Real-world use of nirmatrelvir–ritonavir in outpatients with COVID-19 during the era of omicron variants including BA.4 and BA.5 in Colorado, USA: a retrospective cohort study

Aggarwal et al., The Lancet Infectious Diseases, doi:10.1016/S1473-3099(23)00011-7, Feb 2023

Retrospective 28,167 patients in the USA demonstrating confounding. The large difference shown in Figure 1 at day 0 indicates that the groups are not comparable (32 control hospitalizations versus 0 for paxlovid at day 0) and suggests immortal time bias / confounding by indication. For example, patients in more serious condition may be sent for admission rather than to the pharmacy for paxlovid. ~60% of the difference between the groups at day 28 exists at day 0, and close to 100% by day 4. Confounding may also arise due to inclusion of contraindicated patients in the control group, only partially investigated in sensitivity analysis. Patients that seek out paxlovid may also differ in risk to those that do not.

Authors also note that "The post-hoc sensitivity analysis derived from a cohort of only patients with an observed SARS-CoV-2 postitive [sic] test date was 70% smaller than the primary cohort, and the point estimate for a nirmatrelvir-ritonavir association with reduced 28-day hospitalisation did not reach statistical significance."

Resistance. Variants may be resistant to paxlovid1-6. Use may promote the emergence of variants that weaken host immunity and potentially contribute to long COVID7.

Confounding by contraindication. Hoertel et al. find that over 50% of patients that died had a contraindication for the use of Paxlovid8. Retrospective studies that do not exclude contraindicated patients may significantly overestimate efficacy.

Black box warning. The FDA notes that "severe, life-threatening, and/or fatal adverse reactions due to drug interactions have been reported in patients treated with paxlovid"9.

Kidney and liver injury. Studies show significantly increased risk of acute kidney injury10 and liver injury11.

Important missing comments or errors? Let us know.

1. Zhou et al., Nirmatrelvir-resistant SARS-CoV-2 variants with high fitness in an infectious cell culture system, Science Advances, doi:10.1126/sciadv.add7197.science.org, doi.org.

2. Moghadasi et al., Rapid resistance profiling of SARS-CoV-2 protease inhibitors, npj Antimicrobials and Resistance, doi:10.1038/s44259-023-00009-0.nature.com, doi.org.

3. Jochmans et al., The Substitutions L50F, E166A, and L167F in SARS-CoV-2 3CLpro Are Selected by a Protease Inhibitor In Vitro and Confer Resistance To Nirmatrelvir, mBio, doi:10.1128/mbio.02815-22.asm.org, doi.org.

4. Lopez et al., SARS-CoV-2 Resistance to Small Molecule Inhibitors, Current Clinical Microbiology Reports, doi:10.1007/s40588-024-00229-6.springer.com, doi.org.

5. Zvornicanin et al., Molecular Mechanisms of Drug Resistance and Compensation in SARS-CoV-2 Main Protease: The Interplay Between E166 and L50, bioRxiv, doi:10.1101/2025.01.24.634813.biorxiv.org, doi.org.

6. Vukovikj et al., Impact of SARS-CoV-2 variant mutations on susceptibility to monoclonal antibodies and antiviral drugs: a non-systematic review, April 2022 to October 2024, Eurosurveillance, doi:10.2807/1560-7917.ES.2025.30.10.2400252.eurosurveillance.org, doi.org.

7. Thomas et al., Nirmatrelvir-Resistant Mutations in SARS-CoV-2 Mpro Enhance Host Immune Evasion via Cleavage of NF-κB Essential Modulator, bioRxiv, doi:10.1101/2024.10.18.619137.biorxiv.org, doi.org.

8. Hoertel et al., Prevalence of Contraindications to Nirmatrelvir-Ritonavir Among Hospitalized Patients With COVID-19 at Risk for Progression to Severe Disease, JAMA Network Open, doi:10.1001/jamanetworkopen.2022.42140.jamanetwork.com, doi.org.

9. FDA, Fact sheet for healthcare providers: emergency use authorization for paxlovid, www.fda.gov/media/155050/download.fda.gov.

10. Kamo et al., Association of Antiviral Drugs for the Treatment of COVID-19 With Acute Renal Failure, In Vivo, doi:10.21873/invivo.13637.iiarjournals.org, doi.org.

11. Wang et al., Development and validation of a nomogram to assess the occurrence of liver dysfunction in patients with COVID-19 pneumonia in the ICU, BMC Infectious Diseases, doi:10.1186/s12879-025-10684-1.biomedcentral.com, doi.org.

Aggarwal et al., 9 Feb 2023, retrospective, USA, peer-reviewed, 10 authors.

This Paper Paxlovid All

Real-world use of nirmatrelvir–ritonavir in outpatients with COVID-19 during the era of omicron variants including BA.4 and BA.5 in Colorado, USA: a retrospective cohort study

Dr Neil R Aggarwal, Kyle C Molina, Laurel E Beaty, Tellen D Bennett, PhD Nichole E Carlson, David A Mayer, Jennifer L Peers, Seth Russell, Prof Matthew K Wynia, Adit A Ginde

The Lancet Infectious Diseases, doi:10.1016/s1473-3099(23)00011-7

Background Nirmatrelvir is a protease inhibitor with in-vitro activity against SARS-CoV-2, and ritonavir-boosted nirmatrelvir can reduce the risk of progression to severe COVID-19 among individuals at high risk infected with delta and early omicron variants. However, less is known about the effectiveness of nirmatrelvir-ritonavir during more recent BA.2, BA2.12.1, BA.4, and BA.5 omicron variant surges. We used our real-world data platform to evaluate the effect of nirmatrelvir-ritonavir treatment on 28-day hospitalisation, mortality, and emergency department visits among outpatients with early symptomatic COVID-19 during a SARS-CoV-2 omicron (BA.2, BA2.12.1, BA.4, and BA.5) predominant period in Colorado, USA. Methods We did a propensity-matched, retrospective, observational cohort study of non-hospitalised adult patients infected with SARS-CoV-2 between March 26 and Aug 25, 2022, using records from a statewide health system in Colorado. We obtained data from the electronic health records of University of Colorado Health, the largest health system in Colorado, with 13 hospitals and 141 000 annual hospital admissions, and with numerous ambulatory sites and affiliated pharmacies around the state. Included patients had a positive SARS-CoV-2 test or nirmatrelvir-ritonavir medication order. Exclusion criteria were an order for or administration of other SARS-CoV-2 treatments within 10 days of a positive SARS-CoV-2 test, hospitalisation at the time of positive SARS-CoV-2 test, and positive SARS-CoV-2 test more than 10 days before a nirmatrelvir-ritonavir order. We propensity score matched patients treated with nirmatrelvir-ritonavir with untreated patients. The primary outcome was 28-day all-cause hospitalisation. Findings Among 28 167 patients infected with SARS-CoV-2 between March 26 and Aug 25, 2022, 21 493 met the study inclusion criteria. 9881 patients received treatment with nirmatrelvir-ritonavir and 11 612 were untreated. Nirmatrelvir-ritonavir treatment was associated with reduced 28-day all-cause hospitalisation compared with no antiviral treatment (61 [0•9%] of 7168 patients vs 135 [1•4%] of 9361 patients, adjusted odds ratio (OR) 0•45 [95% CI 0•33-0•62]; p<0•0001). Nirmatrelvir-ritonavir treatment was also associated with reduced 28-day all-cause mortality (two [<0•1%] of 7168 patients vs 15 [0•2%] of 9361 patients; adjusted OR 0•15 [95% CI 0•03-0•50]; p=0•0010). Using subsequent emergency department visits as a surrogate for clinically significant relapse, we observed a decrease after nirmatrelvir-ritonavir treatment (283 [3•9%] of 7168 patients vs 437 [4•7%] of 9361 patients; adjusted OR 0•74 [95% CI 0•63-0•87]; p=0•0002). Interpretation Real-world evidence reported during a BA.2, BA2.12.1, BA.4, and BA.5 omicron surge showed an association between nirmatrelvir-ritonavir treatment and reduced 28-day all-cause hospitalisation, all-cause mortality, and visits to the emergency department. With results that are among the first to..

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