Determining herd immunity thresholds for hepatitis A virus transmission to inform vaccination strategies among people who inject drugs in 16 U.S. States.

BACKGROUND: Widespread outbreaks of person-to-person transmission of hepatitis A virus (HAV), particularly among people who inject drugs (PWID), continue across the United States and globally. However, the herd immunity threshold and vaccination coverage required to prevent outbreaks is unknown. We aimed to use surveillance data and dynamic modeling to estimate herd immunity thresholds among PWID in 16 U.S. states.

METHODS: We used a previously published dynamic transmission model of HAV transmission, calibrated to surveillance data from outbreaks involving PWID in 16 states. Using state-level calibrated models, we estimated the basic reproduction number (R0) and herd immunity threshold for PWID in each state. We performed a meta-analysis of herd immunity thresholds to determine the critical vaccination coverage required to prevent most HAV outbreaks among PWID.

RESULTS: Estimates of R0 for HAV infection ranged from 2.2 (95% CI 1.9-2.5) for North Carolina to 5.0 (95% CI 4.5-5.6) for West Virginia. Corresponding herd immunity thresholds ranged from 55% (95% CI 47-61%) for North Carolina to 80% (95% CI 78-82%) for West Virginia. Based on the meta-analysis, we estimated a pooled herd immunity threshold of 64% (95% CI 61-68%, 90% prediction interval 52-76%) among PWID. Using the prediction interval upper bound (76%) and assuming 95% vaccine efficacy, we estimate a vaccination coverage of 80% could prevent most HAV outbreaks.

CONCLUSIONS: Hepatitis A vaccination programs in the United States may need to achieve vaccination coverage of at least 80% among PWID in order to prevent most HAV outbreaks among this population.