Herd immunity – is it an achievable goal, or even a desirable one?

How to Achieve Herd Immunity?

Herd immunity is based on individual immunity which refers to a physiological function that the body’s immune system recognizes and differentiates its own and alien substances and eliminates the antigenic substances (such as bacteria and viruses) through immune response to maintain health. It may be built up by confronting a disease or infection in the past and recovering from it. Immunity may also be induced by vaccination. Herd immunity is usually achieved by lots of people being infected with the contagious disease (e.g., influenza), or by vaccination (e.g., smallpox vaccine).

COVID-19 and Herd Immunity

Possible Outcomes After Herd Immunity

Since there’s no approved vaccine for COVID-19 yet, any herd immunity cannot be achieved by vaccination. If herd immunity is derived from natural infection, what is the proportion of a population that need to be immunized in order to achieve the effect of protection? We can estimate this ratio based on the basic infection number (R0, the expected average number of additional cases that one case will generate over the course of its infectious period in an otherwise uninfected and generally susceptible population) of COVID-19.

Based on the formula of herd immunity threshold (threshold = 1–1/R0) (Fine et al., 2011), and the R0 of COVID-19 being 2.27 (Zhang et al., 2020), only when about 56% of population get specific immunity to SARS-CoV-2, then transmission-blocking can be achieved with herd immunity.

Herd immunity in measles, for example, suggests the whole population is protected from emerging infections when 90% or more are immunized, whether by vaccination or recovery from natural infection. However, in the case of COVID-19, there are two outcomes when people get naturally infected – recovery and death. It is unclear whether those who recover are free of the virus and exempted from being contagious, which indicates that the percentage of infected people would likely be more than 70%. Looking at the Case Fatality Rate as it stands today of 6%, that is a sizeable chunk of the population.

 

According to an epidemiological analysis of COVID-19 in China, COVID-19 can cause about 15% severe cases and a 2% death rate (World Health Organization., 2020a; Zhang et al., 2020). Particularly, the projections above were based on the existing data in China where the overall isolation and the centralized allocation of medical resources of the whole country are adopted. Without effective medical resources and isolation interventions, natural infection may result in a more severe mortality rate.

What will be the cost of the government’s “herd immunity” or “mitigate” strategy? A simulation study about the pandemic trend by epidemiological model found that an estimated number of 510,000 British people will die if no mitigating strategy is carried out, and about 250,000 British people would also die if mitigation measures were maximized (Ferguson et al., 2020). The study predicted that the peak in mortality would occur after 3 months and, given the estimated R0 of 2.4, 81% of United Kingdom and United States populations would be infected.

Herd Immunity Lessons From Other Viruses

The length of duration of herd immunity was challenged by immune senescence, and the breadth of duration was challenged by the antigenic diversity of a pathogen (Mallory et al., 2018). Over time, the progressive loss of responsiveness to a pathogen and the decreased antibody titer or cellular responses would result in loss of immunity.

Generally, a viral species (especially RNA viruses), consists of multiple antigenically distinct variants resulted from antigenic drift, antigenic shift, and recombination (Pica and Palese, 2013; Payne, 2017). However, most RNA polymerases lack a proof-reading function to solve it. It poses challenging obstacles in eliciting broad immunity through vaccination with a single serotype of attenuated virus, as evidenced by Norovirus (Debbink et al., 2014), dengue (Midgley et al., 2011), and influenza (Wu et al., 2017). Moreover, vaccination with a single serotype may increase the severity of a secondary infection, (this is a well recognised phenomenon) which occurred in Dengue virus with four serotypes (de Alwis et al., 2014). Similarly, vaccination with the bivalent HPV vaccine caused decline in the prevalence of HPV types 16 and 18 and cross-protection against non-vaccine types HPV 31, 33, and 45, but increased prevalence of non-vaccine, non–cross-protective HPV types (Brisson et al., 2016; Cameron et al., 2016; Ribeiro et al., 2020).