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From Elimination to Herd Immunity?

The experience of last year [1, 2, 3] suggested quite strongly in favour of elimination as a strategy for SARS-Cov-2, rather than a suppression strategy. After the experiences of the original and alpha strains of SARS-Cov-2, it should be quite clear by the numbers that "go hard, go fast" is a successful approach. It means restricting movement and interaction between people (the virus doesn't move, people do), putting up strong fences to demarcate an area and block the potential entry of the infected.

The results speak for themselves; from Australian states like Victoria, which remarkably reached elimination from high numbers, Western Australia, and Tasmania, to countries like New Zealand, Taiwan, and China. The strategy of trying to achieve zero cases was a proven success, with the three aforementioned countries having the lowest case numbers per capita in the developed world. As a grim irony to those who recommended that a policy of suppression in fear of the economic damage that an elimination strategy would cause, the numbers do not lie; those countries and regions that adopted an elimination strategy were able to recover quickly and more completely and there suffered less economic damage than those which did not [4].

Two important changes have occurred in 2021. The first was the arrival of effective vaccines, the best of which can reduce symptomatic infection by 61-83%and mortality by 90-92% (AstraZeneca, Pfizer, respectively) [5]. The second was the arrival of the Delta variant; the Alpha variant of the virus was roughly 50% more infectious than the original strain, and Delta is 50% again, making it twice as infectious as the original. The rate of which it spread caused a massive spike in illness and mortality numbers in countries like India, Indonesia, and Thailand [6].

For health policy, these two changes have led to an extraordinary race between vaccination and the spread of the virus, of which Australia is an excellent illustrative example. Here, since June, Delta is increasingly out of control in New South Wales, whilst Victoria is struggling to reduce infections in general but especially community infections. Well worth the short thirty-minute viewing time, an ABC documentary [7] outlines the policy and detection failures in the two states, respectively. New South Wales hesitated against a hard and fast lockdown, and whilst Victoria has attempted this, the virus had escaped initial detection as it it spread among younger people who were often asymptomatic.

Whilst some political leaders have irresponsibility been heralding magic numbers (e.g., a particular percentage level) as a path out of the pandemic, even to the extent of calling a "freedom day"[8]. Sober minds and epidemiologists have been more circumspect and invariably express herd immunity with caveats of other restrictions and contact tracing. Even countries with high vaccination rates among adolescents and mature adults, such as Israel and Singapore [9, 10] are witnessing higher case numbers. What has changed are mortality, hospitalisation, and symptomatic infection numbers, with the vaccinated surviving and contributing to slowing transmission rates [11] with the unvaccinated and immuno-compromised being more susceptible; "a pandemic of the unvaccinated" as has been quipped [12].

The phrase illustrates a two-fold social problem. The first problem is that there is still a large number of people who are not vaccinated, due to either lack of availability, opportunity, or an ill-informed "vaccine hesitancy" [13]. The reality is that even with a strong intervention in public health, the resistance of the latter group threatens themselves and others. The second problem is the challenge to the resilience of individuals and communities. After more than eighteen months of this pandemic the patience of people inevitably wears thin; take a walk around one's own local community, count the people who are not wearing masks, or are not engaging in social distancing. As vaccination rates increase, so are case numbers as there is an ill-founded sense of over-confidence, as if the virus cares at all about how confident its victims are.

For public health policy, this is quite a challenge. At a certain level of vaccination herd immunity can be reached where the rate of infection is below 1; with a natural R0 estimated of 5.08 for the Delta variant [14] (with some saying as high as 8), this would require an effective vaccination rate of approximately 90% of the population to contain outbreaks if R0 = 5.08 (based on q = 1–1/R_0). As such a number is apparently difficult to reach, policymakers must decide at what level of infection and community transmission is considered politically acceptable. Frankly, the lower the better. Whilst effective vaccination does provide a potential path out from most of the effects of existing variants for most people via herd immunity, the strategy of elimination remains the one successful approach that actually works and works for all people.

Image. The R0 (reproduction number) of SARS-CoV-2 variants and other diseases. The higher the R0 number, the more contagious the disease is. Source: Imperial College London, Lancet, Australian Government.


[1] Lev Lafayette, Elimination, not Suppression, July 15, 2020
[2] Lev Lafayette, COVID-19 Options; New Zealand Hard Elimination, Sweden's Light Suppression, August 14, 2020
[3] Lev Lafayette, The Elimination Strategy, November 9, 2020
[4] Joe Hassel, Which countries have protected both health and the economy in the pandemic?, Our World in Data, September 01, 2020
[5] Doherty Modelling Report Revised 10th August 2021
[6] What the Delta variant did to South-East Asia, The Economist, September 7, 2021
[7] Jeremy Fernandez, How Australia lost control of the Delta variant, Australian Broadcasting Commision, September 10, 2021
[8] Freedom Day celebrated by revellers in England, with end of most COVID-19 restrictions
[9] Israel posts record number of COVID-19 cases on day before schools reopen. SBS News. 31 August, 2021
[10] Singapore reaches 80 pc double-vaccination rate but life is not returning to normal, 13 September 2021
[11] Delta Variant: What We Know About the Science, US Center for Disease Control and Prevention
[12] Becky Sullivan, U.S. COVID Deaths Are Rising Again. Experts Call It A 'Pandemic Of The Unvaccinated', US National Public Radio, 16 July 2021
[13] Dying in the Name of Vaccine Freedom, New York Times, August 24, 2021
[14] Ying Liu, Joacim Rocklöv, The reproductive number of the Delta variant of SARS-CoV-2 is far higher compared to the ancestral SARS-CoV-2 virus, Journal of Travel Medicine, 9 August, 2021