Potential risk factors associated with not completing a 2-dose primary series of mRNA COVID-19 vaccination in a large health care system in the United States (Preprint)

BACKGROUND mRNA COVID-19 vaccines were initially authorized as a 2-dose primary series and have been widely used in the US. Understanding risk factors for not completing the 2-dose primary series is critical to evaluate COVID-19 vaccination programs and to promote completion of the 2-dose primary series. OBJECTIVE The aim of this study was to examine potential risk factors for not completing a 2-dose primary series of mRNA COVID-19 vaccination. METHODS We conducted a retrospective cohort study among members aged ≥18 years from a large integrated health care system, Kaiser Permanente Southern California, during 12/14/2020-06/30/2022. Non-completion of the 2-dose primary series was defined as not completing the second dose within 6 months after receipt of the first dose. A logistic regression model was fit to examine associations of individual-level and community-level risk factors with non-completion of the 2-dose primary series. RESULTS Among 2.5 million recipients of ≥1 dose of mRNA COVID-19 vaccines, only 3.3% did not complete the second dose within 6 months. Members aged 25-44 years, 65-74 years, and ≥75 years were less likely to not complete the 2-dose primary series than those aged 18-24 years while members aged 45-64 years were more likely to not complete the 2-dose primary series (adjusted odds ratio (aOR)=1.14 [95% CI, 1.11−1.17]). Male sex was associated with higher odds of non-completion (aOR=1.19 [95% CI, 1.18−1.21]. Hispanic and non-Hispanic Black race/ethnicities were associated with lower odds of non-completion (range of aOR 0.75−0.90). Having Medicaid and prior influenza vaccination were associated with higher odds of non-completion. Having SARS-CoV-2 infection, experiencing an adverse event (AEs), or having an inpatient/emergency department (ED) visit during the minimum recommended dose intervals were associated with higher odds of not completing the 2-dose primary series (aOR=2.33, 2.21, and 2.05, respectively). Those who received the first dose before June 30, 2021 were more likely to complete the 2-dose primary series within 6 months of receipt of the first dose. CONCLUSIONS Male, Medicaid coverage, and SARS-CoV-2 infection, AEs, and inpatient and ED visits during the minimum recommended dose intervals were significantly associated with non-completion of the 2-dose primary series while older age≥65 and Black and Hispanic race/ethnicity were associated with higher completion rate.