The absence of efficient therapeutics for COVID-19 has brought much attention to evaluation of repurposing drugs. Hydroxychloroquine (HC) is an antimalarial drug that affects endosomal function and blocks autophagosome-lysosome fusion (1). Since coronaviruses use the endolysosomal pathway to enter the cell before uncoating, HC has been shown to inhibit SARS-CoV-2 replication in cellular models. The use of HC in the treatment and/or prevention of COVID-19 has been clouded in controversy and contention. Partly, this is because it has become somewhat of a political football, with one side relentlessly touting its value and with the other side just as adamantly claiming that it has no value and is indeed harmful with various side effects. The second reason is the variety of conditions used in the reported tests. The third reason is that there seems to be a great number of risk factors or co-morbidities, and this can result in vastly different results due to differences in demographics. Results from anecdotal versus retrospective versus observational versus controlled versus randomized/blind trials have varied widely. There is also the question of whether it is necessary to include azithromycin (AZM) or other antibiotics, and whether to use zinc (Zn). AZM, a widely used broad-spectrum antibiotic, also blocks autophagosome clearance in human cells and replication of the Zika and influenza viruses in human cells in vitro. A subset of HC advocates also thinks that inclusion of zinc is critical, and that the primary function of HC is to help Zn enter the cells. The related drug, chloroquine, has been reported to act as a Zn ionophore(2), and Zn inhibits RNA chain elongation by SARS-CoV-1 RNA-dependent RNA polymerase in vitro(3). The authors also showed inhibition of viral infection in Vero (African green monkey kidney) cells. However, it has recently been shown that SARS-CoV-2 enters Vero cells by an alternate pathway that is not inhibited by HC. Due to the use of a different pathway for viral entry, HC treatment in human lung cells did not significantly inhibit viral infection (4). These data suggest that if HC has a beneficial effect on treatment of COVID-19, it is either because it affects the host response to the virus or it affects a step unrelated to viral entry.
For all these reasons, studies that include randomized control groups are inherently far more reliable than observational or retrospective studies, in which data from other groups are used as a control. Thus, without randomized trials, it is difficult to draw firm conclusions. Double-blind randomized studies are the most reliable because they eliminate potential placebo effects, which can be substantial. Randomized trials are, however, more expensive and take more time to carry out, so there are far fewer of them than of observational or retrospective studies. Sorting out all these issues is difficult, but let’s give it a try. The sources we will consider will include PubMed (comprised of peer-reviewed published studies), medRvix (preprints not yet peer-reviewed), and the internet in general (with the uncertainty that entails), including press releases and editorials, perhaps the least reliable source. We will not consider in vitro antiviral studies, as these have been performed largely with Vero cells, which as discussed above, are not useful for SARS-CoV-2 studies and HC.
Reviewing the Literature
Let’s first consider results from several randomized double-blind trials with HC. One such study looked at people with documented occupational or household exposure to individuals with confirmed COVID-19 to observe whether HC was effective prophylactically(5). Treatment with HC was for 5 days within 4 days of exposure, and both groups had somewhat more than 400 subjects. There were slightly but insignificantly fewer cases of COVID-19 in the HC arm, as judged by either a positive PCR test or development of symptoms. There was one hospitalization in each group, and no deaths occurred. HC was not associated with any serious side effects. Another randomized double-blind trial looked at patients with early COVID-19(6). Subjects were treated with HC or placebo for 5 days. There was no difference in symptom severity over 14 days between the groups. There were 4 hospitalizations and one death in the HC group compared to 8 hospitalizations and one death in the placebo group; this did not reach statistical significance. Another small randomized double blind study compared two groups (n=40) of COVID-19 patients treated with either high or lower doses of chloroquine for 10 days(7). All were taking AZM. Although there was more mortality in the high dose group, neither differed significantly from what would be expected from an untreated group of similar patients. We note that not having an internal untreated control weakens this study. Taken together, these trials strongly suggest that there is not a significant therapeutic benefit of HC, although they do not completely rule it out. In fact, the authors in these studies generally do not prove a lack of potential benefit of HC, but suggest further similar studies are needed for confirmation. The data also uniformly suggest that HC is reasonably safe.
There have also been several well controlled large randomized open-label (not blinded) trials. Among these was one testing hospital patients with COVID-19 and requiring either no supplemental O2 or <4 l/min O2. More than 500 patients were randomly assigned to 3 groups and treated for 7 days by standard of care (SOC), HC+SOC, and HC+AZM+SOC(8). Clinical status was evaluated at 15 days. There was no improvement in either group receiving HC relative to the SOC group. Elevated Q-T heart intervals and elevated liver enzymes were more prevalent in these two groups, but these were not considered serious. Another randomized open label study looked at patients with mild to moderate COVID-19 and treated them either with SOC (n=75) or SOC+HC (n=75)(9). As judged by conversion to negativity for SARS-CoV-2, judged by RT-PCR of nasal swabs, and by alleviation of symptoms, there were no differences in outcomes by day 28. Some negative events were attributed to HC, primarily diarrhea. Another randomized open label trial researched 293 patients with COVID-19 who were not hospitalized and were symptomatic for fewer than 5 days. Patients were treated either with HC for one week (n=136) or without HC (n=157). As judged by viral RNA loads, hospitalization and time to resolution of symptoms, there were no significant differences. We note that the number of hospitalizations was too small to reach significance. Yet another randomized non-blind trial looked at patients with mild COVID-19. Patients were treated with HC for 6 days or not treated and viral RNA loads, and resolution of symptoms at day=28 were the end points. There were about 140 in each arm. No differences in outcomes were noted, and no adverse effects were reported. In contrast, another small randomized study treated patients with HC (including pneumonia) for 5 days (no AZM or Zn) or SOC (31 subjects per group) and showed a significant improvement in cough and fever resolution. The reasons for the incongruent results are unclear.
Next in degree of reliability are observational or retrospective studies, as they generally rely on statistics from patient groups that are distinct from the groups under study and may differ by genetic factors or by comorbidities. They may also differ by prior exposure to other coronaviruses or by prevalence of recent or childhood administration of vaccines such as BCG, polio or measles, all of which may affect results from SARS-CoV-2 exposure or infection because of immune memory and cross-reactivity. In general, more observational studies report a protective effect than do those that find no benefit of HC. The quality of these studies varies widely. As with randomized studies, in general, the larger the study, the more likely the results are to be accurate. We will concentrate on several of the best and largest of these; they are reasonably representative of the multitude of observational studies.
A large (~3,700 patients) retrospective/observational study reported a benefit of HC plus AZM treatment for 5 days (n=3,119) compared to groups treated with HC alone, AZM alone or SOC (n=619), termed “others.” Better outcomes compared to “others” were reported for mortality, hospitalization, duration of viral RNA shedding, and several other clinical parameters. It is not clear, however, as to how patients were assigned to treatment groups, and patients not receiving HC seemed sicker, judged by prevalence of cancer or hypertension. It should also be noted that there was no report of Zn usage. A large multi-center (~2,500 patients) study treated patients with HC alone, AZM alone, HC+AZM, or neither(10) and looked at mortality rates as the primary outcome. Taking into accounts various clinical parameters from each group, the authors concluded that HC provided a hazard ratio reduction of 66% and HC+AZM a reduction of 71%. This may not account for all possible confounding factors. For example, it is not clear why the untreated patients were not given HC or whether they were sicker at admission. However, the patients not receiving HC were on average 5 years older and had a higher incidence of cancers, which seem to be serious confounding factors. It is also not totally clear how the decision was made to not administer HC. It should also be noted that steroids were administered to patients receiving HC as adjunct therapy at a far greater rate than those who did not; this is another potential substantial confounding effect. Duration of symptoms before admission were not available. Indeed, the authors caution that randomized prospective trials are needed and that their results should be interpreted with caution. There were no reported major safety issues. Again, we note there was no reported use of Zn.
Another observational study supporting a positive effect of HC looked at ~1,600 patients who were treated with one of 16 different treatments, with death or intubation as an endpoint. The only favorable treatments were HC (with a hazard ratio of 0.83) and predisone (HR=0.85). Dexamethasone treatment resulted in slightly worse outcomes. The same caveats mentioned for the previous studies are applicable to this study.
Another retrospective study with 335 subjects were apparently drawn from subjects treated by Dr. V Zelenko, who at one point claimed to have treated 1500 people with COVID-19 successfully, although this was based on symptoms rather than confirmed tests. It seems likely, based on the size of the cohort tested (335), that not all of the original patients were positive. (For comments on this point, please read here). 141 patients were treated with HC+AZM+Zn. Oddly, public reference data on 377 patients from the same community were used as a control rather than patients in the cohort who were untreated, and no clinical or demographic data are available for this group. Although there were significantly fewer hospitalizations and deaths and no serious adverse effects in the treated group, the authors state that no conclusions can be made on efficacy or safety. This study appears to be where the idea that Zn was critical originated.
There are also observational/retrospective studies that show no effect or HC±AZM. One looked at 226 patients with mild to moderate COVID-19 who were either treated or received SOC. No benefit was observed as judged by viral clearance, hospital stay, or duration of symptoms. It should be pointed out the treated group was relatively small (N=31). Salvarani et al. (11) looked at 4,400 people who were being treated with antimalarial drugs (HC or chloroquine) and compared them with the general population in the same geographic areas. There were no significant differences in rates of diagnoses of COVID-19 nor of positive tests for SARS-CoV-2. Geleris et al. (12) studied 1376 hospitalized COVID-19 patients, of whom 811 were treated within 48 hrs with a 5 day course of HC; the remainder were given SOC. There was no significant difference in intubation or death. As with non-randomized studies, differences in characteristics of cohorts can matter greatly. The HC-treated cohort, for example, was older and had more hypertension than the reference cohort. The Geleris study did use propensity score matching to account for these differences. The authors, however, caution that randomized trials are needed to conclude HC has no value for COVID-19.
There are many more studies than we have described, and it would not be feasible to mention them all. We have, however, tried to be as representative as possible. As we can see, there are considerable differences in outcomes reported. Looking at the data in general, it can be seen that the randomized trials have the most agreement, and most (but not all) conclude that there is no significant benefit to HC treatment with or without AZM. Concerning the observational/retrospective studies, there are more positive than negative reports. Why is this? As discussed, these are inherently less reliable than randomized trials. It may thus be easier to get positive results in an observational trial, especially if the study is small or not well controlled by cohort. Also, it is likely that positive studies in general (not simply COVID-19 and HC) are more readily publishable than negative ones. Although Zn is claimed to be critical, it should be noted that many of the positive reports do not use Zn, which would seem to negate this idea. The placebo effect may play a role.
Our literature review has generated somewhat contradictory findings, but strongly suggests that HC is not beneficial for COVID-19 treatment. There are positive data as well, but these come almost entirely from observational/retrospective studies, with their attendant uncertainties. However, it cannot be excluded that HC is of great benefit to an as of yet uncharacterized subset of patients. As to safety issues, there appears to be general agreement that side effects are relatively minor, and HC does not appear to be very dangerous, as would be expected from long experience with HC in connection to malaria and rheumatoid arthritis. There is far more heat than light in the public discourse on HC (internet, editorials, press releases, etc.) It will be difficult to prove benefit or safety to those whose political views inform their judgements, but we believe we have provided a balanced analysis. In addition, the U.S. Food and Drug Administration (FDA) cautions against use of hydroxychloroquine or chloroquine for COVID-19 outside of the hospital setting or a clinical trial due to risk of heart rhythm problems (13). We realize many of the studies represent something akin to battlefield medicine, are meant to save lives, and should generally be applauded, but until more blinded randomized trials are reported, it is difficult for us to ascribe value or harm to HC.
- KIPLIN GUY, ROBERT S. DIPAOLA, FRANK ROMANELLI, REBECCA E. DUTCH. Rapid repurposing of drugs for COVID-19. SCIENCE22 MAY 2020 : 829-830.
- Xue et al., Chloroquine is a zinc ionophore. PLoS One 9, e109180 (2014).
- J. te Velthuis et al., Zn(2+) inhibits coronavirus and arterivirus RNA polymerase activity in vitro and zinc ionophores block the replication of these viruses in cell culture. PLoS Pathog 6, e1001176 (2010).
- Hoffmann et al., Chloroquine does not inhibit infection of human lung cells with SARS-CoV-2. Nature, (2020).
- R. Boulware et al., A Randomized Trial of Hydroxychloroquine as Postexposure Prophylaxis for Covid-19. N Engl J Med 383, 517-525 (2020).
- P. Skipper et al., Hydroxychloroquine in Nonhospitalized Adults With Early COVID-19: A Randomized Trial. Ann Intern Med, (2020).
- G. S. Borba et al., Effect of High vs Low Doses of Chloroquine Diphosphate as Adjunctive Therapy for Patients Hospitalized With Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Infection: A Randomized Clinical Trial. JAMA Netw Open 3, e208857 (2020).
- B. Cavalcanti et al., Hydroxychloroquine with or without Azithromycin in Mild-to-Moderate Covid-19. N Engl J Med, (2020).
- Tang et al., Hydroxychloroquine in patients with mainly mild to moderate coronavirus disease 2019: open label, randomised controlled trial. BMJ 369, m1849 (2020).
- Arshad et al., Treatment with hydroxychloroquine, azithromycin, and combination in patients hospitalized with COVID-19. Int J Infect Dis 97, 396-403 (2020).
- Salvarani et al., Susceptibility to COVID-19 in patients treated with antimalarials: a population based study in Emilia-Romagna, Northern Italy. Arthritis Rheumatol, (2020).
- Geleris et al., Observational Study of Hydroxychloroquine in Hospitalized Patients with Covid-19. N Engl J Med 382, 2411-2418 (2020).
- 2020. https://www.fda.gov/drugs/drug-safety-and-availability/fda-cautions-against-use-hydroxychloroquine-or-chloroquine-covid-19-outside-hospital-setting-or