Herd Immunity: By the Numbers

Jim Reid (Chief Strategist, Deutsche Bank) shared a post in ZeroHedge today outlining his point of view for suppressing the Wuhan/COVID virus in the G10 in the year ahead (https://www.zerohedge.com/markets/herd-immunity-threshold-now-expected-mid-2021) Monday’s are the new Friday’s.

Beginning with AstraZeneca/Oxford Uni’s discussion of vaccine effectiveness earlier this week (70% average/90% effectiveness), his chart of the day outlines a strategy: “Japan and the UK would be first, shortly followed by the US with Australia and the EU lagging a bit but there by around the end of H1.” 

Although logistics may slow things down, remember other vaccines could come on stream and accelerate the process.

Reid’s personal view: “we’ll be getting back very close to normal life in Q2 2021.”


But there are other factors in play here which might actually accelerate the process. 

For example, Lipsitch et al, 2020 reports SARS- CoV-2 cross-reactive memory T cells, very largely CD4+ T-cells arising from previous exposure to circulating common cold coronaviruses, are detectable in ~28–50% of individuals not exposed to SARS- CoV-2. The tissue-resident memory T cells (TRM cells) can mount a fast response, with recirculating TCM and TEM T cells taking several days to start fighting an infection. CD4+ T-cells generally limit disease severity, reduce the viral burden and/or limit the duration of the disease rather than preventing an initial infection.  Their Figure 1 is reproduced below

Writing in judithcurry.com (https://judithcurry.com/2020/10/14/t-cell-cross-reactivity-and-the-herd-immunity-threshold/#more-26655), Nic Lewis further argues that two Lipsitch scenarios may be more consistent with other evidence, with implications for herd immunity of model scenario 3 (possibly involving also model scenario 2). Taking account of variation in viral dose and innate immune system strength, those implications are very likely not already taken into account in simple epidemiological models based on transmission dynamics data.

Lewis goes on to argue that ” … where the viral dose is sufficiently low, a person with cross reactive CD4+ T cells might either be infected so little that – whether or not a PCR test would be positive, at a sufficiently large cycle threshold (high sensitivity) – they not only remain asymptomatic but also have negligible infectivity. In effect, given a sufficiently low viral dose, Lipsitch et al.’s model scenario 3 might produce rather similar effects to what their model scenario 4 would do for a high viral dose”.

Notably, Lewis previously argued that the herd immunity threshold might be substantially lower, and cited Doshi (2020) in support of it being less than 60% (https://judithcurry.com/2020/09/22/herd-immunity-to-covid-19-and-pre-existing-immune-responses/).

Under those conditions, herd immunity may be evident in a growing number of geospatial pockets as early as March or April 2021.

Spring or summer 2021, depending on vaccine deployment and CD-4 immunity to Wuhan/Covid.


Peter Doshi. COVID-19: Do many people have pre-existing immunity?. BMJ 2020;370:m3563 https://dx.doi.org/10.1136/bmj.m3563

Lipsitch, M., Grad, Y.H., Sette, A. et al. Cross-reactive memory T cells and herd immunity to SARS-CoV-2. Nat Rev Immunol 20, 709–713 (2020). https://doi.org/10.1038/s41577-020-00460-4

2 thoughts on “Herd Immunity: By the Numbers

  1. Lewis said they’d reached herd immunity six months ago in Stockholm. Look at their infexrio rates now, in particular as compared to other Nordic countries.

    You may want to take what Nic says with a boulder of salt.


    1. Lewis’s May 10 SEIR analysis observed Sweden’s estimated infection rate was projected to rise to 25% by May 2020. Yet recorded new cases stopped increasing by April 11.

      In fact, Sweden’s cases resumed its upward march later in the spring after his May 10 analysis, and peaked in June before declining in the summer

      Sweden’s population is ~10 million people. As of today, there have been ~225K cases. That’s 2.5% of the population.

      By comparison, Massachusetts has a population of ~6 million people. The state has employed lockdowns and a range of interventionist policies. As of today, there are ~206K cases – a higher percentage of the population than Sweden where lockdowns are not employed.

      That simple comparison doesn’t seem to make a compelling case for government intervention.

      Anyway, I take early trajectories from any SEIR model with a lot of seasoning. What’s relevant is where the threshold of immunity begins to become apparent.

      The key point is how much immunity is needed from a range of sources – CD4, antibody, vaccination.


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