Is “Herd Immunity” indispensible for Covid-19 prevention?

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Dr. Shuvomoy Banerjee, PhD, Neucrad Health Desk, July 27, 2020

Infection and death is becoming unavoidable in case of Covid-19, as already ~10 million people have been infected and ~6 million people have lost their lives. Scientists around the world are working tirelessly to develop vaccines as quickly as feasible.

However, from time to time “Herd Immunity” term has crossed our eyes and ears. According to scientists herd immunity can be a critical weapon to fight against Covid-19 disease. The questions arise then are: 1) What is Herd Immunity? 2) How can it protect us from coronavirus-mediated infections? Let’s try to find the answers through this article:

Herd Immunity:

Herd immunity can also be called as ‘community immunity’. Simply put, when most of the people in a particular area or region develop immunity to an infection, it is called herd immunity. Interestingly, even if there may be a small number of people in the area/region who have not developed immunity to that particular disease (e.g. children, the elderly and patients with low immunity also called ‘at-risk’) but collectively, as most people developed immunity, they will get protected.

How is Herd Immunity Achieved?

According to scientific theory, hard immunity can be achieved in two ways:

1. Natural process: The human body can develop immunity in natural way. When a person is infected by an external pathogen, the immune system gets activated and starts fighting against that pathogen. Interestingly, the body makes antibodies in order to prevent re-infection. That is what happened in Brazil where herd immunity got built in a normal way and protected the people from re-infection of Zika virus.
2. Vaccination process: Vaccination produces active immunity in the body. In this method, a pathogen is inactivated or pathogen’s protein is introduced into the body. As a result, the body begins to produce antibodies against specific pathogen and prevents the infection in the future. Polio was eradicated from India through vaccination only.

Current scientific research indicates that vaccination plays a particularly important role in building herd immunity.

Scientific Analysis of Herd Immunity:

Dr. Tom Britton, a renowned scientist from the Department of Mathematics at Stockholm University in Sweden, has recently (23^rd June, 2020) published a paper in “Science” journal explaining ‘Herd immunity threshold’ by mathematical modeling. Intriguingly, the ‘Herd Immunity Threshold Model’ gives an idea of ​​the effectiveness of herd immunity against any infectious disease! Simply put, the herd immunity threshold depends on a critical parameter called R₀ or ‘Reproduction number’. By definition, R₀ is the average number of people infected in a group from an already infected person. According to Dr. Britton, Infectivity of a pathogen is directly related to its reproduction number. The higher the infectivity of a pathogen, the more people in a group need to develop immunity against such pathogenic infection.

If we do assume that the reproduction number of a pathogen is 4, then it can be said that an average of 4 healthy people can be newly infected by an existing infected person and theoretically, it is assumed that no immunity has been developed against that pathogen before.

According to the mathematical model, herd immunity threshold value can be easily calculated as below:

Herd Immunity Threshold Value = (1-1/R₀)

Henceforth, in case of the above example (R₀ =4), Herd immunity threshold value =1-1/4 = 0.75

Similarly in case of Covid-19, a group of people must first be “immune” with SARS-CoV-2 vaccine. The rest of the people, who haven’t or couldn’t get vaccinated (due to health problems etc.) will get surrounded by a circle of such “immune” people and will be protected from Covid-19 just because of “herd immunity”.

Specifically, research works have shown that the reproduction number of Covid-19 is between 2 and 3. In other words, to create hard immunity in Covid-19 infected areas, 50% to 67% of the people need to develop immunity!

Covid-19 Herd Immunity: The concerning points

In the current situation when we are hoping to build herd immunity in order to defeat Covid-19, scientists are skeptic about the extent to which herd immunity towards novel Coronavirus infection will be successful. Three main points of their concern are as following:

1. The biggest question about the recovering corona patients is if really active antibodies are produced against the novel coronavirus?
2. Even if the active antibodies are generated in the above case, how long will they be effective? The World Health Organization (WHO) has yet to say with certainty whether patients who have recovered will be re-infected or not.
3. Scientists do believe that proper immunity can be achieved through vaccination; the question is whether the Covid-19 vaccine is able to provide complete immunity or only partial immunity (as in case of influenza)?

Taking all these points into account, scientists are strictly advising to maintain social distance to break the chain of Covid-19 infection until an effective vaccine gets developed. Once a vaccination is available, priority will be to vaccinate majority of the population in order to achieve the required level of herd immunity.

References:

1. Britton T, Ball F, Trapman P. A mathematical model reveals the influence of population heterogeneity on herd immunity to SARS-CoV-2 [published online ahead of print, 2020 Jun 23]. Science. 2020;eabc6810. doi:10.1126/science.abc6810
2. Randolph HE, Barreiro LB. Herd Immunity: Understanding COVID-19. Immunity. 2020;52(5):737-741. doi:10.1016/j.immuni.2020.04.012
3. Kim TH, Johnstone J, Loeb M. Vaccine herd effect. Scand J Infect Dis. 2011;43(9):683-689. doi:10.3109/00365548.2011.582247
4. Manners C, Larios Bautista E, Sidoti H, Lopez OJ. Protective Adaptive Immunity Against Severe Acute Respiratory Syndrome Coronaviruses 2 (SARS-CoV-2) and Implications for Vaccines. Cureus. 2020;12(6):e8399. Published 2020 Jun 1. doi:10.7759/cureus.8399
5. Clemente-Suárez VJ, Hormeño-Holgado A, Jiménez M, et al. Dynamics of Population Immunity Due to the Herd Effect in the COVID-19 Pandemic. Vaccines (Basel). 2020;8(2):236. Published 2020 May 19. doi:10.3390/vaccines8020236
6. Kwok KO, Lai F, Wei WI, Wong SYS, Tang JWT. Herd immunity – estimating the level required to halt the COVID-19 epidemics in affected countries. J Infect. 2020;80(6):e32-e33. doi:10.1016/j.jinf.2020.03.027
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