Implementing behavioural science informed letter interventions to increase COVID-19 vaccination uptake in uncontactable London residents: a difference-in-difference study in London, UK.

Background: The UK COVID-19 vaccination programme began in December, 2020. By February, 2021, eight North West London Clinical Commissioning Groups (CCGs) had the lowest vaccination rates nationally. This study evaluated the impact of behavioural science-informed (BI) letters on vaccination uptake.
Methods: Unvaccinated residents of the Central London CCG who were deemed uncontactable (through text messaging and phone calls) were identified with the whole systems integrated care database. BI letters were sent to residents in the intervention CCG between May and June, 2021. Three neighbouring CCGs in London with similar non-responder data were used as control groups. A linear difference-in-difference analysis was undertaken to assess change in vaccine uptake rate across all four CCGs. Percentage point change was adjusted for selected covariates including ethnicity, age, gender, and index of multiple deprivation (IMD) quintiles. Approval was obtained from the quality improvement and audit office of Imperial College Healthcare NHS Trust (London, UK).
Findings: Within the intervention Central London CCG, 10 161 residents received the BI letter. The control CCGs contained 27 383 uncontactable residents. All CCGs showed an increase in vaccination rates in this population. The linear difference-in-difference analysis showed an increase in vaccination uptake in the intervention CCG (relative change 31·9% (95% CI 30·5-33·3; p<0·0001). Residents in IMD quintile 5 (least deprived) showed the largest rate of change (4·1%; p<0·0001). Residents with a mixed or multiple ethnic background were less likely to receive a COVID-19 vaccine (-4·1%, p<0·0001).
Interpretation: BI letters improved the rate of vaccine uptake. The percentage point increase of 31·9% equates to 436 additional previously uncontactable residents being vaccinated. Our data highlighted differences in the effect of BI-informed interventions in population subgroups. BI letters are a cost-effective and trusted communication tool, effectively engaging residents where other communication strategies did not work.
Funding: None.
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