Covid-19 Vaccine Booster Cadence by Immunocompromised Status

BackgroundData suggest that vaccine effectiveness against Covid-19-associated hospital admission and mortality is augmented with booster doses, but the benefit wanes within several months. However, the CDC recently concluded that second doses of bivalent vaccines this Spring were not warranted because existing data were insufficient to analyze the benefits of such a strategy. Therefore, our objective was to assess whether routinely boosting high-risk populations at least every 6 months may be warranted, depending on age and immune status. MethodsUtilizing a database of 3,574,243 members of Clalit Health Services (CHS), we analyzed the medical records of individuals who received none, or at least one dose of the BNT162b2 mRNA COVID-19 vaccine between January 1, 2021, and April 5, 2022. We examined the risk of moderate-to-severe Covid-19 hospitalization or death stratified by age group, immune status and time since receipt of the last vaccine dose during the early Omicron wave in Israel (December 20, 2021 to April 5, 2022). The number needed to vaccinate (NNV) was calculated as the inverse of the absolute risk reduction for various subgroups and Covid-19 waves. ResultsEligibility criteria were met by 3,381,480 CHS members. The absolute risk of Covid-19 moderate-to severe hospitalization or death during the Omicron wave increased with age, immunocompromised status, and time since receipt of the last vaccine dose. The NNVs varied greatly by age and immune status and were contingent on various disease prevalence scenarios. Among the severely immunocompromised, boosting at the start of the Omicron wave had an NNV ranging from 87 (95% CI 70-109) in persons ages [≥]80 to 1,037 (95% CI 999 -1,513) in persons ages 12-59. In the lower prevalence periods, the NNV for 6-month booster cadencing remained favorable for immunocompromised people in all age groups and immunocompetent people ages [≥]60. ConclusionsOur study provides evidence for the potential benefit of a routine 6-month cadence for Covid-19 boosters for the highest-risk groups, and possibly more frequently, even during relatively lower Covid-19 prevalence.