Ensitrelvir and Nirmatrelvir: Comparing Viral Clearance in COVID-19

While COVID-19 generally presents as a milder disease today, it continues to cause substantial morbidity, particularly among vulnerable populations. The need for effective, well-tolerated oral antivirals remains critical, especially as new variants emerge. A recent phase 2 trial published in The Lancet Infectious Diseases offers a head-to-head comparison of two leading options: ensitrelvir and ritonavir-boosted nirmatrelvir.

Evaluating Antiviral Efficacy in Early Infection
This open-label, adaptive pharmacometric platform trial enrolled low-risk adult outpatients (aged 18-60 years) with early symptomatic COVID-19 (less than 4 days of symptoms) across clinics in Thailand and Laos. Researchers intentionally selected patients with high viral burdens to optimize the comparative assessment of the drugs' antiviral effects.

Patients were randomly assigned to receive either oral ensitrelvir, oral ritonavir-boosted nirmatrelvir, or no study drug for 5 days. The primary endpoint focused on the oropharyngeal SARS-CoV-2 viral clearance rate assessed between day 0 and day 5. This approach allowed for a precise measurement of how quickly each treatment reduced the viral load in the upper respiratory tract.

Viral Clearance and Rebound Dynamics
Both antivirals demonstrated significant efficacy in accelerating viral clearance compared to no treatment, though ritonavir-boosted nirmatrelvir showed a slight edge in speed.

Key findings regarding viral clearance half-lives include:

• Ritonavir-boosted nirmatrelvir: 5.2 hours (IQR 3.8-6.6)
• Ensitrelvir: 5.9 hours (IQR 4.0-8.6)
• No study drug: 11.6 hours (IQR 8.1-14.5)

Viral clearance following ensitrelvir was 82% faster than no study drug (95% credible interval 61-104) and 16% slower than ritonavir-boosted nirmatrelvir (95% credible interval 5-25).

Interestingly, the rates of viral rebound were similar among groups, occurring in 7% (15 of 207) of patients receiving nirmatrelvir, 5% (10 of 202) of those receiving ensitrelvir, and 7% (13 of 191) of the group with no study drug. This suggests that rebound is a feature of the disease course rather than a specific drug effect, providing reassurance to clinicians concerned about post-treatment viral resurgence.

Tolerability Separates the Options
While efficacy was comparable, the safety profiles revealed a distinct difference in tolerability, specifically regarding dysgeusia, a bitter or metallic taste. This side effect was reported by 26% (52 of 201) of patients in the ritonavir-boosted nirmatrelvir group, compared to just 1% (two of 197) in the ensitrelvir group and 1% (one of 191) in the no study drug group. This stark contrast in side effects could impact patient adherence and preference in real-world settings.

Contextualizing the Findings
The study's design, focusing on low-risk adults to maximize viral kinetic measurements, means that protection against severe disease could not be assessed. Evaluating clinical recovery would require a substantially larger cohort. Additionally, the open-label nature of the trial could introduce bias, though the objective virological endpoints mitigate this risk. The findings are also limited by the specific demographic and geographic characteristics of the study population, which may not fully represent the global diversity of COVID-19 patients.

For clinicians, these findings confirm that ensitrelvir is an effective alternative to currently available antivirals. Its comparable efficacy in viral clearance and significantly lower rate of dysgeusia may make it a preferable option for patients who struggle with the side effects of ritonavir-boosted nirmatrelvir, expanding the toolkit for managing early COVID-19 infection. As the virus continues to evolve, having multiple effective and well-tolerated treatment options will be essential for comprehensive patient care.

Reference: 
Schilling WHK, Jittamala P, Wongnak P, et al. Antiviral efficacy of oral ensitrelvir versus oral ritonavir-boosted nirmatrelvir in COVID-19 (PLATCOV): an open-label, phase 2, randomised, controlled, adaptive trial. Lancet Infect Dis. 2026;26(2):139-147. doi:10.1016/S1473-3099(25)00482-7