This letter was submitted to the Lancet on 25 October 2020, as a reply to a letter from Alwan et al (Scientific consensus on the COVID-19 pandemic: We need to act now) that had been published online in the Lancet on 15 October and elsewhere as the John Snow Memorandum. The Lancet editors have informed me that they will wait till 15 November before collating replies to the original letter, then send any that are of interest to the authors of the original letter for a reply. Meanwhile they have no objection to my posting this as a preprint.

Paul McKeigue

Usher Institute, College of Medicine and Veterinary Medicine
University of Edinburgh

Alwan et al. [1] denounce proposals for focused protection [2] as “a dangerous fallacy”, asserting that herd immunity would “result in recurrent epidemics”, that there is no evidence for lasting immunity following natural infection, that focused protection would entail “prolonged isolation of large swathes of the population”, and that infection in younger people “risks significant morbidity” from prolonged debility. They conclude that efforts to suppress infection across the entire population should continue “until safe and effective vaccines and therapeutics arrive within the coming months”.

Herd immunity is the goal of most vaccination programs. This is the only way that people who cannot be vaccinated – for instance people with organ transplants who take immunosuppressants – can be protected from infection without being shielded from social contact for the rest of their lives. It is unreasonable to reject the possibility of achieving herd immunity through natural infection while asserting with confidence that it can be achieved through vaccination alone. There is no guarantee that safe and effective vaccines will be available “within the coming months”. Even with vaccine uptake of 90%, to reach a herd immunity threshold of 75% would require vaccine efficacy of at least 84%. For comparison, current vaccine trials are designed to detect 60% efficacy [3], and the FDA defines 50% efficacy as the criterion for success in these trials [4].

Natural infection generally confers better protection than vaccines. Among many millions with positive nucleic acid tests, only a few cases of reinfection with a different strain of SARS-CoV-2 have been documented [5]. Even if IgG antibody levels fall after infection [6], there is no evidence that this is associated with increased susceptibility to re-infection.

A more fundamental point is that the herd immunity threshold – the proportion who have to be immune for herd immunity to be achieved – is lower for natural infection than for vaccination [7]. This is because natural infection selectively immunizes the most susceptible and highly connected individuals. The conclusions of the modelling study cited by Alwan et al as showing that lockdowns were “essential” are not supported by a re-analysis that allows for this heterogeneity of susceptibility [8].

As Alwan et al. note, SARS-CoV-2 would not be eradicated by herd immunity, but would become endemic like other coronaviruses. However, as the Chief Scientific Adviser to the UK government has stated, this transition to an endemic state is the most likely outcome even with a vaccine [9].

The development of risk scoring algorithms based on medical records [10] makes it feasible to identify individuals at high risk who can be offered shielding. Focused protection does not require “prolonged isolation” of these individuals. Because SARS-CoV-2 spreads rapidly in highly connected and susceptible populations with a doubling time of only a few days, if low-risk individuals resume normal social and economic activity the expected time for the epidemic to run to completion is no more than a few months.

As yet there is no evidence that chronic debility is more common after infection with SARS-Cov-2 than with other viral infections; to establish this would require a blinded study of people who reported symptoms in the early stage of the SARS-CoV-2 epidemic, comparing current symptoms in test-positive and test-negative individuals.

Whatever current policy may be, public health professionals have a responsibility to evaluate and advise on alternative policy options calmly and rationally rather than dismissing them as “distractions that undermine an effective response”.


This work was not in any way directly or indirectly supported, funded, or sponsored by any organisation or entity. The author declares no competing interests within the submitted work. The author writes in a personal capacity.


[1] Alwan NA, Burgess RA, Ashworth S, Beale R, Bhadelia N, Bogaert D, et al. Scientific consensus on the COVID-19 pandemic: We need to act now. Lancet 2020. doi:10.1016/S0140-6736(20)32153-X.

[2] Kulldorf M, Gupta S, Bhattacharya J. The Great Barrington Declaration 2020.

[3] Mallapaty S, Ledford H. COVID-vaccine results are on the way and scientists’ concerns are growing. Nature 2020;586:16–7. doi:10.1038/d41586-020-02706-6.

[4] Food and Drug Administration. Development and Licensure of Vaccines to Prevent COVID-19; Guidance for Industry. U.S. Department of Health and Human Services; 2020.

[5] European Centre for Disease Prevention and Control. Reinfection with SARS-CoV-2 : Considerations for public health response. 2020.

[6] Ibarrondo FJ, Fulcher JA, Goodman-Meza D, Elliott J, Hofmann C, Hausner MA, et al. Rapid Decay of AntiSARS-CoV-2 Antibodies in Persons with Mild Covid-19. New England Journal of Medicine 2020;383:1085–7. doi:10.1056/NEJMc2025179.

[7] Novozhilov AS. Epidemiological Models With Parametric Heterogeneity : Deterministic Theory for Closed Populations. Mathematical Modelling of Natural Phenomena 2012;7:147–67. doi:10.1051/mmnp/20127310.

[8] Colombo M, Mellor J, Colhoun HM, Gomes MGM, McKeigue PM. Trajectory of COVID-19 epidemic in Europe. medRxiv 2020:2020.09.26.20202267. doi:10.1101/2020.09.26.20202267.

[9] Vallance P. Oral evidence: Biosecurity and national security 2020.

[10] McKeigue PM, Weir A, Bishop J, McGurnaghan SJ, Kennedy S, McAllister D, et al. Rapid Epidemiological Analysis of Comorbidities and Treatments as risk factors for COVID-19 in Scotland (REACT-SCOT): A population-based case-control study. PLOS Medicine 2020;17:e1003374. doi:10.1371/journal.pmed.1003374.