Use of convalescent plasma for COVID-19 in India: A review & practical guidelines

COVID-19 in India

The cases of COVID-19 in India, the pandemic caused by SARS-CoV-2, has reached to 10,582,647 and total deaths above 152,000 on January 19, 20211. There is currently no proven specific therapy, and multiple novel and repurposed molecules are being used on an experimental basis. The lack of effective therapeutic options has been a major hurdle in our pandemic mitigation measures.

Convalescent plasma (CP), an overview

Convalescent plasma (CP) is a mode of passive immunization wherein preformed antibodies against an infectious agent are infused into a susceptible host with the aim of either preventing or treating the infection2 (Fig. 1). CP was widely used in the past to treat several bacterial (diphtheria, tetanus, pneumococcal pneumonia and meningococcemia) and viral infections (rabies, poliomyelitis and measles) in the pre-antibiotic era345. Safer and more standardized modalities such as hyperimmune globulins and monoclonal antibodies are currently available against diseases caused by agents such as Clostridium tetani, hepatitis A and B, rabies and respiratory syncytial virus4 (Fig. 2). CP still retains an important place in the management of infections caused by novel pathogens because of the rapidity and ease with which it can be put to clinical use (Table I). Hence, it has been widely used during almost all recent epidemics, including H1N1 influenza67, SARS89, Middle East respiratory syndrome (MERS)1011 and Ebola1213. Most of the literature from these epidemics is in the form of case reports or series. There are very few randomized control trials (RCTs)1415 or systematic reviews and meta-analyses1617. Past experience unequivocally attests to CP's safety with very few adverse events reported1718. Pooled analyses suggest the benefit of CP use in diseases such as H1N1, SARS and Argentine haemorrhagic fever, especially when used early in the illness1617. Data from across the globe regarding various aspects of CP use are accumulating at a fast pace. The current review aims to summarize the available evidence in COVID-19 disease and provide the recommendations for setting up CP protocols in clinical and research settings.

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Fig. 1

Overview of convalescent plasma (CP) therapy. The Figure shows the process of CP donation and its use in a patient with COVID-19. CP contains over a thousand types of proteins. Apart from the antibodies against SARS-CoV-2, various other proteins may contribute to the beneficial effects of CP administration, including anti-inflammatory cytokines, complement and clotting factors.

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Fig. 2

Post-donation modifications of convalescent plasma (CP). The Figure shows the possible post-donation modifications to CP to create safer, more potent, standardized passive immunization products such as hyperimmune globulins and monoclonal antibodies against SARS-CoV-2.

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Immunological aspects of convalescent plasma therapy in COVID-19

CP therapy involves collecting plasma from an individual who has recently recovered from infection and infusing it into an at-risk individual219 (Fig. 1). Such plasma contains several humoral factors capable of providing immunity, but the most important one is believed to be the polyclonal antibodies against the target agent18. CP confers immune-protection by numerous mechanisms (Fig. 3). Direct viral neutralization by neutralizing antibodies (NAbs) and the immunomodulatory actions that limit host damage are important mechanisms in COVID-1918.

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Fig. 3

Mechanisms of action of convalescent plasma (CP). The Figure depicts the multiple possible mechanisms of action of CP. Of note, there are multiple non-antibody-based as well as non-viral neutralization-based mechanisms of action. Hence, CP with low or absent anti-SARS-CoV-2 antibodies can theoretically provide beneficial effect when administered to COVID-19 patients. ADCC, antibody dependent cellular cytotoxicity.

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Immunological studies have demonstrated a diverse antibody repertoire in the serum of recently recovered patients20. Antibodies directed against the receptor-binding domain of the viral spike protein (S-RBD) bind them, limit cell entry, and inhibit the virus amplification. Laboratory studies have shown potent neutralizing activity by such antibodies repertoire against the live virus2122232425. The majority of patients achieve seroconversion within two weeks of symptom onset21262728 (Fig. 4). While data regarding the long-term duration of this humoral response in COVID-19 are still evolving, studies conducted during the SARS epidemic have shown that this antibody response is short-lasting, with peak levels at around 3-4 months followed by gradual waning over the next two years29.

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Fig. 4

Current understanding of the relevant immunological concepts in COVID-19. The Figure explains the current concepts in the humoral immune response of COVID-19. (i) IgM antibody production starts at around 4-7 days, peaks by approximate week three and wanes gradually by 3-4 months from symptom onset202128. (ii) IgG antibody production starts at around 10-14 days. The disappearance of IgG response is not exactly known; however, based on data during the earlier SARS epidemic, it is expected to be short-lasting and be undetectable in most individuals by 24-36 months29. (iii) Period of infectivity peaks a couple of days before symptom onset.

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Measurement of antiviral activity of convalescent plasma: Neutralizing antibody titers

The neutralizing activity of CP directly quantifies the plasma's protective activity and has been used as a gold standard for the quantification of its efficacy. These are functional assays that involve incubating serial dilutions of the plasma with a standard dose of the live virus and inoculation of this mixture in a culture medium. The dilution of the plasma that prevents the culture of the virus or its cytopathic effect is defined as the neutralizing titre of the plasma30. However, these assays are not available on a large scale for CP therapy because of accessibility and standardization problems.

The use of commercially available serological assays as alternatives to neutralizing assays is an attractive option. A few recent publications have demonstrated a good correlation of several such serological titres, especially assays targeting the S-RBD domain and neutralizing assays2324313233. The New York Blood Centre (NYBC) found that two commercial assays (by Abbott and ORTHOS) showed a good correlation with NAb titres in a study of serum from 370 CP donors34. The ConCOVID group investigators from the Netherlands found a good correlation of a commercially available ELISA with neutralization assays3235. The assay manufactured by EuroImmun AG has also been shown to correlate with neutralization assays36 and used in a RCT from Spain37. Another issue in the antibody titre measurement of CP is the wide variability in the methodology and cut-offs used in the literature (Table II). Ideally, these parameters need to be standardized and validated in clinical trials before clinical use.

Ideal donor

A potential CP donor for COVID-19 is a healthy adult who has recently recovered from COVID-19 infection and remained asymptomatic for a minimum of 14 days. This period is based on the estimates of antibody kinetics and period of infectivity (Fig. 4). Documented nasopharyngeal swab PCR negativity before CP donation is no longer mandatory in most CP protocols across the globe41. Based on the available antibody kinetics data, the ideal window for donation is 3-4 months after symptom onset29.

There is a wide variability in the titres of anti-SARS-CoV-2 antibodies within donors42. Initial data from China showed that a high proportion of donors (39/40) possessed NAb titres >1:16043. However, recent reports have shown more variations. In the NYBC report, 55 per cent of the screened 370 CP donors did not have high titre antibodies, while 10 per cent had very high levels34. The ConCOVID group reported similar findings in a cohort of 115 plasma donors, with 43 per cent screened donors having neutralizing titres >1:320 and 10 per cent with titres >1:128035. In other studies, the proportion of donors found lacking in high titre antibodies ranged from 25 to 50 per cent234445, with about 10-20 per cent having very high antibody levels of more than 20 times the cut-off limits, the so-called super-donor phenotype45. Older age, male gender and clinical features such as moderate-to-severe disease27, high C-reactive protein (CRP) and lymphocytopaenia2346 have been found to be associated with higher NAb titres. Notably, asymptomatic individuals have been found to mount a very transient and low titre antibody response47. These clinical and demographic factors may help guide the selection of suitable donors.

Ideally, donor selection should be based on their NAb titres, but such an approach is currently not feasible in India (Table II). Due to the unavailability of such assays, most clinical and trial protocols are proceeding with CP therapy without quantification of antibody titres. Of note, the compassionate use programmes in the USA and Israel followed this approach and determined antibody titres of the infused plasma units post hoc, and their findings have been recently published3648. The Indian Council of Medial Research (ICMR)-sponsored PLACID trial also followed this strategy. The antibody titres in this study were found to be low, with a median neutralization titre of 1:40 [interquartile range (IQR) 1:30-1:80]49. These findings have highlighted the hazards of proceeding with CP therapy without appropriate antibody quantification in real time. For now, it seems to be a reasonable strategy to screen potential CP donors for the presence of anti-SARS-CoV-2 antibodies using commercially available serological tests to exclude donors with low anti-SARS-CoV-2 antibodies.

Process of plasma donation

A prospective donor should fulfil the standard donor screening criteria, as per the Drugs and Cosmetics Rules, 194550, and the COVID-19 advisory from National Blood Transfusion Council (NBTC)51. Plasma donation is a voluntary exercise and requires informed consent. Although whole blood can be used in resource-limited settings, plasmapheresis remains the preferred method of CP donation because of the larger collection volume of plasma, the feasibility of repeated collections and the minimal impact on the donor's haemoglobin. Up to 15 per cent of total blood volume (400-800 ml per session) can be safely donated in a single sitting41525354. Recommendations allow for serial plasma donations by a single donor with a minimum gap of seven days between consecutive plasmapheresis sessions5455.

Donated plasma is frozen at less than −30°C (preferably −40 to −80°C) within 8-24 h of collection and can be stored for up to 12 months. Pathogen inactivation by techniques such as photochemical inactivation or solvent detergent treatment and pooling from multiple donors are other post-donation processing options to improve the safety and quality (Fig. 2).

Clinical use of convalescent plasma

Despite the widespread use and publicity of CP, it remains an experimental therapy. Conclusive proof of its efficacy and the parameters of its effective use remain to be firmly established. In such a situation, it is advisable to use CP only under a research protocol after discussing the experimental nature and potential risks of CP use with the recipient.

The use of CP can be envisioned primarily in four clinical settings. The factors related to the dosing of CP in different settings are summarized in Tables III and IV. A cardinal principle is that earlier use in the course of illness is expected to be more beneficial24. Early in the course of the disease, the viral burden is less, and the infection is not yet firmly established. The hypothesized mechanism of action of CP is by prevention or delay in the establishment of infection long enough to allow for the host immune response to clear the infection. In the latter stages of illness, the hyper-inflammatory state is the major driver of morbidity. The ongoing tissue damage maintains the inflammatory state as a vicious cycle, and control of viral replication may have a minimal effect on disease course. As a corollary, CP may be expected to be an effective modality for post-exposure prophylaxis. Table IV shows the suggested dosing for use of CP in different clinical settings.

Accumulating evidence suggests that any potential benefit of CP use may be found in mild and moderate illness rather than the critically ill3343. Studies during the SARS epidemic showed CP to be more useful in symptomatic patients who were seronegative and PCR positive8. Results from the RCT from the Netherlands suggest the need to choose such seronegative COVID-19 patients early in the course of illness35. However, the shortages of high antibody titre donors preclude the use in prophylactic or mild illness settings on a large scale. Thus, in the current scenario, the potential use of CP may be prioritized in the following settings: (i) Post-exposure prophylaxis for healthcare workers; (ii) Post-exposure prophylaxis of individuals with comorbidities at higher risk of severe illness; and (iii) Treatment of symptomatic individuals with moderate disease, especially in the first week of illness.

Risks of convalescent plasma use

Despite the use of guideline-based screening strategies, CP carries a non-zero risk of transfusion-associated infections. Other complications include allergic reactions, transfusion-related acute lung injury (TRALI) and transfusion-associated circulatory overload (TACO). COVID-19 patients with their compromised respiratory reserve are especially vulnerable to TRALI/TACO, which may be difficult to distinguish from the progression of COVID-19. In the periodically published safety analyses from the Expanded Access Program (EAP) in the USA where the safety data of the first 500056 and later 20,000 patients57 have been reported, the incidence of these reactions was very low (<1%), and only two serious adverse events could be conclusively attributed to plasma therapy. Another theoretical concern is the antibody-mediated enhancement of infectivity, a phenomenon described with several viral infections such as dengue5859. It is believed to occur when an individual harbours pre-existing antibodies against a closely related strain of the virus (such as the SARS-CoV-1, MERS or other coronaviruses). It is hypothesized that these pre-existing antibodies are sub-neutralizing in nature and mask the viral epitopes from immune recognition, thus facilitating intracellular entry and survival, leading to a paradoxical worsening of the illness58. There is no evidence available to suggest the role of antibody-mediated enhancement in COVID-19.

Worldwide use of convalescent plasma in COVID-19

The earliest reports of CP use came from China. These and other initial reports have been summarized in Table V. By early March, CP therapy was adopted in the USA, Italy, Israel, Spain and several other countries on compassionate grounds in severely ill patients or as part of clinical trials. Several large single-arm studies from these centres (Table VI) provided a proof of concept and established the feasibility and safety of CP in COVID-19. These have reinforced the general theme that earlier administration is associated with better outcomes. The limitations of these studies included small sample sizes, absent or non-randomized control arms, simultaneous use of multiple experimental therapies and a non-uniform study design and donor selection criteria. Most of these studies were in severe and critically ill patients.

The largest experience of systematic use of CP in COVID-19 comes from the USA. Apart from the on-going clinical trials and the compassionate use under the Emergency Investigational New Drug (eIND) license from FDA, CP has been made accessible to patients and clinicians under the EAP of the USFDA75. Under this programme, more than 80,000 units of CP were infused, and the safety analysis of this programme was mentioned earlier5657. The group recently published their post hoc analysis after determining the NAb titres in 35,322 patients48 (Table VI, row 10). The two main findings were that infusion of CP within three days of diagnosis compared to later showed benefit in seven-day mortality (8.7 vs. 11.9%, P<0.001) and that there was a dose-response curve for mortality with respect to infused antibody titres. The patients who received CP units with low, medium and high antibody titres had a mortality of 13.7, 11.6, and 8.9 per cent. The antibody titres were not known at the time of the administration of CP. Hence, the arm with low-titre CP can be considered an internally blinded control arm76. Similarly, Israel's compassionate CP use programme has also shown a dose-response curve between titres and mortality in their post hoc antibody titre analysis involving 49 patients. Several other single-arm studies have been published, a few with matched control arm with claims of CP use efficacy (Tables V and VI).

This evidence, although compelling, has not been replicated in the initial RCTs on CP. The first RCT on CP therapy in COVID-19 was an open-label RCT from seven centres in Wuhan33 (Table VI, row 5). It was terminated early due to slow recruitment, with only 103 of the target 200 individuals enrolled. This trial employed high titre CP units and showed a higher rate of clinical improvement in severely ill patients but not in the critically ill. There was earlier symptom resolution and viral clearance in the CP arm. The limitations were the lost statistical power due to early termination, lower dosing than other studies and the long median time of >30 days from symptom onset to plasma infusion33.

A multicentre open-label RCT from 14 centres from the Netherlands randomized 86 of the planned 462 patients and gave CP to 4335 (Table VI, row 9). CP with median neutralizing titres 1:640 was infused. The cohort was predominantly with mild illness and admitted early in the course of disease with a median time from symptom onset of 10 days. While there was a trend towards mortality benefit in the CP arm (26 vs. 14%), there was no significant outcome difference between the two arms. Moreover, 80 per cent of the enrolled patients had anti-SARS-CoV-2 antibodies in their serum at baseline with titres comparable to those of the 115 convalescent donors. The titres of the infused CP units were higher than these baseline recipient titres, and more than a four-fold rise of titres within a week could be demonstrated amongst CP recipients. The RCT was halted for revision of design since the investigators reasoned that administering CP to already seropositive patients would confer no additional benefit35.

Another RCT from Spain was halted prematurely due to a fall in recruitment37 (Table VI, row 13). The data on 81 patients enrolled who received high titre CP with median titres of 1:292 (IQR 168-882) showed a mortality benefit at day 14 compared to the control arm; however, the number of events was very low.

Most recently, the ICMR-sponsored PLACID trial also published its findings49 (Table VI, row 15). This open-label RCT could recruit its target, unlike the earlier RCTs and randomized 462 patients. There was no difference in mortality or progression to severe disease between the plasma and the control arm. A high proportion of participants had anti-SARS-CoV-2 antibodies at the time of enrolment, and their antibody titres were higher than those of CP donors. This trial replicated the real-world use of CP, wherein the antibody titres of CP were not known a priori. However, the median antibody titres in the infused CP units were significantly lower than the EAP or the Israel compassionate use data3648. The median NAb titres of infused plasma in this study were 1:40. Whether this reflects the heterogeneity of antibody responses in the Indian population versus the Western population remains to be seen. This experience established the need to have adequate antibody testing for CP done before its use in any trial setting.

Another important RCT on CP use was published in November 2020 by the PlasmAr group from Argentina⁷³ (Table VI, row 17). They enrolled 333 patients of mild to moderately severe COVID acute respiratory distress syndrome (ARDS). However, no mortality or difference in clinical outcome could be demonstrated between the two groups. The median titre of neutralizing antibody used in the study was 1:300, although the median time of administration was eight days, a relatively later time period in the course of disease. Only 39 patients were enrolled within 72 h of symptoms, a number too small to test the hypothesis of administration of CP within first three days of illness. Conversely another double blind RCT from Argentina by the INFANT-COVID-19 group conducted in elderly patients with mild COVID disease who were given high titre CP72. The results showed decreased progression to severe disease in patients who received CP and favoured the use of CP in this group (Table VI, row 16). In particular, all the recipients of CP received the infusion within 72 h of diagnosis. Another smaller RCT conducted in a single centre in Kolkata in 80 patients demonstrated a significant decrease in the hyperinflammatory cytokine response in patients with mild to moderate COVID ARDS74 (Table VI, row 18). Although there was no demonstrable clinical benefit in terms of survival, duration of hypoxia or duration of hospital stay, there was some benefit noted in the subgroup of patients aged below 67 years74.

CP therapy has been granted the Emergency Use Authorization in the USA on August 23, 202040. The high titre CP units in this authorization have been defined as having antibody titre with signal/cut-off ratio >1240.

Is there a need for further randomized control trial for convalescent plasma use in COVID-19?

Until now, in the RCTs the infusion of high-quality CP in the requisite number of patients could not be done either due to poor recruitment or poor titres of antibodies. The two RCTs from Argentina7273 which could infuse high antibody titre plasma to the target population showed contrasting results. While there was no benefit in infusion of plasma in moderate to severely ill patients in the PlasmAr study73, there was benefit when it was given in mildly ill elderly pateints within three days of diagnosis in INFANT-COVID-19 group72. While more than 100,000 units of CP has been used worldwide, there is still not unequivocal evidence for or against CP use. Therefore, there remains a need for further scientifically designed RCTs to answer these questions. There are hypotheses to suggest that even seropositive patients may derive the benefit of CP use by augmentation of natural antibody response, immunomodulatory actions and avoidance of cytokine storm77. However, these aspects can be explored once the efficacy of CP's use has been established.

At present, there are over 100 registered trials worldwide investigating CP in COVID-19757879. Apart from the recently published PLACID trial from India⁴⁹ including a large-scale trial called the PLATINA, which is an open-label RCT of CP use in severe COVID-19 illness, recruiting in 21 centres across Maharashtra (CTRI/2020/06/026123).

Convalescent plasma use in India

If proven beneficial in COVID-19, CP may be a promising treatment option for India. However, efforts will have to be made to make antibody testing available before any large-scale CP use programme in the country, after learning from the PLACID trial experience49. Setting up a centralized antibody titre determination system under the aegis of a competent authority may help overcome such problems. Another operational requirement would be to ensure that patients receive CP units as soon as possible, ideally targeting infusions within three days of diagnosis. The scarcity of donors is another major problem. Creating a CP stockpile large enough to treat patients, provide prophylaxis for healthcare workers and provide enough raw material for producing purified products in the future requires a major public health initiative. Motivating eligible donors for multiple sessions of plasmapheresis is necessary to overcome these shortages.

Voluntary blood donation has always been a challenge in India80, and these problems are compounded further for CP. Multiple barriers for CP donation exist among donors. The most common reasons for reluctance to donate include fear of visiting a healthcare facility during an epidemic, fear of waning of immunity and risk of reinfection due to CP donation. The imposition of lockdowns and curfews also leads to restricted mobility of donors to visit CP donation sites.

The Drug Controller General of India has approved the drug to be administered only under a trial protocol. The Ministry of Health and Family Welfare (MoHFW) guidelines for the treatment of COVID-19 lists CP as an off-label experimental therapy option81. These guidelines recommend using CP after the measurement of antibody titres by neutralization assays or anti-S-RBD IgG ELISA, which may not be possible in most centres of the country. For the design of clinical and research protocols guidelines issued by global regulatory bodies such as WHO82, International Society of Blood Transfusion (ISBT)5283, USFDA38 and European Commission39 may be useful.

Social and ethical implications of convalescent plasma in India

CP therapy for COVID-19 presents with its own unique social and ethical challenges. Although unproven in efficacy, there is a demand for CP therapy, especially in critically ill hospitalized patients. Avoidance of monetary or other coercion is necessary to avoid the exploitation of CP donors. Plasma donation appeals should be only of pro-social altruistic nature and should be completely voluntary. It is necessary to ensure that available plasma is rationed in an unbiased and evidence-based manner. Documentation of outcomes should be mandatory. The formation of a nationwide registry of CP use may go a long way to address these issues.

Conclusions

CP may be a promising and safe treatment option for COVID-19 and is feasible in the Indian setting. Its efficacy in this setting remains to be unequivocally established. Further research on donor selection, antibody cut-offs, precise indications of use and dosing is required before more widespread CP use becomes possible. Emerging evidence points out the necessity of measuring antibody titres in infused plasma units and selecting high titre units for infusion. The recommendations regarding various aspects of CP therapy are summarized in Table VII. The development of more potent modalities such as hyper-immune globulins would be the next step in enhancing passive immune transfer-based therapeutics for COVID-19 (Table VIII).