Prophylaxis against covid-19: living systematic review and network meta-analysis

BMJ. 2021 Apr 26:373:n949. doi: 10.1136/bmj.n949.


Updates: This is the second version (first update) of the living systematic review, replacing the previous version (available as a data supplement). When citing this paper please consider adding the version number and date of access for clarity.

Objective: To determine and compare the effects of drug prophylaxis on severe acute respiratory syndrome coronavirus virus 2 (SARS-CoV-2) infection and coronavirus disease 2019 (covid-19).

Design: Living systematic review and network meta-analysis (NMA).

Data sources: World Health Organization covid-19 database, a comprehensive multilingual source of global covid-19 literature to 4 March 2022.

Study selection: Randomised trials in which people at risk of covid-19 were allocated to prophylaxis or no prophylaxis (standard care or placebo). Pairs of reviewers independently screened potentially eligible articles.

Methods: After duplicate data abstraction, we conducted random-effects bayesian network meta-analysis. We assessed risk of bias of the included studies using a modification of the Cochrane risk of bias 2.0 tool and assessed the certainty of the evidence using the grading of recommendations assessment, development and evaluation (GRADE) approach.

Results: The second iteration of this living NMA includes 32 randomised trials which enrolled 25 147 participants and addressed 21 different prophylactic drugs; adding 21 trials (66%), 18 162 participants (75%) and 16 (76%) prophylactic drugs. Of the 16 prophylactic drugs analysed, none provided convincing evidence of a reduction in the risk of laboratory confirmed SARS-CoV-2 infection. For admission to hospital and mortality outcomes, no prophylactic drug proved different than standard care or placebo. Hydroxychloroquine and vitamin C combined with zinc probably increase the risk of adverse effects leading to drug discontinuation—risk difference for hydroxychloroquine (RD) 6 more per 1000 (95% credible interval (CrI) 2 more to 10 more); for vitamin C combined with zinc, RD 69 more per 1000 (47 more to 90 more), moderate certainty evidence.

Conclusions: Much of the evidence remains very low certainty and we therefore anticipate future studies evaluating drugs for prophylaxis may change the results for SARS-CoV-2 infection, admission to hospital and mortality outcomes. Both hydroxychloroquine and vitamin C combined with zinc probably increase adverse effects.

Systematic review registration: This review was not registered. The protocol established a priori is included as a supplement.

Funding: This study was supported by the Canadian Institutes of Health Research (grant CIHR-IRSC:0579001321).

Publication types

  • Meta-Analysis
  • Research Support, Non-U.S. Gov't
  • Systematic Review

MeSH terms

  • Anti-Infective Agents / pharmacology
  • COVID-19* / prevention & control
  • Carrageenan / pharmacology*
  • Chemoprevention / methods
  • Chemoprevention / statistics & numerical data
  • Global Health / statistics & numerical data*
  • Humans
  • Hydroxychloroquine / pharmacology*
  • Ivermectin / pharmacology*
  • SARS-CoV-2
  • Treatment Outcome
  • Uncertainty


  • Anti-Infective Agents
  • Hydroxychloroquine
  • Ivermectin
  • Carrageenan