Development of Herd Immunity Against COVID-19: When Do We Know That an Adequate Level is Reached to Lift Lockdown?

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Social interaction and vaccination both contribute to development of herd immunity however development of herd immunity as a result of social interaction is directly proportional to the number of secondary infections that arise from the primary cases. Herd immunity is said to have established when a critical percentage of people in a population gets infected, when we can say that the lockdown can be lifted for the normal social life to resume. Partial herd immunity against COVID-19 can also occur in individuals who had contracted a less severe form of the virus and if individuals have been infected previously with a related family of influenza viruses.

Herd Immunity

Herd immunity’ is defined as protection against infection that a population acquires after exposure to the disease-causing germs in the normal social interactional environment or when people are inoculated with attenuated or weakened forms of the disease-causing germs by using a vaccine produced against that particular disease. In both the situations, the body develops and learns to develop antibodies for protection against any future infection by the same germs. Thus, in social interaction healthy people catch infection from infected people in the normal course of social life but in vaccination non-infected healthy people are artificially administered vaccines as a therapy to trigger the body to produce antibodies thereby preventing infection.

Thus, both ‘social interaction’ and ‘vaccination’ are significant tools in development of herd immunity against a disease in a population; the former comes at no price nor disrupts economy or society but it subjects some members of the society to negative selection pressures and thus may cost lives. On the other hand, vaccine development is time consuming and incurs huge investments of money and so is administering vaccination. Because of these contradictions, it is not easy for policy makers to formulate strategies to optimize the best use of the two tools of herd immunity development. Where to strike a balance between the ‘two’ for a minimal loss of lives and in a very fast evolving pandemic scenario like that of COVID-19 is a very difficult decision to make – if you allow ‘social interaction’ for herd immunity to develop, you keep the economy running but it can lead to high mortality hence practice of ‘social distancing becomes imperative till vaccines and therapeutics become available. Added to this is the problem of knowing exactly when adequate level of herd immunity has developed in the population to allow limited or full social interaction after the lockdown.

One of the key concerns globally at the moment with regard to COVID-19 pandemic, is knowing when herd immunity has been/will be achieved so that scheduling a time frame to resume “normal living” in each of the countries affected by the pandemic.

In the ‘Letter to the Editor’ posted on 21 March 2020 in the ‘Journal of Infection’ by Kwok KO., Florence Lai F et al., describe that the magnitude of secondary infections caused by primary cases is a useful indicator of both the risk of an epidemic and the effort required to control an infection. This is defined as the reproductive number R, that can be calculated using mathematical modelling taking into account the number of new cases being developed per unit time, number of cases getting recovered and the rate of mortality associated with the infection. Once R is known, the critical percentage of population (Pcrit) that needs to be infected to develop herd immunity can be calculated using the following formula.

Pcrit = 1-(1/R)

In addition, if a person has recently been infected with any type of influenza virus, they may become prone to less severe form of COVID-19. This can explain why some individuals who may have had a recent flu are asymptomatic and may not get a serious full blown COVID-19 disease.

Another recent study posted on 27 March 2020 in the preprint server, Kamikubo and Takahashi talk about epidemiological tools for prediction of partial herd immunity. They describe another factor that contributes to the development of herd immunity for COVID-19 when an individual contracts the disease with a less replicative and ancient form of the virus known as type S as opposed to type L (a more recent version which is capable of replicating and transmitting rapidly), it becomes partially immune to further infection with other influenza viruses as well as type L (2). The development of herd immunity can be confirmed by performing the serological tests to identify antibodies to COVID-19. This may pose a financial hurdle to developing countries but can certainly be adopted by the developed world to kick start the normal life and reduce economic losses going forward.

These studies suggest that by categorizing the population that has been previously infected and by knowing the critical percentage of people infected with COVID-19 concomitant with adequate and precise serological testing, one can formulate and adapt strategies to lift lockdown in a partial and/or complete manner so as to resume normal social life going forward.

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References:

Kwok KO., Florence Lai F et al., 2020. Herd immunity – estimating the level required to halt the COVID-19 epidemics in affected countries. Journal of Infection. Published: March 21, 2020. DOI: https://doi.org/10.1016/j.jinf.2020.03.027

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