Estimating vaccination threshold and impact in the 2017–2019 hepatitis A virus outbreak among persons experiencing homelessness or who use drugs in Louisville, Kentucky, United States. Issue 49 (3rd December 2021)

• Record Type: Journal Article
• Authors: Dankwa, Emmanuelle A.; Donnelly, Christl A.; Brouwer, Andrew F.; Zhao, Rui; Montgomery, Martha P.; Weng, Mark K.; Martin, Natasha K.
• Is Part Of: Vaccine. Volume 39:Issue 49(2021)
• Page Start: 7182
• Page End: 7190
• Publication Date: 2021-12-03
• Subjects: Hepatitis A -- Critical vaccination coverage -- Persons who experience homelessness or who use drugs -- Herd immunity -- Dynamic modeling; Vaccines -- Periodicals; 615.372
• DOI: 10.1016/j.vaccine.2021.10.001
• Languages: English
• ISSNs: 0264-410X

Abstract

Highlights: We estimate vaccination impact in the 2017/19 hepatitis A virus outbreak in Louisville. Critical vaccination threshold among persons who experience homelessness or who use drugs was 77% assuming 90% vaccine efficacy. Vaccination during the outbreak accounted for at least US$490000 in cost savings. Earlier and faster vaccination would have led to greater impact. Abstract: Background: Between September 2017 and June 2019, an outbreak of hepatitis A virus (HAV) occurred in Louisville, Kentucky, resulting in 501 cases and 6 deaths, predominantly among persons who experience homelessness or who use drugs (PEH/PWUD). The critical vaccination threshold ( V c ) required to achieve herd immunity in this population is unknown. We investigated V c and vaccination impact using epidemic modeling. Methods: To determine which population subgroups had high infection risks, we employed a technique based on comparing the proportion of cases arising before and after the epidemic peak, across subgroups. We also developed a dynamic deterministic model of HAV transmission among PEH/PWUD to estimate the basic reproduction number ( R 0 ), herd immunity threshold, V c and the effect of timing of the vaccination intervention on epidemic and economic outcomes. Results: Of the 501 confirmed or probable cases, 385 (76.8%) were among PEH/PWUD. Among PEH/PWUD and within the general population, homelessness was a significant risk factor for infection in the initial stages of the outbreak (odds ratios for homeless versus not homeless: 2.62; 95% confidence interval (CI): 1.62–4.25 for PEH/PWUD and 2.39; 95% CI: 1.51–3.78 for all detected cases). Our estimate for R 0 ranges between 2.85 and 3.54, corresponding to an estimate of 69% (95% CI: 65–72) for herd immunity threshold and 76% (95% CI: 72%-80%) for V c, assuming a vaccine with 90% efficacy. The observed vaccination program was estimated to have averted 30 hospitalizations (95% CI: 19–43), associated with over US$490 000 (95% CI: $310 000–700 000) in hospitalization cost. Greater impact was observed with earlier and faster vaccination implementation. Conclusions: Vaccination coverage of at least 77% is likely required to prevent outbreaks of HAV among PEH/PWUD in Louisville, assuming a 90% vaccine efficacy. Proactive hepatitis A vaccination programs among PEH/PWUD will maximize health and economic benefits of these programs and reduce the likelihood of another outbreak.