THE herd immunity, an epidemiological term, was introduced approximately a century ago. It was widely used following an increase in the use of vaccines and vaccination in the process of eradication of diseases. A key question is whether herd immunity can protect us from the ongoing COVID-19 pandemic that has been taking a heavy toll on human lives across the world or not. COVID-19 is caused by severe acute respiratory syndrome coronavirus 2, SARS-CoV-2, and has posed an unprecedented challenges to global public health — with more than 12 million confirmed cases and more than 0.55 million deaths globally as of July 12, 2020.
Herd immunity is a form of indirect form of protection against an infectious disease. Herd immunity is believed to work when the majority of a population becomes immune to the disease either through natural infection with a pathogen or through immunisation with a vaccine. Since there is no effective and confirmed vaccine against COVID-19 yet, natural infection is the only way to develop immunity in a population. The immunity development in a community depends on the basic reproduction number, known as R0, which refers to an average number of infections caused by a single infectious individual introduced into a completely susceptible population. Various studies have estimated the R0 of COVID-19 to be in the range of 2–6. This means that on an average, one infected individual will infect two to six others during the infectious period, assuming no immunity exits in the population. However, a single pathogen may have multiple R0 values due to differences in population density, age structure, cultural behaviour, underlying comorbidity rate and differences in contact rates across demographic groups, which may, directly and indirectly, affect herd immunity. The principle underlying herd immunity is that the presence of enough immune persons in a community interrupts the transmission of infectious agents and thereby provides an indirect protection to susceptible persons.
COVID-19 has been, and is still, spreading fast globally since the first reported case was confirmed in December 2019 in Wuhan, China. Vaccination is a powerful public health tool to protect us from infectious diseases. However, we do not yet have any vaccine against COVID-19. A key question is if and when Bangladesh will reach herd immunity. Under the simplest model, herd immunity depends on a single parameter, R0. When the number of immune cases reaches a significant percentage of the population, the population will be shielded from having more infected cases. Such percentage is called herd immunity threshold. Mathematically, the herd immunity threshold is defined by 1–1/ R0, indicating that the more infectious a disease is, the higher proportion of people would need to be immune to the infection to reach herd immunity. If we consider the report by Liu et al that SARS-CoV-2 spreads fast with R0 of 3, the corresponding herd immunity threshold would be 1-1/3=0.67. This means that herd immunity should be achieved when around 67.0 per cent of the population becomes immune to SARS-CoV-2. Bangladesh, with a population of about 160 million, approximately will require about 107 million people to be infected with COVID-19 to reach herd immunity. We do not yet know how many individuals have been infected, how many are immune and how far we are from reaching the herd immunity threshold. Mass serological testing is now needed to clarify these issues.
Most patients with COVID-19 exhibit mild to moderate symptoms — approximately 15 per cent of the infected progress to severe pneumonia, about 5 per cent eventually develop acute respiratory distress syndrome, septic shock, and/or multiple organ failure, and about 2 per cent die. This means that, to reach the herd immunity threshold, about 16 million cases have to go to the hospital and about five million will need to use the intensive care unit and ventilation, and, it is expected, that the total number of death will be more than two million. If the infected need medical care at once, the healthcare system could quickly become overwhelmed in any country. One expert analysis found that creating herd immunity in the UK would require more than 47 million people to be infected. With a 2.3 per cent fatality rate and a 15 per cent severe cases, this could result in more than a million people dying and a further eight million needing critical care. A recent study has reported that if a uniform herd immunity threshold of COVID-19 is 67 per cent with the R0 being three, the absolute number of expected deaths across the globe would exceed 30 million people. Besides, it is not yet clear whether infection with COVID-19 makes a person immune to reinfection and how long they can persist to be immune. Therefore, the concept of herd immunity has been criticised by the World Health Organisation that states that it is dangerous and unethical to rely on this concept to combat the disease. This indicates that in the absence of a vaccine, a herd immunity concept is highly risky.
The best way to rapidly develop herd immunity is through vaccination. A vaccine delivers a small amount of a virus or fraction of a virus into the body, and the immunity system learns how to fight it off without having to get sick. However, we do not yet have a vaccine against the virus. In this situation, the expert opinion is to build natural immunisation through slowing the pace of the spread. The virus is needed to pass through the population at a delayed speed, prolonging the initiation of the outbreak and decreasing the pick to a lower level as much as possible. Delaying the spread of the pandemic to the point when and where the healthcare system can accommodate those cases is important. At the same time, we have to make sure to protect the most vulnerable people in the community. The risk is higher among people with comorbidities such as cardiovascular disease, hypertension, chronic respiratory disease, cancer and obesity. This suppression strategy is reasonable given the constraints of the healthcare system and the hope for an effective vaccine and medication shortly. The social distancing protocol, home isolation of suspected cases, household quarantine of their family member, and closure of educational institutes are the important steps to delay the spread of COVID-19 in population. At the same time, it is also important to educate people the significance of the natural means of boosting immunity. Stress reduction, physical activity, adequate sleep, healthy and antioxidant-rich diet can boost body’s immunity.
Dr Mohammad Shah Alam is head and an associate professor at the department of anatomy and histology at Bangabandhu Sheikh Mujibur Rahman Agricultural University.
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