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Around the world, countries are having to strike a balance between COVID-19 cases and restrictions. In the UK and the US, daily new cases number in the thousands, but restrictions and limitations are being lifted. In contrast, New Zealand has started a short national lockdown to contain just a handful of cases.

For the past 20 months, New Zealand, Australia and several other east Asian countries have pursued tough policies aiming to completely eradicate COVID-19. The hallmarks of these “zero COVID” approaches are strict border controls and quarantine arrangements as well as the early introduction of lockdowns when discovering cases.

So far they’ve helped minimise infections and deaths. The economic impacts experienced by countries adopting these approaches have also been less severe than those who have not. New Zealand has said it intends to continue its COVID-19 eradication strategy indefinitely.

Is this sustainable? In an ideal world, completely eliminating COVID-19 is what all countries would aim to do, and earlier on in the pandemic I supported this strategy. But now the pandemic has evolved, the approach makes less sense.

Many countries now have high levels of the virus circulating and aren’t aiming to eradicate it. And it’s unlikely countries with few infections, such as New Zealand or Australia, can continue to keep COVID-19 at bay indefinitely in a world where the virus circulates.

There will always be the risk of importation by infected travellers from other regions. And in a globalised world, isolating a country from most others long term would likely be too costly and unpopular to maintain. Plus, it requires a degree of luck. Vietnam, Thailand and South Korea, once cited as zero COVID success stories, have struggled to contain the virus when it’s been imported despite various border controls being in place.

The fact that the virus is mutating may explain why they’ve found it harder to keep it at bay. The virus is becoming better at spreading in humans. The alpha variant is around 50% to 100% more transmissible than the original virus that emerged in late 2019, and delta around 50% more transmissible than alpha. The more infectious the virus becomes, the more that has to be done to enforce suppression.

There are other factors to consider too. The virus can infect both domestic and wild animals. If humans give the virus to animals, such that new viral reservoirs are created, then the virus could then potentially spill back into humans after being suppressed.

On top of this, a large proportion of human infections are asymptomatic. These infections are difficult to detect early and so are likely to spread. Both factors raise the likelihood of COVID-19 being reintroduced at some point — unless high levels of restrictions in zero COVID countries persist.

But how long will people continue to support a zero COVID approach if it means periodically entering lockdown to handle a small numbers of cases? In Australia, people appear to be growing tired of repeated restrictions, especially as the virus seems to be taking hold anyway. Trust in the government’s COVID-19 response is waning and stress is building. That said, support is still there for New Zealand’s strict approach.

An important counterpoint is that countries such as Australia and New Zealand still have low vaccine coverage. Relaxing current policies may see the virus spread quickly and cause large amounts of disruption, illness and death that could be avoided.

And although zero COVID policies are costly, one thing this pandemic has taught us is that in the short term, adopting a stringent approach leads to the least damage to the health and wealth of societies. In countries with low levels of infection and low vaccine coverage, there’s a very good case for continuing with maximum suppression.

But it’s less clear what the optimal long-term solution is. The virus has yet to settle into its ecological niche, so it’s unclear what behaviours it will eventually fall into. There are several possible outcomes, and they depend on to what degree vaccines block people from catching and spreading the virus, as opposed to simply stopping them from getting ill.

If vaccines protect well enough against infection, and enough of the population is vaccinated, then cases should subside to low levels. It may then be possible to achieve eradication of COVID-19 across much of the world through immunisation, much like measles. There will remain the risk of the virus being reintroduced from areas where cases are higher or of the virus persisting in unvaccinated groups — which is how measles behaves today.

However, it’s not known how long vaccine protection lasts, and substantial inequity in global vaccine distribution is a significant barrier to widespread suppression of COVID-19. Increasingly, the view of public health experts is that reaching population-level immunity is unachievable at the present time.

The other possibility is that vaccines do not sufficiently block the spread of infection. In this scenario, the virus would continue to circulate, but with severe disease, hospitalisations and deaths reduced. We would see periodic outbreaks and probably seasonal epidemics, similar to flu. This is the more likely scenario. The focus then would be less on trying to stop the spread of infection and more on protecting vulnerable individuals through immunisation.

Accepting that COVID-19 will become endemic — as many already are — and preparing for that eventuality may be the only realistic endgame strategy for all countries. As such, countries with low levels of infection and immunity, like Australia and New Zealand, should urgently immunise their populations. This is key if they want to avoid the considerable COVID-19 mortality and morbidity seen in Europe and the Americas.

But once this is done, continuing with recurrent lockdowns could be both socioeconomically disruptive and challenging to maintain public support for. Together with the virus being more transmissible, the near impossibility of having completely closed off borders long term, and the fact that other countries aren’t pursuing zero COVID, these factors likely will make completely eliminating the virus unachievable.