Objectives: Ethnic minority groups experience disproportionate influenza morbidity and mortality. Ethnic differences in response to the inactivated influenza vaccine (IIV) are not fully understood. We evaluated IIV serological response in UK healthcare workers (HCWs).

Methods: We conducted a prospective cohort study of HCWs from September 2020 to August 2022. We measured haemagglutination inhibition assay (HAI) titres to influenza A/H1N1 and A/H3N2 pre-vaccination, 1-month and 6-months post-vaccination. We compared geometric mean titres between ethnic groups using t-tests, and examined the boosting(pre-vaccination to 1-month) and waning(1-month to 6-months) phases using multivariable linear regression adjusting for previous titre, sampling interval, age, sex, and prior influenza vaccinations.

Results: Of 815 participants, 639(78%) were White, 133(16%) were South Asian and 43(5%) were of Black/Mixed/Other ethnicity. In year 1 we found no significant differences by ethnicity. In year 2, South Asian participants had a greater increase in A/H1N1 HAI titre during the boosting phase compared to White participants (adjusted geometric mean ratio 1.77[95%CI 1.26,2.48]p=0.001). Adjustment for prior influenza vaccination attenuated the association between the Black/Mixed/Other group and greater A/H1N1 and A/H3N2 HAI titre increase during the boosting phase compared to White participants.

Conclusions: We found ethnic differences in IIV serological responses, with evidence that pre?existing immunity contributes to this variation.