How much does herd immunity help?

I was asked in a recent email about the potential contribution of (partial) herd immunity to controlling Covid-19.  This seemed a question that many may be asking, so here is the original question and my reply (expanded slightly).

We know that the virus burns itself out if R remains < 1.

There are 2 processes that reduce R, both operating simultaneously:

1) Containment which limits the spread of the virus.

2) Inoculation due to infection which builds herd immunity.

Why do we never hear of the second process, even though we know that both processes act together? What would your estimate be of the relative contribution of each process to reduction of R at the current state of the pandemic in Wales?

One of the UK government’s early options was (2) developing herd immunity1.  That is you let the disease play out until enough people have had it.
For Covid the natural (raw) R number is about 3 without additional voluntary or mandated measures (depends on lots of factors).   However, over time as people build immunity, some of those 3 people who would have been infected already have been.  Once about 2/3 of the community are immune the effective R number drops below 1.  That corresponds to a herd immunity level (in the UK) of about 60-70% of the population having been infected.  Of course, we do not yet know how long this immunity will last, but let’s be optimistic and assume it does.
The reason this policy was (happily) dropped in the UK was the realisation that this would need about 40 million people to catch the virus, with about 4% of these needing intensive care.  That is many, many times the normal ICU capacity, leading to (on the optimistic side) around half a million deaths, but if the health service broke under the strain many times that number!
In Spain (with one of the larger per capita outbreaks) they ran an extensive antibody testing study (that is randomly testing a large number of people whether or not they had had any clear symptoms), and found only about 5% of people showed signs of having had the virus overall, with Madrid closer to 10%.  In the UK estimates are of a similar average level (but without as good data), rising to maybe as high as 17% in London.
Nationally these figures (~5%) do make it slightly easier to control, but this is far below the reduction needed for relatively unrestricted living (as possible in New Zealand, which chose a near eradication strategy)   In London the higher level may help a little more (if it proves to offer long-term protection).  However, it is still well away from the levels needed for normal day-to-day life without still being very careful (masks, social distancing, limited social gatherings), however it does offer just a little ‘headroom’ for flexibility.  In Wales the average level is not far from the UK average, albeit higher in the hardest hit areas, so again well away from anything that would make a substantial difference.
So, as you see it is not that (2) is ignored, but, until we have an artificial vaccine to boost immunity levels, relying on herd immunity is a very high risk or high cost strategy.  Even as part of a mixed strategy, it is a fairly small effect as yet.
In the UK and Wales, to obtain even partial herd immunity we would need an outbreak ten times as large as we saw in the Spring, not a scenario I would like to contemplate 🙁
This said there are two caveats that could make things (a little) easier going forward:
1)  The figures above are largely averages, so there could be sub-communities that do get to a higher level.  By definition, the communities that have been hardest hit are those with factors (crowded accommodation, high-risk jobs, etc.) that amplify spread, so it could be that these sub-groups, whilst not getting to full herd-immunity levels, do see closer to population spread rates in future hence contributing to a lower average spread rate across society as a whole.  We would still be a long way from herd immunity, but slower spread makes test, track and trace easier, reduces local demand on health service, etc.
2)  The (relatively) low rates of spread in Africa have led to speculation (still very tentative) that there may be some levels of natural immunity from those exposed to high levels of similar viruses in the past.  However, this is still very speculative and does not seem to accord with experience from other areas of the world (e.g. Brazilian favelas), so it looks as though this is at most part of a more complex picture.
I wouldn’t hold my breath for (1) or (2), but it may be that as things develop we do see different strategies in different parts of the world depending on local conditions of housing, climate, social relationships, etc.

Update

Having written the above, I’ve just heard about the following that came out end of last week in BMJ, which suggests that there could be a significant number of mild cases that are not
detected on standard blood test as having been infected.
Burgess StephenPonsford Mark JGill DipenderAre we underestimating seroprevalence of SARS-CoV-2? https://www.bmj.com/content/370/bmj.m3364
  1. I should say the UK government now say that herd immunity was never part of their planning, but for a while they kept using the term! Here’s a BBC article about the way herd immunity influenced early UK decisions, a Guardian report that summarises some of the government documents that reveal this strategy, and a Politco article that reports on the Chief Scientific Adviser Patrick Vallance ‘s statement that he never really meant this was part of government planning.  His actual words on 12th March were “Our aim is not to stop everyone getting it, you can’t do that. And it’s not desirable, because you want to get some immunity in the population. We need to have immunity to protect ourselves from this in the future.”  Feel free to decide for yourself what ‘desirable‘ might have meant.[back]

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