How and when will the Covid-19 pandemic end?

After basking in glorious sunshine and spending summer time outdoors, quite a few people are feeling fairly optimistic that the end of the Covid-19 pandemic is in sight. So how and when is this likely to happen?

The “when” is almost impossible to answer. Experience from the past four pandemics would suggest that viruses morph from pandemic drivers to endemic sources of disease within two years of emerging. But all of those pandemics were influenza pandemics. This time we are dealing with a novel coronavirus, which probably means we will see a different pattern.

There are significant differences between Covid infection and transmission on the one hand and influenza infection and transmission on the other. The incubation period – the time from exposure to illness – is longer with Covid. People are sick for longer and they are infectious for longer, too. The flu historian John Barry says this pandemic "is like influenza moving in very slow motion", which makes predicting the timing of an endgame hugely uncertain.

Given the most likely endpoint is that the risk of infection will be manageably low, and that Covid-19 will become another influenza, occurring seasonally and requiring annual immunisation, where to next?

Uneven vaccine access, persistent vaccine hesitancy, emerging viral variants, and global disease waves will all play a part in shaping this endgame. Writing in the Journal of the American Medical Association this month, infectious-disease experts from Emory and Brown universities, in the United States, outlined four possible scenarios for how the pandemic might end: eradication, elimination, cohabitation and conflagration.

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What would it take to eradicate Sars-CoV-2? “Both vaccine- and infection-derived immunity would have to be highly effective, long-lasting, adept at preventing secondary transmission and reinfection, and protective against all manner of present and future viral variants,” they say, adding that “eradication may prove to be too aspirational a goal even as a thought experiment, let alone as a public-health strategy”.

Elimination may well constitute a more realistic near-term goal for Sars-CoV-2, especially if booster vaccines become necessary to target emerging viral variants. But, despite early advances in some countries, without future vaccination efforts against Sars-CoV-2, durable elimination may prove infeasible, they say.

Cohabitation with Sars-CoV-2, an outcome short of all-out eradication or substantial elimination, may be acceptable. In this scenario, vaccine-mediated protection would go so far as to prevent the most severe manifestations of Covid-19, interrupt the chain of viral transmission, and counter the majority of emerging viral variants. A more tolerable endemicity may replace the volatility of the pandemic phase if cohabitation prevails.

Notwithstanding its apocalyptic undertones, conflagration is defined as a steady state characterised by a moderate level of Sars-CoV-2 endemicity. With large segments of the global population undervaccinated due to access constraints, or vaccine hesitancy, the circulation of Sars-CoV-2 would remain robust. This would give the virus continuous opportunities to replicate and create new variants that would evade our immune responses. With conflagration, ongoing outbreaks on a wider scale are likely.

Whichever scenario eventually prevails, we must put more emphasis on how we wish to respond. Until now, we have been largely guided by the expert members of Nphet. Given the most likely endpoint is that the risk of infection will be manageably low, and that Covid-19 will become another influenza, occurring seasonally and requiring annual immunisation, where to next?

It’s time for a debate on how we wish, as a society, to navigate our way to this point. Can we better balance the health of the community with the burdens of interventions? Perhaps the key point is one of proportionality. When does it become disproportionate to continue to enforce social distancing, limit public gatherings, and tell people where they can travel?

Four major forces are driving the “how” dynamic of Sars-CoV-2: human behaviour, seasonality, viral evolution and rates of immunisation in the population.

Let's add a fifth dimension – what shape do we want to put on a post-pandemic world? And in what way would we like to get there as humanely and holistically as possible?

muirishouston@irishtimes.com, muirishouston.com