination status. Under-vaccination was defined as failure to receive all doses of vaccine for which a person was eligible; that is, COVID-19 vaccines of any type licensed in the UK, namely Pfizer-BioNTech (BNT162b2), Oxford-AstraZeneca (ChAdOx1) and Moderna (mRNA-1273).
The vaccine deficit was defined as the number of doses recommended by the JCVI minus the number of doses taken.
The number of under-vaccinated individuals as of 1 June 2022 was 26,985,570 (45.8%) in England, 938,420 (49.8%) in Northern Ireland, 1,709,786 (34.2%) in Scotland, 773,850 (32.8%) in Wales, and 30,407,626 (44.4%) in the entire aggregate population of the four nations.
The highest rates of under-vaccination were found in younger people, men, people living in areas of higher poverty and people of non-white ethnicity. Vaccine under-vaccination was associated with an elevated risk of severe COVID-19 outcomes (hospitalisation and death) in all age groups and countries, particularly in people aged 75 years and older.
The aHR (adjusted hazard ratios) for severe COVID-19 outcomes in the meta-analysis for the age group 16-74 years were 1.26 (95% CI 1.19-1.32) for one dose deficit, 1.88 (1.71-2.06) for two doses and 1.50 (1.42-1.57) for three doses. In the age group 75 years and older, it was 2.70 (2.61-2.78) for a one-dose deficit, 3.13 (2.93-3.34) for two doses, 3.61 (3.13-4.17) for three doses and 3.08 (2.89-3.29) for four doses.
According to the study authors, if all study participants had been fully vaccinated by 1 June 2022, there would have been an associated reduction in severe COVID-19 outcomes by the end of follow-up of 210 (95% CI 94-326) in the age group 5-15 years, 1,544 (1399-1689) in the age group 16-74 years, and 5,426 (5340-5512) in the age group 75 years and older, out of a total of 40,393 severe COVID-19 outcomes (14,156 in the under-vaccinated).
Mathematical modelling thus indicated that 7,180 hospitalisations and deaths out of approximately 40,400 cases of severe COVID-19 during the four months of summer 2022 could have been avoided if the UK population had been fully vaccinated.
Despite the limitations of the study, the authors believe that the results are useful to begin analytical work that can guide future health policies. They believe there is a need to expand this work by better understanding the barriers to adequate vaccination, particularly in the subpopulations identified, and to formulate health policies and public health interventions aimed at improving vaccination coverage. This could include, for example, the need to address vaccine misinformation more directly.