Real-world effectiveness of nirmatrelvir/ritonavir against COVID-19 hospitalizations and severe COVID-19 in community-dwelling elderly Singaporeans during Omicron BA.2, BA.4/5, and XBB transmission

Objectives: Real-world data on continued effectiveness of nirmatrelvir/ritonavir against hospitalization and severe COVID-19 in the context of widespread booster mRNA vaccine uptake and more immune-evasive Omicron sub-variants are lacking. We conducted a retrospective cohort study in adult Singa-poreans aged >60 years presenting to primary care with SARS-CoV-2 infection, during waves of Omicron BA.2/4/5/XBB transmission.Methods: Binary logistic regression was used to estimate the effect of treatment (receiving nirmatrelvir/ ritonavir) on outcomes (hospitalization, severe COVID-19). Additional sensitivity analyses, including inverse-probability-of-treatment-weighting-adjusted analysis and adjustment using overlap weights, were performed to account for observed differences in baseline characteristics among treated/untreated cohorts.Results: We included 3959 nirmatrelvir/ritonavir recipients and 139 379 untreated controls. Almost 95% received >3 doses of mRNA vaccines; 5.4% had preceding infection. Overall 26.5% of infections occurred during the Omicron XBB period and 1.7% were hospitalized. On multivariable logistic regression, receipt of nirmatrelvir/ritonavir was independently associated with lower odds of hospitalization (adjusted odds ratio [aOR] = 0.65, 95% CI = 0.50-0.85). Consistent estimates were obtained after inverse-probability-of-treatment-weighting adjustment (aOR for hospitalization = 0.60, 95% CI = 0.48-0.75) and adjustment using overlap weights (aOR for hospitalization = 0.64, 95% CI = 0.51-0.79). Although receipt of nir-matrelvir/ritonavir was associated with lower odds of severe COVID-19, it was not statistically significant.Discussion: Outpatient usage of nirmatrelvir/ritonavir was independently associated with reduced odds of hospitalization amongst boosted older community-dwelling Singaporeans during successive waves of Omicron transmission, including Omicron XBB; however, it did not significantly reduce the already low risk of severe COVID-19 in a highly vaccinated population. Liang En Wee, Clin Microbiol Infect 2023;29:1328 (c) 2023 European Society of Clinical Microbiology and Infectious Diseases. Published by Elsevier Ltd. All rights reserved.