Herd immunity

Delaying the First Lockdown May Have Saved Lives

Dr Raghib Ali, a Senior Clinical Research Associate at the MRC Epidemiology Unit, University of Cambridge and an Honorary Consultant Physician in Acute Medicine at the Oxford University Hospitals NHS Trust, has written a comment piece for the Telegraph in which he argues that if the UK had locked down earlier, it might have meant fewer people dying in the first wave, but that saving would have been more than offset by a greater number of people dying in the second wave.

It is true that two countries – Norway and Finland – who locked down a week before the UK (and closed their borders) have been remarkably successful in having both small first and second waves and no excess mortality to date.

But they are the exceptions – what happened in many other countries in Europe who also locked down (and closed their borders) at the same time is that they did have very small first waves in Spring 2020 but this was followed by much larger second waves in autumn/winter 2021 (and now into spring 2021, too).

And this has happened despite second and third lockdowns in many of these countries as people understandably struggled to maintain compliance with restrictions for months on end.

Dr Ali’s argument is that suppressing infections more effectively in the spring of last year would have meant more people becoming infected in the autumn and winter when the NHS would have been less able to cope. Thanks to the comparatively high number of infections in the spring, the British population had more herd immunity going into the autumn and winter which meant a lower rate of infection than in other parts of Europe.

Worth reading in full.

What Happened in South Dakota?

We’re publishing a piece today by Dr Noah Carl, an independent scholar, on South Dakota. As Noah points out, South Dakota had some of the lightest restrictions in the Western world and its death toll is high compared to other US states – the eighth highest, in fact. But it has also seen cases decline rapidly since November in spite of the Governor’s laissez-faire approach, which is puzzling given that the herd immunity threshold doesn’t appear to have been reached. In the following Extract, Noah speculates about why this could be.

So, why did case numbers fall in South Dakota? I’m afraid I don’t have the answer. But here are a few possibilities. First, the herd immunity threshold is lower than 66%. This could be because the threshold has been overestimated in general, or because it is lower specifically in South Dakota, perhaps due to the state’s geography.

Second, the Google mobility index is a poor measure of the behaviours that drive transmission (as Philippe Lemoine has suggested). Perhaps South Dakotans were extra careful to practice social distancing during the month of November, even though they didn’t stop going out for retail and recreation. Weighing against this interpretation is the fact that there were dramatic changes at the start of the pandemic. Notice the precipitous decline in the retail index, and concomitant rise in the residential index, on the left-hand side of the chart.

Third, the level of immunity at which cases start declining (even if true herd immunity has not yet been reached) is much lower than 66%. This could be the case if there is substantial heterogeneity in the behaviours that drive transmission. Suppose that 80% of infections are caused by 20% of people. (Perhaps these ‘super-spreaders’ are particularly sociable, careless, or likely to interact with others by nature of their work.) Once a large enough share of the 20% has been infected, case numbers may begin falling rapidly. (This point has been made by David Dowdy.)

Worth reading in full.

Noah’s piece originally appeared in his Substack newsletter, which is worth subscribing to. He writes regularly about the pandemic.

Has South Africa Reached Herd Immunity?

Last month a pre-print was published that showed COVID-19 antibodies in South Africa had reached remarkably high levels. It was ignored by most of the media, but given the concern over the South Africa variant being “more transmissible” and “evading vaccine immunity” it shouldn’t have been as it gives an indication of what we might expect from the variant.

Extrapolating from antibody testing on blood donors, the researchers found antibody levels of 63% in Eastern Cape province (EC), 46% in Free State (FS), 52% in KwaZulu Natal (ZN) and 32% in Northern Cape (NC). These figures were between 15 and 22 times higher than the percentage of the population that had tested positive for the virus to date.