Dr. Muhammad Torequl Islam :
Herd immunity (HdI) is the indirect protection from infection conferred to susceptible individuals when a sufficiently large proportion of immune individuals exist in a population. The time to reach HdI of a community depends on the reproduction number (R0). It means the average number of people that a single infected person with the virus can infect those aren’t already immune. The higher the R0, the more people need to be resistant to reach HdI. According to the scientific reports, the R0 for novel coronavirus disease 2019 (COVID-19) is within 2 to 6. This means that one infected person can infect two to six other persons. It also means 17 to 50% of the population would need to be resistant before HdI kicks in and the infection rates start to go down. However, a single pathogen may have multiple R0 values depending on the characteristics and transmission dynamics of the population. Therefore, the HdI threshold value (1 – 1/R0) may vary between populations.
The communicability of an infectious disease depends on many factors, such as population density and age structure, cultural behaviors, underlying comorbidity rates, differences in contact rates across demographic groups, which may affect the HdI threshold. The effective reproduction number (Re or Rt) is also important to understand the population-level immunity. It is the average number of secondary cases generated by a single index case over an infectious period in a partially immune population. Thus, the goal of vaccination programs is to bring the value of Re below 1 will be possible only when the HdI threshold exceeded. The pathogen spread cannot be maintained, therefore, a decline in the number of infected individuals will be seen within the population.
The challenges of HdI in case of COVID-19 are: (i) less effectiveness, periodic outbreaks can still occur, (ii) unevenly distributed within a population, clusters of susceptible hosts that frequently contact one another may remain, (iii) the proportion of immunized individuals surpasses the HdI threshold, susceptible individuals will be found in the risk zone for local outbreaks, (iv) non-relevant infection fatality rate (IFR) and case fatality rate (CFR). Still there is no straightforward, ethical path to reach the goal with HdI in case of COVID-19, due to the societal consequences of achieving it are devastating. A non-uniform COVID-19 case fatality rate (CFR) has been reported across age groups, with the vast majority of deaths occurring among individuals 60 years old or greater. Sex- and ethnicity-specific CFRs suggest that genetic, environmental, and social determinants may affect in susceptibility to COVID-19 and the severity of novel coronavirus 2019 (nCoV-19) infections. Upon going through the current scientific aspects, there are two possible approaches to build widespread nCoV-19 immunity: (i) mass vaccination campaign with an effective and safe vaccine and/or (ii) natural immunization of global populations with the nCoV-19 over time.
On the other hand, the latest Chinese guideline emphasized that convalescent plasma therapy (CPT) may be an emergent treatment for serious COVID-19 cases. However, one study indicates that CPT reduces serum cytokine response with uncertain implementing time. Therefore, it can result hyperimmune attacks. CPT should be more effective in the earlier stages of in COVID-19. The therapeutic effect of CPT on COVID-19 is determined by the level of nCoV-19 neutralizing antibody titer (NAT). One study suggests that the specific IgG began to increase around the 3rd week after onset, and peaked in the 12th week. The convalescent plasma (CP) from donors who have recovered and who are at the 12th week after onset with a NAT level of not less than 1:160 is expected to be more effective.
Some other limitations of acquiring CPT such as age, weight, health state, informed consent, required CP volume, recovered patient ratio needing the CP are should be taken into account. Therefore, CPT limits its wide application, especially in countries which are in the acceleration stage and late accumulation stage of nCoV-19 infection. The CPT also results some common adverse reactions, including chills, fever, anaphylactic reactions, transfusion-related acute lung injury, circulatory overload, hemolysis, and so on. Additionally, the risk of transfusion-transmitted infections, such as human immunodeficiency virus (HIV), hepatitis B and C viruses, and syphilis, should be taken into deliberate account.
(Dr. Muhammad Torequl Islam is Assistant Professor, Department of Pharmacy, Bangabandhu Sheikh Mujibur Rahman Science and Technology University. E-mail: [email protected])
