Chloroquine and hydroxychloroquine: Current evidence for their effectiveness in treating COVID-19

March 25, 2020

Kerstin Frie and Kome Gbinigie

Oxford COVID-19 Evidence Service Team
Centre for Evidence-Based Medicine, Nuffield Department of Primary Care Health Sciences
University of Oxford
Correspondence to

Lay Summary by Mandy Payne, Health Watch

Several in vitro studies report antiviral activity of chloroquine and hydroxychloroquine against SARS-CoV-2. In vivo data, although promising, is currently limited to one study with considerable limitations. On the basis of the weak evidence available to date, treatment guidelines have already incorporated the usage of chloroquine/hydroxychloroquine for certain patients with COVID-19.

Further research should address the optimal dose and duration of treatment, and explore side effects and long-term outcomes.

There is a higher risk of side effects in the presence of renal and liver impairment, and there have been isolated reports of COVID-19 disease-causing renal and hepatic injury.

Over twenty in vivo clinical trials have already been registered to test the use of chloroquine and hydroxychloroquine for the treatment of COVID-19.

Contraindications for the use of these drugs must be checked for each individual before treatment. Empirical evidence suggests that hydroxychloroquine has a better safety profile, and it might therefore be preferable to focus research efforts on this less toxic metabolite.

Chloroquine (CQ) was first used as prophylaxis and treatment for malaria. Hydroxychloroquine (HCQ) is a more soluble and less toxic metabolite of chloroquine, which causes less side effects and is, therefore, safer (1-3).
More recently, CQ/HCQ has been used to manage conditions such as systemic lupus erythematosus and rheumatoid arthritis.  CQ/HCQ has been used in the treatment of HIV with mixed results (4).  The ability of  CQ/HCQ to inhibit certain coronaviruses, such as SARS-CoV-1, has been explored with promising results (5, 6). Both drugs are affordable and widely available internationally. With decades of experience administering these drugs, their safety profiles are well-established. It is likely to take many months for novel, specific treatments of COVID-19 to become available. As a result, there has been growing interest in the use of CQ and HCQ as potential treatments in the interim.

Results from In Vitro and In Vivo Research

In Vitro Studies

There is preliminary in vitro evidence of the ability of CQ and HCQ to inhibit SARS-CoV-2 activity. Liu et al  (7) found a similar 50% cytotoxic concentration (CC50 – the concentration which results in 50% cell death) for the two drugs, however, the 50% maximal effective concentration (EC50 – the concentration at which viral RNA increase is inhibited by 50%) was lower for CQ than HCQ, irrespective of the multiplicity of infection (MOI – the ratio of virions to host cells) (7).

By contrast, Yao et al (1) found that HCQ was more potent against SARS-CoV-2 than CQ in vitro (EC50 of 0.72 μM and 5.47 μM, respectively. MOI = 0.01). Wang et al reported in vitro antiviral activity of CQ, with an EC50 of 1.13μM and CC50 >100μM at an MOI of 0.05, and with high selectivity for SARS-CoV-2 rather than host cells (8).

In Vivo Clinical Trials

The empirical evidence for the effectiveness of CQ/HCQ in COVID-19 is currently very limited. First clinical results were reported in a news briefing by the Chinese government in February 2020, revealing that the treatment of over 100 patients with chloroquine phosphate in China had resulted in significant improvements of pneumonia and lung imaging, with reductions in the duration of illness (9). No adverse events were reported. It appears that these findings were a result of combining data from several ongoing trials using a variety of study designs. No empirical data supporting these findings have been published so far.

On the 17th of March 2020, the first clinical trial data were published by Gautret and colleagues in France (2). The researchers conducted an open-label non-randomised controlled trial with 36 patients diagnosed with SARS-CoV-2. Six of these patients were asymptomatic, 22 had upper respiratory tract infection symptoms and eight had lower respiratory tract infection symptoms. Twenty patients were assigned to the treatment group, and received HCQ 200mg three times a day for ten days. The control group received usual care. Six of the patients in the treatment group were also prescribed azithromycin to prevent bacterial superinfection.

The main outcome of the trial was SARS-CoV-2 carriage at Day 6, tested using PCR of SARS-CoV-2 RNA from nasopharyngeal swabs. The results showed that patients in the treatment group were significantly more likely to test negative for the virus on Day 6 than patients in the control group (70% vs 12.5% virologically cured, p<0.001). Moreover, all of the six patients who were treated with a combination of HCQ and azithromycin tested negative on Day 6. The authors argue that this finding speaks to the effectiveness of HCQ and a potential synergistic effect of its combined treatment with azithromycin.

Following the promising results of these first clinical trials, official guidelines recommending the treatment of COVID-19 using CQ/HCQ were published. The National Health Commission of the People’s Republic of China published their recommendation mid-February, suggesting to treat patients with 500mg chloroquine phosphate (300mg for CQ) twice per day, for a maximum of 10 days (10). In Italy, the L. Spallanzani National Institute for the Infectious Disease published their recommendations for treatment on the 17th of March, which included the provision of 400mg of HCQ per day or 500mg CQ per day, in combination with another antiviral agent (11).

While the results of these clinical trials sound promising, there are several limitations.

No data from the Chinese research have been published as yet and the results and conclusions can therefore not be peer-reviewed. The trial by Gautret and colleagues also has some limitations. The authors state that an additional six patients were recruited to the trial, but were lost to follow-up for various reasons. The authors excluded these six patients and did not perform intention-to-treat analysis, which may have introduced bias (12). Moreover, the researchers did not recruit the 48 patients necessary to achieve 85% power, as per their own calculations. With a sample size of 36 the trial was therefore underpowered, which may cause exaggeration of effect sizes and false-positive results (13). With viral PCR status at Day 6 as the primary outcome, the trial lacks medium and long-term follow-up data. The authors report that one patient tested negative for the virus on Day 6, but subsequently tested positive on Day 8. Such recurrences of positive test results demonstrate that long-term data is necessary to properly assess whether CQ/HCQ are effective treatments. Finally, the trial did not randomise patients to the control and treatment group, thus potentially introducing allocation bias. Given these considerable limitations of the current evidence, further clinical trials are urgently needed so that we can better understand the effectiveness of CQ and HCQ for the treatment of COVID-19. Fortunately, more than twenty clinical trials are already registered to this purpose (14). The trials employ a variety of study designs (including open-label randomised, open-label non-randomised, and single-blinded randomised), exclusion criteria, and treatment approaches (only CQ/only HCQ/either in combination with another drug) (15). It remains to see whether the results of these trials support further usage of CQ/HCQ for the treatment of COVID-19.

Biological Mechanism of Chloroquine

A number of potential mechanisms of action of CQ/HCQ against SARS-CoV-2 have been postulated. The virus is believed to enter cells by binding to a cell surface enzyme called angiotensin-converting enzyme 2 (ACE2) (16). ACE2 expression is also believed to be upregulated by infection with SARS-CoV-2 (17). Chloroquine may reduce glycosylation of  ACE2, thereby preventing SARS-CoV-2 from effectively binding to host cells (18). Furthermore, Savarino et al (19) hypothesise that CQ might block the production of pro-inflammatory cytokines (such as interleukin-6), thereby blocking the pathway that subsequently leads to acute respiratory distress syndrome (ARDS). Some viruses enter host cells through endocytosis; the virus is transported within the host cell in a cell-membrane derived vesicle called an endosome, within which the virus can replicate (19). When the endosome fuses with the acidic intracellular lysosome, this leads to rupture of the endosome with the release of the viral contents (19). Chloroquine has been found to accumulate in lysosomes, interfering with this process (20). Chloroquine is also believed to raise the pH level of the endosome, which may interfere with virus entry and/or exit from host cells (6).

Side Effects of Chloroquine

Both CQ and HCQ have been in clinical use for several years, thus their safety profile is well established (18). Gastrointestinal upset has been reported with HCQ intake (21). Retinal toxicity has been described with long-term use of CQ and HCQ (22, 23), and may also be related to over-dosage of these medications (23, 24). Isolated reports of cardiomyopathy (25) and heart rhythm disturbances (26) caused by treatment with CQ have been reported. Chloroquine should be avoided in patients with porphyria (27). Both CQ and HCQ are metabolised in the liver with renal excretion of some metabolites, hence they should be prescribed with care in people with liver or renal failure (27, 28). In a letter to the editor, Risambaf et al (27) raise concerns about reports of COVID-19 causing liver and renal impairment, which may increase the risk of toxicity of CQ/HCQ when it is used to treat COVID-19.

CHLOROQUINE | Drug | BNF content published by NICE

Disclaimer:  the article has not been peer-reviewed; it should not replace individual clinical judgement and the sources cited should be checked. The views expressed in this commentary represent the views of the authors and not necessarily those of the host institution, the NHS, the NIHR, or the Department of Health and Social Care. The views are not a substitute for professional medical advice.

Kerstin Frie* is a postdoctoral researcher in the Health Behaviours team of the Nuffield Department of Primary Care Health Sciences, University of Oxford

Kome Gbinigie* is a General Practitioner and doctoral researcher based at the Nuffield Department of Primary Care Health Sciences, University of Oxford

*Co-first authors

We searched Pubmed and Google Scholar on 21st March 2020 using the search terms *chloroquine, coronavirus, SARS-Cov-2, 2019-NCov, and COVID-19. We screened titles and abstracts and included in vitro and in vivo studies of CQ/HCQ for the treatment of SARS-CoV-2. In addition, we included reviews of the existing literature on this topic. We provide a narrative summary of the current literature.


1)    Yao, X., Ye, F., Zhang, M., Cui, C., Huang, B., Niu, P., Liu, X., Zhao, L., Dong, E., Song, C. and Zhan, S., 2020. In vitro antiviral activity and projection of optimized dosing design of hydroxychloroquine for the treatment of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Clinical Infectious Diseases.
2)    Gautret, P., Lagier, J.C., Parola, P., Meddeb, L., Mailhe, M., Doudier, B., Courjon, J., Giordanengo, V., Vieira, V.E., Dupont, H.T. and Honoré, S., 2020. Hydroxychloroquine and azithromycin as a treatment of COVID-19: results of an open-label non-randomized clinical trial. International Journal of Antimicrobial Agents, p.105949.
3)    Sahraei, Z., Shabani, M., Shokouhi, S. and Saffaei, A., 2020. Aminoquinolines Against Coronavirus Disease 2019 (COVID-19): Chloroquine or Hydroxychloroquine. International Journal of Antimicrobial Agents, p.105945.
4)    Chauhan, A. and Tikoo, A., 2015. The enigma of the clandestine association between chloroquine and HIV‐1 infection. HIV medicine, 16(10), pp.585-590.
5)    Keyaerts, E., Li, S., Vijgen, L., Rysman, E., Verbeeck, J., Van Ranst, M. and Maes, P., 2009. Antiviral activity of chloroquine against human coronavirus OC43 infection in newborn mice. Antimicrobial agents and chemotherapy, 53(8), pp.3416-3421.
6)    Vincent, M.J., Bergeron, E., Benjannet, S., Erickson, B.R., Rollin, P.E., Ksiazek, T.G., Seidah, N.G. and Nichol, S.T., 2005. Chloroquine is a potent inhibitor of SARS coronavirus infection and spread. Virology journal, 2(1), p.69.
7)    Liu, J., Cao, R., Xu, M., Wang, X., Zhang, H., Hu, H., Li, Y., Hu, Z., Zhong, W. and Wang, M., 2020. Hydroxychloroquine, a less toxic derivative of chloroquine, is effective in inhibiting SARS-CoV-2 infection in vitro. Cell Discovery, 6(1), pp.1-4.
8)    Wang, M., Cao, R., Zhang, L., Yang, X., Liu, J., Xu, M., Shi, Z., Hu, Z., Zhong, W. and Xiao, G., 2020. Remdesivir and chloroquine effectively inhibit the recently emerged novel coronavirus (2019-nCoV) in vitro. Cell research, 30(3), pp.269-271.
9)    Gao, J., Tian, Z. and Yang, X., 2020. Breakthrough: Chloroquine phosphate has shown apparent efficacy in treatment of COVID-19 associated pneumonia in clinical studies. BioScience Trends.
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12) Ranganathan, P., Pramesh, C.S. and Aggarwal, R., 2016. Common pitfalls in statistical analysis: Intention-to-treat versus per-protocol analysis. Perspectives in clinical research, 7(3), p.144.
13) Dumas-Mallet, E., Button, K.S., Boraud, T., Gonon, F. and Munafò, M.R., 2017. Low statistical power in biomedical science: a review of three human research domains. Royal Society open science, 4(2), p.160254.
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16) Zhou, P., Yang, X.L., Wang, X.G., Hu, B., Zhang, L., Zhang, W., Si, H.R., Zhu, Y., Li, B., Huang, C.L. and Chen, H.D., 2020. A pneumonia outbreak associated with a new coronavirus of probable bat origin. Nature, pp.1-4.
17) Wang, P.H., 2020. Increasing Host Cellular Receptor—Angiotensin-Converting Enzyme 2 (ACE2) Expression by Coronavirus may Facilitate 2019-nCoV Infection. bioRxiv.
18) Devaux, C.A., Rolain, J.M., Colson, P. and Raoult, D., 2020. New insights on the antiviral effects of chloroquine against coronavirus: what to expect for COVID-19?. International Journal of Antimicrobial Agents, p.105938.
19) Savarino, A., Boelaert, J.R., Cassone, A., Majori, G. and Cauda, R., 2003. Effects of chloroquine on viral infections: an old drug against today’s diseases. The Lancet infectious diseases, 3(11), pp.722-727.
20) Golden EB, Cho HY, Hofman FM, Louie SG, Schonthal AH, Chen TC. Quinoline-based antimalarial drugs: a novel class of autophagy inhibitors. Neurosurg Focus. 2015;38(3):E12.
21) Srinivasa, A., Tosounidou, S. and Gordon, C., 2017. Increased incidence of gastrointestinal side effects in patients taking hydroxychloroquine: a brand-related issue?. The Journal of rheumatology, 44(3), pp.398-398.
22) Mavrikakis, M., Papazoglou, S., Sfikakis, P.P., Vaiopoulos, G. and Rougas, K., 1996. Retinal toxicity in long term hydroxychloroquine treatment. Annals of the rheumatic diseases, 55(3), pp.187-189.
23) Easterbrook, M., 1993, October. The ocular safety of hydroxychloroquine. In Seminars in arthritis and rheumatism (Vol. 23, No. 2, pp. 62-67). WB Saunders.
24) Browning, D.J., 2002. Hydroxychloroquine and chloroquine retinopathy: screening for drug toxicity. American journal of ophthalmology, 133(5), pp.649-656.
25) Cubero, G.I., Reguero, J.R. and Ortega, J.R., 1993. Restrictive cardiomyopathy caused by chloroquine. Heart, 69(5), pp.451-452.
26) Costedoat-Chalumeau, N., Hulot, J.S., Amoura, Z., Leroux, G., Lechat, P., Funck-Brentano, C. and Piette, J.C., 2007. Heart conduction disorders related to antimalarials toxicity: an analysis of electrocardiograms in 85 patients treated with hydroxychloroquine for connective tissue diseases. Rheumatology, 46(5), pp.808-810.
27) Rismanbaf, A. and Zarei, S., 2020. Liver and Kidney Injuries in COVID-19 and Their Effects on Drug Therapy; a Letter to Editor. Archives of Academic Emergency Medicine, 8(1), p.17.
28) Wang, Y. and Zhu, L.Q., 2020. Pharmaceutical care recommendations for antiviral treatments in children with coronavirus disease 2019. World Journal of Pediatrics, pp.1-4.