The way that our Prime Minister and the Director General of Heath described the use of the four alert levels, and moving up and down over time with the waves, is classic herd immunity strategy. It stands in stark contrast to stamping the disease out.
No-one has specifically said that the policy is to rely on development of immunity through manipulation of infection rates. This reflects that it is the operative rules rather than stated or unstated objectives that count.
The difference between words and actions often arises because the words are aimed at maintaining social licence and avoiding panic, rather than being consistent with underlying realities. On other occasions, the discordance between words and action is because, in a context of a crisis, the Government, relying on advisers and with everyone under stress, has actually got itself in a muddle and does not understand its own strategy.
I see elements of both right now here in New Zealand. The one thing I am sure of is that, for whatever reason, the COVID-19 rules that we are all being asked to apply are consistent with a philosophy of herd immunity that is discredited in relation to COVID-19. Conversely, the rules are inconsistent with stamping the disease out. And that is real frightening.
Ten days ago, proposals for herd immunity strategies were gaining support in key parts of Europe. But then wiser counsel warned that it was an incredibly dangerous policy that could go horribly wrong. I don’t think our Government has caught up with the change in thinking.
Herd immunity is a well-known concept both in health and animal science. For each disease, once a specific proportion of immune individuals is reached, then the disease no longer transmits at a level that allows to disease to be sustained in the population. In humans, we typically aim to achieve this with vaccination.
For some diseases, such as common flu, the natural transmission rate is sufficiently low that once 30 to 50 percent of the population has immunity, then the disease dies out. This is achieved because a situation is reached where each infected person on average infects less than one other person. But typically, the disease still festers away in small sub-populations and then breaks out again when population immunity dies away.
A fundamental issue with COVID-19 is that the natural transmission rate is very high. Early estimates from Chinese data were that it might only be of the order of ‘2’. If correct, this would have meant that without intervention, each infected person on average infects two others. However, a subsequent paper from American scientists at Los Alamos has indicated that the transmission rate without intervention lies between 4.7 and 6.6. Whatever the precise number, it is now very clear that it is much higher than previously thought. This is why the number of infections has been exploding in countries like Italy, Spain and the USA, and indeed almost everywhere else in the world.
If the natural transmission rate is of the order of say five, then around 80 percent of a population have to acquire immunity before the disease dies away. There lies the rub. Even then, it will fester and breakout in sub-populations.
Our Government has been talking about waves of the disease, and turning the control knobs up and down to keep the disease at acceptable levels without society becoming totally disrupted. The problem with that is that there are big lags between shifts in the control knobs and when changes in disease levels occur.
For example, whatever our Government does today will have minimal impact on disease levels for the next week. The exception to that statement is that changes in testing level will impact on the number of confirmed cases but not the number of actual cases.
In relation to ICU cases, the delay between shifting the control knobs and any effect on the number of cases is at least two weeks and more likely three weeks. This disease starts gentle before it builds up. For death rates, the lag is also about three weeks. And somewhat like a nuclear reactor, some of the knobs may not work at all after a certain point.
The key concepts for analysing and modelling these sorts of relationships comprising stocks, flows, feedbacks and control knobs were developed way back in the 1960s by Jay Forrester at MIT in the United States and are known as ‘system dynamics’. There are various software packages which facilitate the modelling. I know something about this, having used the concepts within my own PhD in relation to growth models more than 20 years ago.
The most important contribution of system dynamics is not the specific model outputs, but as a way of thinking, and also as a way of testing different scenarios. As famous British econometrician George Box said some forty or so years ago: “all models are wrong, but some models are useful”. In particular, they can alert us as to what are some of the critical issues that influence outcomes, and the over-arching importance of uncertainty.
One of the problems with any modelling is that people with inadequate understanding look for simple answers and typically under-estimate the uncertainties and hence the risks.
In the case of trying to achieve herd immunity for COVID-19 by any other means than vaccination, the scope for making a catastrophic stuff-up is huge. And that is where we are right now.
The idea of over-70s staying in their homes is also a herd immunity concept. The idea is that younger people will get sick and then recover, and herd immunity will be achieved. At that stage the over-70s can come out again.
The first problem is that it is increasingly clear that not only over-70s die of this disease. In fact, it is apparent from China and elsewhere, but with Italy possibly an exception, that the average age of those who die is typically less than 70, largely because the biggest population groups are in younger-age cohorts.
If we are going to require over-70s to stay at home, then it is going to be many months, perhaps well over a year before population immunity is reached. In that time, a lot of under-70s are still going to die. And in the meantime, the over-70s are going to suffer huge mental health issues. I am already hearing cases of what we call ‘cabin fever’. Wherever possible, these people should be outside walking in the fresh air, interacting socially but with social distancing, just like everyone else.
The chances of population immunity protecting those who are in rest homes or in hospitals is close to zero. Young people may well be less likely to die from COVID-19, but they are definitely able to infect others. That means it is inevitable that carers will seed infections in these institutions. The Chinese experience has been that hospitals are a terrible place for the non-infected during a community outbreak, as they soon become infected.
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As each day goes by, the difficulties of stamping out COVID-19 increase. Today’s big news is that there are now four people who attended the recent World Hereford Conference in Queenstown who are now confirmed infected, with two of these being Kiwis, together with another two now back in their own countries and hence not included in the New Zealand count.
The frightening issue with the World Hereford Conference is that it is only today that attendees have become eligible for testing, despite a number of attendees reporting as being sick. This is because being sick was an insufficient trigger for being tested, and even now is only possible because all attendees are now classed as ‘close contacts’. I have been looking at the events and timing of that conference, plus pre and post conference tours, and it meets all of the preconditions for becoming a super-spreading event. The attendees are now spread to the four winds.
If we are to get things back under control then we have to tighten the screws. That means absolutely no events of more than say eight or ten people. It means no restaurants except for take-aways. It means all new arrivals hereafter must go into Government quarantine for 14 days. Of course, it also means still keeping all of our essential industries going. That includes health care and everything to do with food, together with those wonderful trades people who are at the heart of a functioning society, plus police and army who also have a role to play.
On a related tack, I have increasing confidence that we are close to having drugs that reduce the severity of this disease. Chloroquine, which is the old-time favourite for treating malaria and has saved millions of lives, looks increasingly promising. There is also a new Japanese drug called favipiravir that is claimed to have greatly reduced the intensity of the disease in a Chinese clinical trial with 340 persons. While all of this gets sorted out, we have to hold the line.
Come on! Let’s do it!
*Keith Woodford was Professor of Farm Management and Agribusiness at Lincoln University for 15 years through to 2015. He is now Principal Consultant at AgriFood Systems Ltd, and has had a longstanding interest in epidemiology. He can be contacted at email@example.com. Keith’s previous COVID-19 articles are available here.