Background: Oral nirmatrelvir/ritonavir (Paxlovid) aims to avoid severe COVID-19 in asymptomatic people or those with mild symptoms, thereby decreasing hospitalization and death. It remains to be evaluated for which indications and patient populations the drug is suitable.

Objectives: To assess the efficacy and safety of nirmatrelvir/ritonavir plus standard of care (SoC) compared to SoC with or without placebo, or any other intervention for treating COVID-19 or preventing SARS-CoV-2 infection. To explore equity aspects in subgroup analyses. To keep up to date with the evolving evidence base using a living systematic review (LSR) approach and make new relevant studies available to readers in-between publication of review updates.

Search methods: We searched the Cochrane COVID-19 Study Register, Scopus, and World Health Organization COVID-19 Research Database, identifying completed and ongoing studies without language restrictions and incorporating studies up to 15 May 2023. This is a LSR. We conduct update searches every two months and make them publicly available on the open science framework (OSF) platform.

Selection criteria: We included randomized controlled trials (RCTs) comparing nirmatrelvir/ritonavir plus SoC to SoC with or without placebo, or any other intervention for treatment of people with confirmed COVID-19 diagnosis, irrespective of disease severity or treatment setting, and for prevention of SARS-CoV-2 infection. We screened all studies for research integrity. Studies were ineligible if they had been retracted, or if they were not prospectively registered including appropriate ethics approval.

Data collection and analysis: We followed standard Cochrane methodology and used the Cochrane RoB 2 tool. We rated the certainty of evidence using the GRADE approach for the following outcomes: 1. to treat outpatients with mild COVID-19; 2. to treat inpatients with moderate to severe COVID-19: mortality, clinical worsening or improvement, quality of life, (serious) adverse events, and viral clearance; 3. to prevent SARS-CoV-2 infection in postexposure prophylaxis (PEP); and 4. pre-exposure prophylaxis (PrEP) scenarios: SARS-CoV-2 infection, development of COVID-19 symptoms, mortality, admission to hospital, quality of life, and (serious) adverse events. We explored inequity by subgroup analysis for elderly people, socially-disadvantaged people with comorbidities, populations from low-income countries and low- to middle-income countries, and people from different ethnic and racial backgrounds.

Main results: As of 15 May 2023, we included two RCTs with 2510 participants with mild and mild to moderate symptomatic COVID-19 in outpatient and inpatient settings comparing nirmatrelvir/ritonavir plus SoC to SoC with or without placebo. All trial participants were without previous confirmed SARS-CoV-2 infection and at high risk for progression to severe disease. Randomization coincided with the Delta wave for outpatients and Omicron wave for inpatients. Outpatient trial participants and 73% of inpatients were unvaccinated. Symptom onset in outpatients was no more than five days before randomisation and prior or concomitant therapies including medications highly dependent on CYP3A4 were not allowed. We excluded two studies due to concerns with research integrity. We identified 13 ongoing studies. Three studies are currently awaiting classification. Nirmatrelvir/ritonavir for treating people with asymptomatic or mild COVID-19 in outpatient settings Nirmatrelvir/ritonavir plus SoC compared to SoC plus placebo may reduce all-cause mortality at 28 days (risk ratio (RR) 0.04, 95% confidence interval (CI) 0.00 to 0.68; 1 study, 2224 participants; low-certainty evidence) and admission to hospital or death within 28 days (RR 0.13, 95% CI 0.07 to 0.27; 1 study, 2224 participants; low-certainty evidence). Nirmatrelvir/ritonavir plus SoC may reduce serious adverse events during the study period compared to SoC plus placebo (RR 0.24, 95% CI 0.15 to 0.41; 1 study, 2224 participants; low-certainty evidence). Nirmatrelvir/ritonavir plus SoC probably has little or no effect on treatment-emergent adverse events (RR 0.95, 95% CI 0.82 to 1.10; 1 study, 2224 participants; moderate-certainty evidence), and probably increases treatment-related adverse events such as dysgeusia and diarrhoea during the study period compared to SoC plus placebo (RR 2.06, 95% CI 1.44 to 2.95; 1 study, 2224 participants; moderate-certainty evidence). Nirmatrelvir/ritonavir plus SoC probably decreases discontinuation of study drug due to adverse events compared to SoC plus placebo (RR 0.49, 95% CI 0.30 to 0.80; 1 study, 2224 participants; moderate-certainty evidence). No studies reported improvement of clinical status, quality of life, or viral clearance. Nirmatrelvir/ritonavir for treating people with moderate to severe COVID-19 in inpatient settings We are uncertain whether nirmatrelvir/ritonavir plus SoC compared to SoC reduces all-cause mortality at 28 days (RR 0.63, 95% CI 0.21 to 1.86; 1 study, 264 participants; very low-certainty evidence), or increases viral clearance at seven days (RR 1.06, 95% CI 0.71 to 1.58; 1 study, 264 participants; very low-certainty evidence) and 14 days (RR 1.05, 95% CI 0.92 to 1.20; 1 study, 264 participants; very low-certainty evidence). No studies reported improvement or worsening of clinical status and quality of life. We did not include data for safety outcomes due to insufficient and inconsistent information. Subgroup analyses for equity For outpatients, the outcome 'admission to hospital or death' was investigated for equity regarding age (less than 65 years versus 65 years or greater) and ethnicity. There were no subgroup differences for age or ethnicity. For inpatients, the outcome 'all-cause mortality' was investigated for equity regarding age (65 years or less versus greater than 65 years). There was no difference between subgroups of age. No further equity-related subgroups were reported, and no subgroups were reported for other outcomes. Nirmatrelvir/ritonavir for preventing SARS-CoV-2 infection (PrEP and PEP) No studies available.

Authors' conclusions: Low-certainty evidence suggests nirmatrelvir/ritonavir reduces the risk of all-cause mortality and hospital admission or death in high-risk, unvaccinated COVID-19 outpatients infected with the Delta variant of SARS-CoV-2. There is low- to moderate-certainty evidence of the safety of nirmatrelvir/ritonavir. Very low-certainty evidence exists regarding the effects of nirmatrelvir/ritonavir on all-cause mortality and viral clearance in mildly to moderately affected, mostly unvaccinated COVID-19 inpatients infected with the Omicron variant of SARS-CoV-2. Insufficient and inconsistent information prevents the assessment of safety outcomes. No reliable differences in effect size and direction were found regarding equity aspects. There is no available evidence supporting the use of nirmatrelvir/ritonavir for preventing SARS-CoV-2 infection. We are continually updating our search and making search results available on the OSF platform.

SR: none.

MIM: none.

RK: is a CIDG editorial team member, but was not involved in the editorial processing of this review. She has no known conflicts of interest to declare.

MP: none.

IG: none.

PK: none.

PM: none.

NS: none.

SW: none.