Convalescent Plasma for COVID-19: Brief Review of Current Evidence

Words by Sargam Raja Mahat, PharmD; Digbijay Raja Mahat, PhD; Yug Bijay Mahat, MD; and Subhas Bijay Mahat, MBBS

Introduction
Our world has been battling against Severe Acute Respiratory Syndrome Coronavirus – 2 (SARS-CoV-2), the cause of Coronavirus Disease – 19 (COVID-19), as this pandemic has spurred a global health crisis. According to Johns Hopkins University Resource Center, the number of deaths attributed to COVID-19 worldwide at the time of this writing is around 1.27 million,¹ and it continues to rise as doctors and scientists still scramble to find proven options for a vaccine or antivirals. Recently, passive antibody administration through transfusion of convalescent plasma (CP) has raised curiosity, as it may offer some degree of immediate immunity to susceptible individuals exposed to the pathogen or as a therapy for those who develop COVID-19. The therapeutic potential of CP, in the current absence of a vaccine or medicines, has generated immense optimism. CP therapy has been used in recent epidemics like Ebola, MERS, 2009 H1N1, and SARS.² Deployment of plasma therapy against an increasing number of cases of COVID-19, given its historical precedent, shows a viable option for mitigation and treatment. However, various controlled studies or the lack of (randomized controlled trials) point to data being very mixed regarding its efficacy as an antiviral agent.
Historical Use
Convalescent plasma therapy is one of the oldest known treatment modalities for infectious diseases. Emil von Behring first showed its efficacy in the management of diphtheria, a serious infectious disease, which won him the inaugural Nobel Prize in Physiology or Medicine in 1901.³ Convalescence pertains to a phase where a person is recovering after an illness or an injury, whereas plasma is the liquid component of a person’s blood that contains such antibodies, clotting factors, electrolytes, and other proteins. When people get infected by a pathogen exposure, the body’s immune system gets activated to produce antibodies against the specific molecular area of such pathogens. These antibodies could develop immunity against those pathogens by neutralizing them and/ or signaling other immune cells to destroy specific pathogens. It takes about 1 to 2 weeks on average to elicit an antibody response in the form of IgG for people not previously vaccinated or exposed to a pathogen. Providing antibodies in the form of plasma might prevent infections, make it milder, or reduce the duration of symptoms.⁴
CP has been used as post-exposure prophylaxis for hepatitis, mumps, measles, rabies, and polio. Hepatitis B immunoglobulin (HBIG) is still used in addition to the hepatitis B vaccine as a post-exposure prophylaxis in some population.⁵ Human rabies immunoglobulin (HRIG) is also a mainstay along with the vaccine in post-exposure prophylaxis of rabies.⁶ Similarly, this therapy has also been used to treat a myriad of infectious diseases such as Severe Acute Respiratory Syndrome, Middle East Respiratory Syndrome (MERS), Influenza A (H5N1), Influenza A (H1N1), Argentine hemorrhagic fever, and Ebola.⁷ Even though the idea sounds very plausible, the data have been mixed regarding its efficacy in treating those diseases.
A meta-analysis of 8 studies on CP use in patients with Spanish influenza concluded that while there were methodological issues with the studies analyzed, patients who received CP may have had lower mortality rate than the patients who did not receive it.⁸ A small non-randomized study of convalescent plasma administration on the Ebola virus by Van Griensven et al. showed that there were no significant improvements in survival of patients treated with CP as compared to historical controls.⁹ The patients who received plasma were within 2 days of the diagnosis. Since the neutralizing antibody levels were unknown, the dose-response of the antibody titers were inconclusive. In another small retrospective study of convalescent plasma on SARS-CoV-1 by Soo et al., patients receiving convalescent plasma had shorter hospital stays and lower mortality by day 22.¹⁰ There were some limitations to this study. It was non-randomized, the neutralizing antibody levels were not standardized, and the patients in the control group seemed to be sicker than the treatment group. Another non-randomized study of CP on 80 patients with SARS-CoV-1 in Hong Kong by Cheng et al. showed that in patients receiving CP before 14 days of symptoms onset compared to patients receiving CP after 14 days of onset, there was a statistically significant difference in the rate of “good outcomes”. Good outcome was defined as discharge by day 22 of symptoms onset. Patients who benefitted most from CP also were PCR positive and seronegative for SARS at the time of administration. There was also a trend towards lower mortality rate amongst those who received the treatment within 14 days of onset, but it did not reach statistical significance.¹¹ Once again, it was not a randomized controlled trial. The study did not establish a control group between patients receiving plasma, so the evidence for its efficacy was inconclusive. Also, with the antibody titers not standardized, the results for patients’ health improvements were ambiguous. This means that the patients who had positive effects might have received plasma with high neutralizing antibody titers or patients who did not see benefits might have received plasma with low neutralizing antibody titers. It could also be possible that some people did not see any benefits despite receiving high neutralizing antibody titers. Nothing is conclusive. This is why reporting of neutralizing antibody titers is very important.
Current Evidence for COVID-19
There have been a handful of studies about the use of plasma in COVID-19 patients, including limited number of randomized controlled trials. The data cannot determine whether it is effective. Most of those studies are observational, which limits the generalizability of the results. A recent Cochrane review, including 2 randomized controlled trial (RCT), 8 controlled non-randomized study of intervention (NRSI), and 9 non-controlled NRSIs, concluded that the efficacy of convalescent plasma in improving mortality or clinical improvement in COVID-19 patients is uncertain.¹² There was also a major preprint study by Mayoclinic that had a report of over 35000 people comprising mostly of critically ill patients which led the FDA to grant Emergency Use Authorization to this therapy.¹³ It showed that the patients receiving plasma with a higher level of neutralizing antibodies (nAbs) had 35% of relative improvement in the seven-day mortality rate compared to patients receiving plasma with lower nAbs. It is worth emphasizing here that this is a relative improvement and not absolute. When you look at the absolute percentages, the seven-day mortality rate was 8.9% in patients receiving higher nAb titers and 13.7% in patients receiving lower nAb titers, which is not that huge of a difference. Besides that, there was also a significant mortality benefit seen in patients receiving plasma within 3 days of diagnosis (8.7%) than patients getting it after 4 days or later (11.9%).¹⁴ After the results, even the FDA commissioner, Dr. Stephen Hahn, erroneously stated that there was a 35% reduction in mortality rate, which he later retracted on his Twitter.¹⁵ It was a comprehensive study, but the lack of a comparator group weakens these findings as to the evidence of efficacy. The patients were also on other medications like remdesivir, steroids, lopinavir-ritonavir, and hydroxychloroquine amongst many others. Although hydroxychloroquine and lopinavir-ritonavir have fallen out of favor in recent times, there is good data regarding the benefits of dexamethasone and . The mortality benefit of dexamethasone amongst hospitalized patients on invasive mechanical ventilation came to light in the RECOVERY trial.¹⁶ Remdesivir also has shown to improve time to clinical recovery in the ACTT-1 trial in hospitalized patients requiring oxygen.¹⁷ The recently published pre-print of SOLIDARITY trial, however, did not see any benefit of remdesivir in mortality or hospital duration, but there were some methodological issues with the trial which is beyond the scope of this paper.¹⁷ So, is the benefit because of plasma therapy, or could it be because of other medications? It is very uncertain, so to establish cause and effect, we still need more well designed randomized controlled trials.
At the time of writing, there have been 4 published randomized controlled trials. Unfortunately, all those studies suffer from at least one of these issues: low sample size, under-enrollment of participants, premature termination of trials, and inadequate blinding. In an open-label randomized controlled trial conducted in Wuhan, Li et al. studied 103 patients with severe and life-threatening COVID-19. All of those patients were receiving the usual standards of care, but one group received convalescent plasma along with the standards of care. The protocol was to enroll 200 patients to maintain adequate power, but because of the disease being well-controlled in Wuhan at the time of the study, only 103 people were eventually enrolled, and the trial ended rather prematurely. Compared with controls, patients receiving convalescent plasma had a trend towards lower mortality, greater likelihood of hospital discharge by day 28, and faster improvement in symptoms without reaching statistical significance.¹⁸ In another trial (ConCOVID) in the Netherlands, Gharbharan et al. studied 86 hospitalized patients. They randomized the patients to receiving plasma and standards of care versus patients only receiving standards of care. At the time of inclusion in the study, the patients were hospitalized for a median of 2 days and showed symptoms for a median of 10 days. They reported seeing no difference in 60-day mortality, length of hospital stay, or disease severity by day 15. The trial stopped prematurely because they found out that most of the patients enrolled already had high anti-SARS-CoV-2 antibodies at the time of enrolment and about 41% of them had high nAb titers of 1:320. To put things into perspective, the plasma that they were providing had nAb titers of 1:80 and above. So, some enrolled patients already had higher nAb titers than the plasma that they were going to receive.¹⁹ Avedano-Sola et al. conducted another randomized study of 81 hospitalized patients in Spain. They concluded that convalescent plasma could be superior to standards of care in avoiding progression to mechanical ventilation or death in hospitalized patients with COVID-19, but the difference did not reach statistical significance because of premature termination as the disease got relatively well-controlled by that time. The mortality rate was also lower in the plasma group, but that was also not statistically significant.²⁰ Finally, the most recent randomized controlled trial (PLACID), which was published in the prestigious BMJ, came from India, where Agarwal et al. studied 464 hospitalized patients. They were randomized into receiving CP and standards of care versus standards of care only. They concluded that CP was not associated with a reduction in all cause mortality or progression to severe COVID-19 by day 28.²¹ Having said that, even though the patients were supposed to be early on in their disease course, a lot of them already had high level of nAbs before treatment. This could mean that those patients were probably not that early in their disease course. In addition to that, the donated CP also did not have therapeutic levels of nAb with the median of just 1:40. The combination of these issues could have negated any benefits that CP could have possibly had.
As mentioned earlier, CP therapy has over 100 years of history, and is generally considered to be a safe practice. Mild allergic reactions like fever, urticaria, erythema, etc. are always possibilities. However, it can also cause various severe transfusion reactions including anaphylaxis, hemolysis, transfusion-related acute lung injury (TRALI), and transfusion-associated circulatory overload (TACO). It could also lead to thromboembolic complications because of the presence of coagulation factors in the plasma. It is worth emphasizing that patients with COVID-19 are at a hypercoagulable state which could already increase the risk of thromboembolic complications. According to the safety update including 20000 patients published by Mayoclinic, the rate of TACO was 0.18%, TRALI was 0.1%, and a severe allergic reaction was 0.1%. The rate of thromboembolic complications was 0.19% and cardiac events were 0.4%.²² There is also a theoretical possibility of antibody-dependent enhancement (ADE), where the antibodies could lead to a paradoxical increase in viral uptake and worsen the clinical outcomes, but so far, there have been no reports of such phenomenon for COVID-19.
Discussion
This is not a comprehensive review, but it is an amalgamation of some significant studies on this topic. Based on the current evidence, it is unclear whether convalescent plasma offers mortality benefits or clinical improvements to the patients. Some observational studies show that there could be a mortality benefit, but because of the confounding variables and other limitations on the trials, it is not certain if benefits can be attributed solely to plasma therapy.¹⁴ We believe that CP or antibody therapy may have its place in COVID-19 treatment if administered soon after the onset of symptoms or as a prophylactic intervention after COVID-19 exposure.
It is very important to understand that COVID-19 is a biphasic disease. The first phase is the “viral” phase where the virus gets into the body and starts replicating. The second phase is the “cytokine storm” phase which occurs in about a week or later when the body cannot clear the virus, resulting in the immune system having a highly uncontrolled response releasing high levels of interleukins, chemokines, and other pro-inflammatory mediators. We also know this as the cytokine storm, which is a life-threatening condition characterized by exaggerated systemic inflammation, Acute Respiratory Distress Syndrome, multi-organ failure, and if left untreated, death.²³
The distinction between the “viral” and “cytokine” phase is very important because the treatment modalities should be based on the phase that the patient is in. Early on in the disease course, the primary focus should be on clearing the virus as soon as possible and preventing the progression to the cytokine storm phase. That is where antivirals like remdesivir, interferons, CP, or antibodies come into play. Failure to clear the virus results in the pathogen multiplying, wherein the immune system goes into hyper-drive resulting in the cytokine storm.²³ Theoretically, antivirals or antibodies will not have much role here because, by this time, COVID-19 would have already progressed to the cytokine storm phase. Here, it becomes paramount to dampen the immune system to limit as much damage as possible. Therefore, in this phase, therapy that could suppress the immune system would lead to better outcomes. This is where immunosuppressants come into play. As mentioned earlier, it was shown in the RECOVERY trial that dexamethasone was effective in reducing mortality rate of hospitalized patients on invasive mechanical ventilation. 16Dexamethasone is a glucocorticoid that suppresses the immune system, and it is perhaps not surprising to see its benefits in the patients on invasive mechanical ventilation because dampening the immune system will help prevent further damage in this group of patients. Conversely, the administration of dexamethasone on patients not receiving oxygen trended towards poorer outcomes. This is also not surprising because patients receiving oxygen might still be in the viral phase, so suppressing the immune system using dexamethasone could be counterproductive because the body is relying on the immune system to clear the virus at that point. The key point of this is, one should be highly cognizant of the fact that different treatment modalities could be more effective at one phase but not as effective or even possibly detrimental at a different phase.
As mentioned earlier, some antibodies in plasma can bind to a specific molecular area of the viral antigen and neutralize them by blocking its entry or signaling other immune cells to destroy them. That is why CP therapy could be more beneficial in the viral phase of the illness. Theoretically, it could be the most effective soon after the onset of symptoms or as a prophylactic therapy because it will prevent the virus from replicating and infecting the cells. The trials mentioned in this paper have not given conclusive results, but overall, benefits of CP was mostly seen amongst patients who were very early in their disease course. The Mayoclinic study showed that when administered within 3 days of diagnosis, the CP therapy provided significant mortality benefit over administering it after 4 days. There was also significantly improved mortality benefit in patients receiving plasma with a high dose of neutralizing antibodies compared to patients receiving a lower dose of neutralizing antibodies. This also aligns with what is already known about antibodies therapy based on its mechanism and history of use.
Now, the challenge is getting the right plasma. What is considered to be an optimal plasma? It is generally the one with higher nAb titers. The optimal level of nAb titer in the plasma is unknown, but in the United States, the Food and Drug Administration (FDA) recommended a neutralizing antibody titer of greater than or equal to 250 in the Broad Institute’s neutralizing antibody assay or a signal-to-cut-off ratio (S/C) of greater than or equal to 12 in ortho VITROS IgG assay.13 Therefore, just giving CP may not be enough. We also need to know the dose of nAbs being administered to study its efficacy and improve reproducibility. Unfortunately, not every convalescent patient has the same level of nAb titers. It has now been seen in multiple studies that the antibody levels wane over a few months. It has also been seen that sicker and hospitalized patients generally seemed to produce more nAbs than mildly infected or asymptomatic patients. Older patients also seemed to produce more nAbs than younger patients.²⁴ So, nAbs seem to depend upon multiple variables like age, disease severity, hospitalization, duration, and even sex. Just because someone is recovering from COVID-19, does not mean that they will have high enough nAb titers to prevent others from getting the infection.Also, if the convalescent patient is already months after the diagnosis, there may not be high enough nAb titers present in the plasma. ^25,26
In Nepal, most of the COVID-19 cases have been on the milder side, and as mentioned earlier, the nAb titers are lower amongst mildly ill or asymptomatic patients as compared to sick and hospitalized patients. The PLACID trial enrolled mostly young donors who only had mild illness. That is why the nAb titers they collected were not of therapeutic levels. This could seriously impact the results from the therapy because if the dose is not high enough, then the response may not be that significant either. Therefore, finding CP may not be difficult, but finding the CP with all the right characteristics may prove to be an arduous challenge. Can our medical facilities measure nAb titers on every single donated plasma? If the answer is no, then we are essentially trying to hit the bull’s eye blindfolded, hoping and praying to reach anywhere close to the target.
It was also seen in the aforementioned studies that a lot of the patients who received CP already had fairly high levels of nAb titers entirely from their immune system. Sometimes, they had just as much, if not more, nAb titers in their plasma compared to the therapeutic plasma that they were going to receive as in the cases of ConCOVID and PLACID trials. This means they were probably not that early in their disease course. It is unlikely that administering CP therapy could provide any benefit to these groups of people. The optimal COVID-19 CP can be considered as a limited resource. It is not prudent to waste such a limited resource on something that is doomed to fail. This is why it is also very important to select the right group of patients because CP therapy could really be beneficial early on when the body has not had time to mount an immune response or if the body is not capable of mounting an immune response at all as in the case of immunocompromised patients. It was also seen on Regeneron’s antibody cocktail, REGN-CoV2, which came to fame because President Trump received it during his recent hospitalization from COVID-19. Regeneron’s cocktail of 2 monoclonal antibodies, REGN-CoV2, led to a faster reduction in viral load and improvement in clinical symptoms as compared to placebo amongst non-hospitalized seronegative SARS-CoV-2 patients who also had higher viral load. The same improvement was not seen amongst patients who were seropositive and had lower viral load.²⁷

Conclusion
In summary, convalescent plasma is a safe treatment modality and has years of safety data behind its use, but its efficacy in the treatment of COVID-19 is still unclear among hospitalized patients. Its efficacy may depend on the correct dose of nAbs administered soon after the onset of symptoms or as a prophylactic therapy soon after exposure to SARS-CoV-2. However, it needs to be re-emphasized that data supporting this claim are lacking because studies have mostly been done on hospitalized patients. More randomized controlled trials are needed, which account for all confounding variables in the patient population such as age, underlying comorbidities, the volume of antibody titers infused, concomitant treatments, disease severity, disease duration among many others. Plasma therapy is not end-all-be-all, but it has the potential to be considered as a stop-gap therapy until effective vaccine or antiviral drugs become available. Till then, the focus should be on things that have shown to make difference so far like masks, social distancing, hand hygiene, testing, and contact tracing. ^28,29

1. “COVID-19 Map.” Johns Hopkins Coronavirus Resource Center, 10 Nov. 2020, coronavirus.jhu.edu/map.html.
2. Marano, Giuseppe et al. “Convalescent plasma: new evidence for an old therapeutic tool?.” Blood transfusion = Trasfusione del sangue vol. 14,2 (2016): 152-7. doi:10.2450/2015.0131-15
3. The Nobel Prize in Physiology or Medicine 1901. NobelPrize.org. Nobel Media AB 2020. Thu. 29 Oct 2020. https://www.nobelprize.org/prizes/medicine/1901/summary/
4. Casadevall A, Pirofski L. The convalescent sera option for containing COVID-19. J Clin Invest. 2020;130(4): 1545-1548. https://doi.org/10.1172/JCI138003.
5. Schillie, Sarah, and Claudia Vellozzi. “Prevention of Hepatitis B Virus Infection in the United States: Recommendations of the Advisory Committee on Immunization Practices.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 29 Mar. 2018, www.cdc.gov/mmwr/volumes/67/rr/rr6701a1.htm?s_cid=rr6701a1_w.
6. “Rabies Postexposure Prophylaxis (PEP).” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 11 June 2019, www.cdc.gov/rabies/medical_care/index.html.
7. Bloch, Evan M., et al. “Deployment of Convalescent Plasma for the Prevention and Treatment of COVID-19.” The Journal of Clinical Investigation, American Society for Clinical Investigation, 2 June 2020, www.jci.org/articles/view/138745.
8. Luke TC, Kilbane EM, Jackson JL, Hoffman SL. Meta-analysis: convalescent blood products for Spanish influenza pneumonia: a future H5N1 treatment? Ann Intern Med. 2006 Oct 17;145(8):599-609. doi: 10.7326/0003-4819-145-8-200610170-00139. Epub 2006 Aug 29. PMID: 16940336.
9. Griensven, Johan van, et al. “Evaluation of Convalescent Plasma for Ebola Virus Disease in Guinea.” New England Journal of Medicine, vol. 374, no. 1, 2016. Crossref, doi:10.1056/nejmoa1511812
10. Soo, Y. O. Y., et al. “Use of Convalescent Plasma Therapy in SARS Patients in Hong Kong.” European Journal of Clinical Microbiology & Infectious Diseases, vol. 24, no. 1, 2004.Crossref.doi:10.1007/s10096-004-1271-9.
11. Cheng, Y., et al. “Use of Convalescent Plasma Therapy in SARS Patients in Hong Kong.” European Journal of Clinical Microbiology & Infectious Diseases, vol. 24, no. 1, 2004, pp. 44–46., doi:10.1007/s10096-004-1271-9.
12. Chai KL, Valk SJ, et al. Convalescent plasma or hyperimmune immunoglobulin for people with COVID-19: a living systematic review. Cochrane Database of Systematic Reviews 2020, Issue 10. Art. No.:CD013600. DOI: 10.1002/14651858.CD013600.pub3
13. Commissioner, Office of the. “FDA Issues Emergency Use Authorization for Convalescent Plasma as Potential Promising COVID–19 Treatment, Another Achievement in Administration’s Fight Against Pandemic.” U.S. Food and Drug Administration, FDA, www.fda.gov/news-events/press-announcements/fda-issues-emergency-use-authorization-convalescent-plasma-potential-promising-covid-19-treatment.
14. Joyner, Michael J, et al. “Effect of Convalescent Plasma on Mortality among Hospitalized Patients with COVID-19: Initial Three-Month Experience.” 2020,
doi:10.1101/2020.08.12.20169359.
15. Hahn, Stephen (SteveFDA). “I have been criticized for remarks I made Sunday night about the benefits of convalescent plasma. The criticism is entirely justified. What I should have said better is that the data show a relative risk reduction not an absolute risk reduction.” 24 Aug, 2020, 9:36 pm. Tweet.
16. The Recovery Collaborative Group. “Dexamethasone in Hospitalized Patients with Covid-19 — Preliminary Report.” New England Journal of Medicine, 17 July 2020, doi:10.1056/nejmoa2021436.
17. Beigel, John H., et al. “Remdesivir for the Treatment of Covid-19 — Final Report.” New England Journal of Medicine, vol. 383, no. 19, 2020, pp. 1813–1826., doi:10.1056/nejmoa2007764.
18. Li, Ling, et al. “Effect of Convalescent Plasma Therapy on Time to Clinical Improvement in Patients With Severe and Life-Threatening COVID-19.” Jama, vol. 324, no. 5, 2020, p. 460., doi:10.1001/jama.2020.10044.
19. Gharbharan, A., et al. “Convalescent plasma for COVID-19. A randomized clinical trial. medRxiv.July3,2020.”
20. Avendano-Sola, Cristina, et al. “Evaluation of Convalescent Plasma versus Standard of Care for the Treatment of COVID-19 in Hospitalized Patients: Study Protocol for a Phase 2 Randomized, Open-Label, Controlled, Multicenter Trial.” Medrvix.Org, 2020. Crossref, doi:10.1101/2020.07.31.20165720
21. Agarwal, Anup, et al. “Convalescent Plasma in the Management of Moderate COVID-19 in adults in India: open label Phase II multicentre randomised controlled trial (PLACID Trial).” BMJ.Org, 2020. Crossref, doi:10.1101/2020.09.03.20187252.
22. Joyner, Michael J., et al. “Safety Update.” Mayo Clinic Proceedings, vol. 95, no. 9, 19 July 2020, pp. 1888–1897., doi:10.1016/j.mayocp.2020.06.028.
23. Tang, Yujun, et al. “Cytokine Storm in COVID-19: The Current Evidence and Treatment Strategies.” Frontiers in Immunology, vol. 11, 10 July 2020, doi:10.3389/fimmu.2020.01708.
24. Klein, Sabra, et al. “Sex, Age, and Hospitalization Drive Antibody Responses in a COVID-19 Convalescent Plasma Donor Population.” 2020, doi:10.1101/2020.06.26.20139063.
25. Long, Quan-Xin, et al. “Clinical and Immunological Assessment of Asymptomatic SARS-CoV-2 Infections.” Nature Medicine, vol. 26, no. 8, 2020, pp. 1200–1204., doi:10.1038/s41591-020-0965-6.
26. Sun, Baoqing, et al. “Kinetics of SARS-CoV-2 Specific IgM and IgG Responses in COVID-19 Patients.” Emerging Microbes & Infections, vol. 9, no. 1, 2020, pp. 940–948., doi:10.1080/22221751.2020.1762515
27. “Regeneron’s REGN-COV2 Antibody Cocktail Reduced Viral Levels and Improved Symptoms in Non-Hospitalized COVID-19 Patients.” Regeneron Pharmaceuticals Inc., investor.regeneron.com/news-releases/news-release-details/regenerons-regn-cov2-antibody-cocktail-reduced-viral-levels-and.
28. Chu, Derek K, et al. “Physical Distancing, Face Masks, and Eye Protection to Prevent Person-to-Person Transmission of SARS-CoV-2 and COVID-19: a Systematic Review and Meta-Analysis.” The Lancet, vol. 395, no. 10242, 2020, pp. 1973–1987., doi:10.1016/s0140-6736(20)31142-9.
29. Kucharski, Adam J, et al. “Effectiveness of Isolation, Testing, Contact Tracing, and Physical Distancing on Reducing Transmission of SARS-CoV-2 in Different Settings: a Mathematical Modelling Study.” The Lancet Infectious Diseases, vol. 20, no. 10, 2020, pp. 1151–1160., doi:10.1016/s1473-3099(20)30457-6.