As the pandemic races towards an unenviable two-year milestone, the Otago Global Health Institute’s Covid-19 Masterclass Series is bringing together a network of experts to discuss key Covid-19 topics. Today’s is the final piece in a two-week series.
Two years after the emergence of the first Covid-19 cases, we are better placed to identify paths out of the pandemic. Aotearoa New Zealand is one of the few countries that has successfully eliminated and then tightly controlled its pandemic. University of Otago experts Professor Michael Baker, Dr Amanda Kvalsvig and Professor Nick Wilson outline how we can continue to respond strategically to Covid-19 to minimise its health burden.
There are two main ways a pandemic such as Covid-19 can end: either the disease is eradicated, or it becomes endemic. Global eradication usually requires vigorous interventions, as was the case with smallpox and the cattle disease rinderpest.
In some cases, a newly introduced organism may be only moderately well adapted for human transmission. This appears to be what happened with the coronavirus that causes SARS (SARS-CoV-1).
After emerging in 2002 and causing a pandemic in 2003, it was eradicated with the last case identified in 2004. Because of the transmission dynamics of this virus, it was possible to end the pandemic using basic public health measures and before any vaccine was developed.
The other main trajectory for a new infection is that it becomes endemic. During this process the organism spreads widely and infects most people in a population. The majority mount a successful immune response and recover but some may become seriously ill and die.
The survivors are left with a degree of immunity, which may wane over time. At a point when close to everyone has been exposed, the infection can be considered endemic. It would most likely continue to cause localised outbreaks as it infects vulnerable sub-populations, particularly new cohorts of children.
In general, a new organism will continue to evolve to produce more infectious variants that generate more copies of themselves (i.e. evolution by natural selection). This process typically occurs until the organism reaches a point of maximum fitness – where it is optimally adapted to being transmitted by its human host.
This process is the likely origin of the four coronaviruses that currently cause the common cold, along with many other human infections that have emerged from animal reservoirs and become established human pathogens. Contrary to popular perception, there is no guarantee that the virulence (severity) of an emerging infection declines over time.
Influenza typically causes pandemics several times each century. These have varying impacts on human health ranging from catastrophic (in the year 1918) to relatively mild (2009). In general, the effects of a new strain of pandemic influenza take several years to subside, during which time it circulates across the globe and infects most people.
It is important to remember that endemic does not mean benign. Almost all serious infectious disease cases and deaths are caused by endemic infections. Seasonal influenza (non-pandemic) causes around 500 deaths a year in New Zealand, which is almost 2 percent of all deaths. We should certainly try to avoid allowing Covid-19 to add a similar mortality burden or worse.
Another consideration is the chronic effects of infection. Covid-19 is a multi-organ infection which appears to produce a much higher burden of long-term health harm (including long Covid) than influenza.
Our increasing capacity to determine our pandemic destiny
In the modern age we have the ability to alter the biological destiny of pandemics using a range of tools:
- Public health and social measures, notably border management, case isolation, quarantine of contacts, use of masks, ventilation and physical distancing. These measures can be used to slow the spread of a pandemic to make it more manageable, or stop it entirely as was done with SARS.
- Vaccines and therapeutics which can greatly reduce the impact of a pandemic or even end it entirely if they can interrupt transmission.
- Modern analytical and information tools, including epidemiological analysis, disease modelling, and genomics that help us track pandemics and identify the best ways of controlling them.
The key to successful pandemic control is selecting an appropriate strategy based on evidence about the emerging pathogen and societal values like equity, economic wellbeing, and freedom of movement.
We published a typology of such strategies in the BMJ last year, ranging from elimination through to suppression and mitigation. Evidence suggests that elimination performed much better than the alternatives during the first year of the pandemic. The widely divergent pandemic outcomes across the world (see Figure) are a testament to the impact of effective government strategy – or lack of it – during a public health emergency.
Now that New Zealand is in the second year of its pandemic response, we have a greater range of interventions, notably safe and highly effective vaccines. Partly because of these new interventions, the difficulty of containing the more infectious Delta variant, and the desire to open up to greater international travel, the Government decided to transition away from elimination. The resulting strategy could be described as tight suppression.
Under this strategy, the end of the Covid-19 pandemic here might be reached when the population is highly vaccinated, and the SARS-CoV-2 virus circulates in a controlled way without causing many deaths or threatening to overwhelm the health care system. Immunity could be maintained with boosters using a vaccine that is regularly reformulated to keep it effective against new variants of concern like the new Omicron variant.
Ultimately, we may decide that the optimum pandemic strategy is to return to elimination, which is how we approach other serious infectious diseases such as measles and polio. This decision could be based on consideration of the desirability of achieving this goal (particularly if Covid-19 causes a large burden of chronic illness) and its feasibility (particularly if vaccines and therapeutics become highly effective at blocking virus transmission). These considerations might even suggest that global eradication could be attempted.
Conclusions
The New Zealand experience of managing the Covid-19 pandemic reinforces a number of key lessons. It shows that in the 21st Century, pandemic outcomes are shaped by government action as much as by the characteristics of the organism.
This country greatly benefitted from having a decisive, science-informed strategic response to Covid-19. The resulting elimination strategy appears to be the optimal health (and probably economic) response to a new emerging pandemic disease with the severity of Covid-19.
We should not assume that a state of endemicity is inevitable, sustainable, or desirable. Instead we should continue to keep a close eye on new evidence and take a purposeful approach to identifying and implementing the optimal endgame strategy for New Zealand. Increasingly, we can free ourselves from the pandemic destiny endured by preceding generations.
