Herd immunity for COVID-19 is said to be achieved when 67% of the population is immune to the virus through infection and/or vaccination, whilst the pathogen remains well-characterized (unmutated) throughout transmission in the population that is well-characterized. In the case of SARS CoV-2 infection, achievement of herd immunity is challenging due to the emergence of new variants of concern (VoC), that lead to VoC’s becoming unresponsive to the antibodies generated against the parent strain. The data shows that Israel may have achieved herd immunity as it has reached a figure of 67.7% of population that is immune while UK’s immune population is 53.9% and that of USA is 50.5%. Despite a higher infection rate in Brazil initially, herd immunity has still not been reached. This suggests that the population should adhere to social distancing, washing hands and wearing of masks and the unlock guidelines and ease of restrictions should be carefully thought of to prevent any further catastrophic event from COVID-19.
In order to reach the “normal” scenario the world was in pre-COVID-19, herd immunity needs to be developed within the population that will allow people to move out and roam freely as before. Herd immunity can either be reached by people getting naturally infected by the virus or by vaccinating a certain percentage of people. Let’s look at how vaccination and infection together can lead to herd immunity and lead us back to the life without masks and social distancing that we were living earlier.
Herd immunity1, 2 refers to an estimate of how many people have to be vaccinated or infected to ensure that the virus is no longer transmissible to humans. This means that there are no longer susceptible individuals that will get the infection and propagate them further. Although herd immunity (PI, proportion of the population that is immune) can be calculated based on a simple mathematical formula1, 2, PI = 1-1/Ro, where Ro (“R-naught”) signifies the number of secondary cases caused by the infection, also referred to as the basic reproduction number when infection happens in immunologically naïve population (population that has not been infected or vaccinated by the virus). In case of SARS CoV-2, Ro has been estimated to be around 3, which means that each person will infect an average of 3 people3, 4. On substituting this in the above formula we get a PI figure of 0.67 which means that if 67% of the population is either infected and/or has been vaccinated, then herd immunity is said to have reached.
Does that mean that countries like Israel have achieved herd immunity as 67.7% (58.2% fully vaccinated plus 9.5% infected) of the population in Israel5 are immune while countries such as the UK and the USA will achieve herd immunity once they have 67% of their population is either infected and/or has been vaccinated, which is currently at 53.9% (47.3% fully vaccinated plus 6.6% infected) in the UK6, and 50.5% (40.5% fully vaccinated plus 10% infected) in the USA7?
It is difficult to answer this question because the calculation of herd immunity (PI) is based on assumptions that the pathogen is a well characterized one and it is infecting a well-characterized population. Unfortunately, both are not true in this case as this is a novel virus and the population being infected is very heterogeneous. It is further complicated by the fact that there are new variants of the SARS CoV-2 virus appearing in the population which may or may not respond to the vaccine in the same manner as the original virus strain against which vaccine has been designed. Moreover, the new variants of the virus are not even the same affecting all the countries. While UK predominantly has the B.1.1.7 variant, India, Singapore and other countries have the B1.617 variant, Brazil has B.1.351, P.1 and the P.2 variant while Middle East has the B.1.351 variant in addition to others. Does that mean that more people are getting infected by the new variants irrespective of being vaccinated against the original strain pushing Ro to a higher number? A Ro of 5 would mean that 80% of the population should be immune to prevent further infection. Nevertheless, these countries (Israel, UK and the USA) have started to unlock and lift the restrictions based on the fact that at least 50% of their population is fully vaccinated. Is it too early in the cases of UK and USA as PI has not reached even 67% based on the simple calculation with the assumptions mentioned above? Israel can still boast of saying it has reached this number. However, there has been an increase in the number of cases in the UK this week by 23.3% (compared to the previous week) with concomitant increase in mortality as well6, while in USA, there has been a decrease in the number of cases by 22% this week7 (compared to previous week). The data over the next few months will determine whether the decision of these countries to unlock and lift restrictions was correct or not?
With all these factors related to the complexity of the virus (different strains) along with the population heterogeneity, it is impossible to predict the correct PI number. It is worth mentioning here about the infection rates in Brazil, one of the worst affected countries in the initial stages pf COVID-19 infection. Despite a high percentage of estimated seroprevalence (76%)11 in Manaus and 70% in Peru12, both are witnessing a fierce second wave. While this may partly be attributed to ease of restrictions and elections that were held, numerous other factors could be responsible for the same. One could be the overestimation of the seroprevalence which was observed to be 52.5% in June 2020. Second could be the advent of new and more transmissible strains (P.1, P.2, B.1.351, B.1.1.7), each with its own unique set of mutations that causes high disease severity. Thirdly, the presence of these mutations may also lead to evading the immune response generated against the original strain12.
Another question is about the efficacy of the currently available vaccines in terms of the protection they can offer. It has been estimated on an average that the vaccine efficacy in terms of protection against deaths is 72%8 which means that there is a 28% chance of an individual dying even after being fully vaccinated (after taking the required doses of the vaccine). More specifically, Pfizer-BioNTech BNT162b2 was 85% effective after a single dose while Oxford-AstraZeneca ChAdOx1-S vaccine was 80% effective after a single dose9. Both these vaccines were also effective against the B.1.1.7 strain9. Another important point to keep in mind here is that vaccination does not mean that you won’t get infected with the pathogen, it means that you will be protected as mentioned above and will develop either mild or no symptoms of the disease. Furthermore, there is no evidence yet that the immunity provided by infection and/or vaccines against SARS CoV-2 is long lasting or not?10 This means that there needs to be proper surveillance in place and the vaccination program may need to be extended should this may be the case.
In addition to the achievement of herd immunity by the population by infection and by virtue of full vaccination, certain individuals are still likely to be affected and suffer morbidity or even mortality attributed to COVID-19. Such people can be identified using Electronic Health Records (EHRs) and provided appropriate preventive care as described13.
In summary, predicting herd immunity for SARS CoV-2 is an unsurmountable challenge due to the nature of the mutations acquired by the virus that are making it more transmissible coupled with the heterogeneous population that is being infected. It is surmised that until Ro becomes closer to or less than 1 (that means achieving a herd immunity of 100%), the population should continue to adhere to measures of social distancing, washing hands whenever possible and wearing masks in public in order to avoid contracting the disease. This means that countries should think thoroughly before deciding to ease down restrictions before achievement of 100% herd immunity (on the safer side) to avoid more catastrophic events caused by COVID-19.
- McDermott A. Core Concept: Herd immunity is an important—and often misunderstood—public health phenomenon. Proc. Natl. Acad. Sci. 118 (21), (2021). DOI: https://doi.org/10.1073/pnas.2107692118
- Kadkhoda K. Herd Immunity to COVID-19: Alluring and Elusive, American Journal of Clinical Pathology, 155 (4), 471-472, (2021). DOI: https://doi.org/10.1093/ajcp/aqaa272
- Liu Y, Gayle AA, Wilder-Smith A, Rocklöv J. The reproductive number of COVID-19 is higher compared to SARS coronavirus. J Travel Med. 2020 Mar 13;27(2): taaa021. DOI: https://doi.org/10.1093/jtm/taaa021 . PMID: 32052846; PMCID: PMC7074654.
- Billah M A, Miah, M M, Khan M N. Reproductive number of coronavirus: A systematic review and meta-analysis based on global level evidence. PLoS One 15, (2020). Published: November 11, 2020. DOI: https://doi.org/10.1371/journal.pone.0242128
- Ministry of Health Israeli Government. Press release – Israel to Lift All Coronavirus Restrictions. Publish Date 23.05.2021. Available online at https://www.gov.il/en/departments/news/23052021-02
- Gov.UK – Coronavirus (COVID-19) in the UK. Available online at https://coronavirus.data.gov.uk
- CDC COVID Data Tracker – COVID-19 Vaccinations in the United States. Available online at https://covid.cdc.gov/covid-data-tracker/#vaccinations
- Jablonska K, Aballea S, Toumi M. The real-life impact of vaccination on COVID-19 mortality in Europe and Israel medRxiv (2021). DOI:https://doi.org/10.1101/2021.05.26.21257844
- Effectiveness of the Pfizer-BioNTech and Oxford-AstraZeneca vaccines on covid-19 related symptoms, hospital admissions, and mortality in older adults in England: test negative case-control study BMJ, 373, (2021). DOI: https://doi.org/10.1136/bmj.n1088
- Pennington T H. Herd immunity: could it bring the COVID-19 pandemic to an end? Future Microbiology, 16 (6), (2021). DOI: https://doi.org/10.2217/fmb-2020-0293
- Buss L F, Prete C A, Abrahim C M M et al. Three-quarters attack rate of SARS-CoV-2 in the Brazilian Amazon during a largely unmitigated epidemic. Science. 371, 288-292, (2020). DOI: https://doi.org/10.1126/science.abe9728
- Sabino E., Buss L., et al. 2021. Resurgence of COVID-19 in Manaus, Brazil, despite high seroprevalence. (2021). DOI:https://doi.org/10.1016/S0140-6736(21)00183-5
- Estiri H., Strasser Z H, Klann J G et al. Predicting COVID-19 mortality with electronic medical records. npj Digit. Med. 4, 15 (2021). DOI: https://doi.org/10.1038/s41746-021-00383-x
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