key: cord-0855180-z20zha8t authors: Syal, Kirtimaan title: COVID‐19: Herd Immunity and Convalescent Plasma Transfer Therapy date: 2020-04-13 journal: J Med Virol DOI: 10.1002/jmv.25870 sha: e3c6ac259b9a111a9db4c252aa83afa0a78f8b8c doc_id: 855180 cord_uid: z20zha8t Coronavirus disease 2019 (COVID‐19), caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS‐CoV‐2), has spread worldwide triggering a pandemic. Its first case was reported in Wuhan, the capital of China's Hubei province, and since then it has spread across world becoming one of the worst pandemics in the history of mankind(1). Its basic reproductive number (R(0)) has been determined to be 2‐3(2). This article is protected by copyright. All rights reserved. then it has spread across world becoming one of the worst pandemics in the history of mankind 1 . Its basic reproductive number (R 0 ) has been determined to be 2-3 2 . Here, basic reproductive number is the average number of secondary cases generated by infected subject in the completely susceptible population. Any pandemic with basic reproductive number less than one will die out on its own 3 . To give a perspective to this number, the 1918-influenza pandemic which infected one third of world's population and resulted in 50 million casualties had reproductive number of 1.8 4 . Majority of COVID-19 patients show mild symptoms and recover on their own in two to three weeks, so it is likely that cases could go undetected and the actual basic reproductive number is much higher than stated above. COVID-19 This article is protected by copyright. All rights reserved. symptoms include fever, cough, sore throat, diarrhoea, shortness of breath, loss of smell/taste, pneumonia and in severe cases, it can lead to multiple organ failure and death 5 . The overall rate of mortality per confirmed cases has been reported to be nearly 4.5 percent 6 . The rate of recovery and severity of the disease depend on the age and health of the subjects. The younger age group without any pre-health condition is expected to have a low death rate (0.2 percent) whereas older population (above 80) have much higher death rate (15 percent) 6 . Research is being carried out for validating the available potential treatment regime and vaccines, and for developing a new one. Herd immunity is an age-old concept. It is an indirect protection conferred by immune individuals to the susceptible ones in a given population against a specific pathogenic infestation. Herd immunity protects by limiting the spread of the disease 7 . R 0 number determines the minimum percentage (Y) of population required to be immune for achieving the herd immunity for the entire population 7 . Here, Y= (R 0 -1)/R 0 X 100 7 As described before, R 0 = 2-3 as per recent reports. Similarly, when R 0 =3, then Y=[(3-1)/3*100=66.66 % Therefore, for R 0 = 2-3, nearly 50 to 66.66 percent* (threshold) of the population is required to be immune against COVID-19 for the protection of susceptible individuals in a given population through herd immunity. *As discussed previously, it can be higher as cases are most likely under-reported. In emergency situations, recovered subjects who are negative for COVID-19 can be considered for voluntary temporary employment at sensitive locations such as hospitals and airport as per their capability and skill set. Such locations have the potential to become the hot bed for spread of infections. In presence of immune volunteer workers, spread of the disease would be restricted. The recovered This article is protected by copyright. All rights reserved. individuals if placed in high enough number at sensitive location can act as a source of indirect immunity 8 and protect the target population. The coronavirus cannot reinfect the recovered subjects. Though few reports from Japan suggest the possibility of reinfection but it has been disputed by some experts who believed that redetection was due to the error in diagnosis and not the reinfection 9 . Some health officials also considered it to be the cases of re-activation of latent COVID-19 9 . In China, nearly Physical distancing, quarantine and other sanitizing habits have showed some success in slowing down the pandemic but it is still far from being contained in most countries. The antimalarial drug chloroquine and antibiotic azithromycin have showed potential against COVID-19 but its efficacy has been recently disputed 19 . Other potential therapeutic agents tried so far include ritonavir, favipiravir, remdesivir, ribavirin, sarilumab and tocilumab, but their usefulness also needs more evidence 20 . Considering the plight of undeveloped and developing countries where most people cannot afford ventilators or lock down for extended period of time, pandemic must be slowed down so that health infrastructure can handle it efficiently. In the absence of validated treatment and tested vaccine, localized herd immunity can help in slowing down its spread at sensitive locations. Here, convalescent plasma transfer could be the most critical weapon in the fight with COVID-19 in severe cases. Here, survivors have a key role to play in both herd immunity as well as plasma transfer. In order to tap the potential, all survivors should be well-documented. Their convalescent plasma should be checked for neutralizing antibodies and stored in blood banks, and spread up across countries so that it can be delivered when need arises. From the ethical point of view, there is a road-block for taking help from survivors to this extent, but Effectiveness of convalescent plasma therapy in severe COVID-19 patients Johns Hopkins gets FDA approval to test blood plasma therapy to treat COVID-19 patients Convalescent Plasma to Treat COVID-19: Possibilities and Challenges Convalescent plasma as a potential therapy for COVID-19. The Lancet Infectious Diseases Investigational-covid-19-convalescent-plasma-emergency-inds. US Food and Drug Administration Anti-Ebola Virus Antibody Levels in Convalescent Plasma and Viral Load After Plasma Infusion in Patients With Ebola Virus Disease No Evidence of Rapid Antiviral Clearance or Clinical Benefit with the Combination of Hydroxychloroquine and Azithromycin in Patients with Severe COVID-19 Infection Potential therapeutic agents against COVID-19: What we know so far considering the necessity to safeguard the health of immunocompromised and older population, it should be considered and steps must be taken to explain its importance to the survivors. Sun P, Lu X, Xu C, Sun W, Pan B. Understanding of COVID-19 based on current evidence. J Med Virol. 2020.