Written by Dr David Owens
The impact of any infectious illnesses must be assessed in terms of information relating to the disease and information relating to the epidemic. For individuals it is the impact of the disease that matters, in population terms the impact of infectious disease will be driven by the epidemic process. Ultimately control of all epidemics requires a balance between the cost of the disease (human and economic) and the economic and political costs of the public health interventions.
What is an elimination strategy?
I previously described the Covid-19 epidemic using the analogy of a fire. I will continue this theme to illustrate the transition from the current elimination (Zero Covid) phase to that of ongoing management which involves a shift towards ongoing control and mitigation.
When this fire started to rage in January 2020 we didn’t (and still don’t) have methods to put it out (drugs to treat the illness) so we needed to focus on containment. The fire will need to burn out by itself. This is how the public health measures work. We build a wall around the fire to contain it. As often happens in this situation, some embers escape. Some land on the ground and burn out (this is the majority of cases of Covid, probably around 70%) but others start smaller, little fires. There is a danger that an ember lands in a particularly dry and vulnerable place and triggers another larger fire (or series of small fires). We know that 10 - 20% of embers from Covid cause 80% of future fires and smaller numbers (the super spreaders) can trigger huge infernos. How well the fires are contained in new locations will depend on a number of factors, including population immunity, environmental and political factors that influence the ability of the medical firemen to do their job of containment and the capacity of the health system to manage the resulting complications of the illness. Infectious diseases (fire in this analogy) always do the greatest damage in poor communities with low capacity health systems.
When the fire first broke out the containment response meant stopping people around the fires from moving so that they couldn't spread the fire (restriction of movement and quarantine). The principle then would be to do the same thing around all the smaller fires. The containment response also involved wearing of masks, social distancing and reducing population mixing (restricting crowds and working from home). Early in the process it seemed reasonable to advise people to not move and stay in their fireproof homes. The firemen (doctors, nurses, workers in energy and transport) need to keep working throughout. But what happens if the fire seems to be coming under control or what if the fire seems to be dangerous but only in certain circumstances? It is well recognised that poverty is the single biggest factor that determines health in population terms. At some point, we need to keep the economy functioning or more people will die as a result of the economic fallout than from the impact of the disease. This is the dilemma of negotiating the transition from the early aggressive containment phase to managed containment and eventually, depending on the unfolding scenario of the epidemic, either back to the old normal, or the new normal of control and mitigation.
Methods of containment invariably involve political decisions. Restriction of movement and quarantine are decisions which balance the rights of the individual against the rights of the majority. This process can be critiqued in philosophical and political terms. There have been criticisms that public health interventions have been both slow and inadequate but also that they have been excessive. The historical bias of public health experts including the WHO was that once a disease had broken out of geographic boundaries the focus should shift to mitigation. There was an unwritten rule that we don’t close borders as the economic impact of such measures would invariably lead to higher disease impact. What we saw with Covid was essentially a divide between countries that continued with an elimination strategy with a focus on aggressive and early control of community transmission and those which transitioned to early mitigation. Continuing the metaphor, the traditional view was that once the fires had escaped, the best outcome was to let the fires burn whilst maintaining economic activity and living as normal a life as possible.
In general countries which continued an elimination strategy invoked early border controls and put significant resources into testing capacity. The focus was on control of epidemic size with test, trace and isolate as the core goals. They put a huge wall around their territory and put significant resources into finding and testing every ember or potential ember and putting barriers around areas at potential risk of fire even if this meant locking up people who never posed a risk. 95% of close contacts quarantined in Hong Kong never develop the infection. Countries which eschewed elimination strategies varied between levels of mitigation from masks, social distancing and intermittent lockdowns (a suppress and lift strategy) through to open borders and letting the fire burn through.
Do elimination strategies work?
In short, the evidence now shows that elimination strategies do indeed work. Hong Kong is a prime example of an effective response.
A recent review in the Lancet compared the health and economic outcomes of OECD countries that had pursued an elimination strategy in comparison to those which focused on early mitigation. Elimination was associated with a 25 fold reduction in population mortality in addition to better measures of economic performance. One historical argument against an elimination strategy is the potential restriction of civil liberties. In this analysis, the authors assessed that elimination ultimately resulted in less restrictions on individuals in comparison to mitigation. This is easily appreciated by considering the relative inconvenience to life in Hong Kong of quarantine risk and limited restaurant capacity in comparison to the extended lockdowns common in much of the world.
So is Zero Covid a potential long term strategy?
Although desirable, Zero Covid is not a viable long term solution to the pandemic. The evidence would suggest that countries which excelled in implementing Zero Covid as a short term policy, now need to focus on boosting vaccine uptake, and therefore immunity in populations.
Elimination is ultimately a short term strategy. Unless Covid dies, which is considered unlikely now, the only possible long term solution is the development of population immunity. This can only happen in one of two ways. Either we allow this disease to burn through or we develop high levels of vaccine coverage. The goal of elimination is to buy time until an effective vaccine can be developed in order to establish population immunity. Metaphorically, vaccination is like spraying water on the ground. We can prevent the fire from catching. If we can cover enough of the ground (herd immunity) we can also protect dry and vulnerable areas which we can not otherwise reach.
The recent surge of cases in Taiwan and Singapore have shown how brittle elimination is as a long term strategy. The evolving variants are more transmissible. The fires are now throwing off more and larger embers and the ground is getting drier. Vaccines are available, they are safe and highly effective yet we are facing high levels of vaccine hesitancy. I discussed this issue recently in a podcast with Professor Cowling. Elimination has served Hong Kong well, it has been a very effective strategy. However, repeating the simple analogy from the podcast we are now sitting in a tinder box covered in petrol and large numbers of people are not smelling the petrol because they are all wearing masks.
So how does Hong Kong get back to normal?
Managing this process involves accepting the evolving risk/benefit in the context of the competing needs of the population. This requires a change in messaging and a clear explanation of the reasons for the changing public health strategy.
Hong Kong has done very well during the pandemic in comparison to most countries. A Hong Kong resident is 70 times less likely to have died of Covid than a resident of the UK or USA. We have also, in the vast majority of cases, had less impact on our daily lives. However, we are facing a difficult choice in the context of high vaccine hesitancy. How long do we continue with a zero Covid strategy, accepting both its brittle nature and likely long term economic costs?
Early in this process the arguments for public health controls in the strategy of elimination were based around the social contract. We were all in the same boat. The restriction of civil liberties was an acceptable cost for the greater good. Time has passed by. Hong Kong has procured adequate supplies of effective vaccinations. There is a highly efficient system in place to provide vaccinations free of charge. For the vast majority of people there are no barriers to entry to vaccination. There are high levels of vaccine hesitancy. We are no longer all in the same boat. An individual who is vaccinated poses almost no risk to a non-vaccinated person because the vaccinations have been shown to reduce both infections and transmission. An individual who is not vaccinated poses almost no risk to somebody who has been vaccinated because the vaccinated person is immune. Therefore, in reality, the unvaccinated pose the risk to the unvaccinated. It is true that not everybody can be vaccinated. Children cannot yet be vaccinated but the risk of serious illness in young people is extremely small.
Zero Covid was always intended to be a short term strategy. The countries which have pursued this strategy have been shown to be correct so far. However, we are only at the end of the first act and without high vaccine coverage Hong Kong remains vulnerable. We are now faced with a dilemma. We either continue with the current elimination strategy or we accept that we can never completely eliminate risk and transition towards mitigation using evidence-based risk management strategies. It seems likely that the negative messaging and actions around the current elimination strategy are adversely impacting vaccine uptake. All health interventions involve a risk-benefit analysis. Decisions which ostensibly reduce population risk, such as lengthening quarantine or persisting with public health restrictions may well increase harm by adversely impacting vaccine uptake and negatively impacting population immunity. What is the incentive for having a vaccination if a vaccinated individual is treated as if they present the same risk as a non-vaccinated person? At the same time what is the incentive of having a vaccination if there is no immediate risk of disease in the community? This second question requires continued education and positive messaging around risk-benefit. In our recent survey, the largest discriminator in belief between vaccinated and vaccine-hesitant individuals was the belief in the importance of high population immunity.
Zero Covid has been a very successful strategy for Hong Kong but it was always a waypoint on the journey to population immunity via vaccination. Increasing vaccine uptake involves not only education but the explicit recognition that absolute risk can never be eliminated, it can only be mitigated. The recent announcement of reduced quarantine for antibody-positive arrivals is an early, cautious move towards risk-based mitigation. In Hong Kong we discharge Covid patients from hospital on the basis of evidence of immunity, why should vaccine-induced immunity be treated any differently? Treating people who are vaccinated as if they are immune poses a minimal threat to the population and is likely to increase vaccine uptake by providing measurable benefits to vaccinated individuals. This will require a transition to consistent positive messaging around the benefits of vaccination and a progressive reduction in public health restrictions including masks and quarantine for individuals who are vaccinated.
Returning to normal will be determined by an increasing understanding and recognition of the balance between the threat of the disease and the cost of the public health controls in both human and economic terms. The move to local mitigation will involve a shift in messaging and will require education and communication. It will require an explanation of the changing public health strategy. This will involve taking the wall from around Hong Kong and accepting that every individual has the option to protect not only themselves but also other more vulnerable members of the community by getting vaccinated. Little fires may continue to break out but we are fortunate to have one of the best medical fire departments in the world. We will all eventually, both literally and metaphorically need to take our masks off, start smiling at each other and focus on health not disease.
1. Oliu-Barton M; Pradelski BSR; Aghion P; Artus P; Kickbusch I; Lazarus JV; Sridhar D; Vanderslott S; (n.d.). SARS-CoV-2 elimination, not mitigation, creates best outcomes for health, the economy, and civil liberties. Lancet (London, England). https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(21)00978-8/fulltext