Can we catch Covid-19 again, like the flu? Will it be back next year? And the year after that? Are the billions being spent on economic stimulus at risk? Epidemiologists are beginning to think just that.
A group of Harvard University epidemiologists plugged what we know about Covid-19 and its close family members into a variety of simulations in an attempt to understand our future.
The answer is we don't know enough to be sure. But it doesn't look good.
There's a good chance those who've had it could catch it again in a few short months. Eventually, the pandemic may settle down into a seasonal crisis - like the flu.
Social distancing - and the associated economic lockdowns - may become the centrepiece of every winter to come. If no vaccine is found.
"It is urgent to understand the future of severe acute respiratory syndrome–coronavirus 2 [SARS-CoV-2] transmission," the report begins.
Professor Nikolai Petrovsky, who is working on a vaccine, agrees. The devil is in the detail. But the detail we know is pointing in a disturbing direction.
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"Essentially what they show is that, over the next two years, it's going to be a constant battle - irrespective of the season," he says. "Each time you lift social isolation measures, the virus is going to come back and bite."
The research director of Vaxine Pty Ltd and professor at the College of Medicine and Public Health at Australia's Flinders University says the Harvard modelling is a call to action.
"We should be throwing everything at a vaccine," he says.
University of Melbourne epidemiologist Professor Nancy Baxter says the study is illuminating: "Irrespective of the models, one period of physical distancing unfortunately is not going to be all we need to endure to deal with this pandemic. The researchers have made a compelling argument to that effect."
Right now, everything hinges on one key question: how long does any acquired immunity last?
Scientists don't know the answer to that yet, but early reports aren't looking good.
That means we need to urgently start serological testing of those who have recovered from the disease.
Either way, the building blocks of Covid-19 mean it's not likely to go away any time soon.
Expectations of a lifting of social distancing measures early next year may be premature.
"If immunity to SARS-CoV-2 wanes in the same manner as related coronaviruses, recurrent wintertime outbreaks are likely to occur in coming years," the study finds.
Covid-19, instead, will likely be slowly settling itself down into a new routine.
"You'll see more and more reinforcement of a seasonal pattern, but that could be five years out," Petrovsky says. "And of course, that would give you a bit of a reprieve because, what they're essentially saying, is that in five years, during the summer season, you can relax a bit. But during winter, you'll have to go back into isolation."
With some nations beginning to "crest the wave" of the pandemic, some certainty on such a scenario is vital for recovery plans.
What can realistically be expected?
History is an excellent place to start.
Covid-19's closest genetic relative is Sars. Community lockdowns and drastic public health measures across Asia in 2003 not only contained the epidemic, but they also eradicated it.
Can this happen with Covid-19?
"Increasingly, public health authorities consider this scenario unlikely," the Harvard study warns.
"Alternatively, the transmission of SARS-CoV-2 could resemble that of pandemic influenza by circulating seasonally after causing an initial global wave of infection."
That means we could eventually face a new epidemic every 12 months. If immunity is a problem.
"So we know with these two other human coronaviruses that you can get reinfected after 10 months," Petrovsky says. "Now, we don't know how long Covid-19 immunity lasts: maybe it will be even shorter, or it may be longer."
The Harvard modelling takes this into account.
"Most crucially, serological studies could indicate the extent of population immunity, whether immunity wanes, and at what rate," the Harvard team say. "In our model, this rate is the key modulator of the total SARS-CoV-2 incidence in the coming years."
But the outcome is always the same: a diminishing pandemic tidal wave that settles into a seasonal pattern.
"This prospect is frankly overwhelming to me as a health professional," Baxter says. "Healthcare would change in a profound way."
Respiratory disease management and treatments - and intensive care - would need significant expansion. Not just for patient care - but to contain the risk of infecting others.
"We would essentially have to shut down the care for other conditions for a period every year, or even for a prolonged period of time. So no cancer surgery for months every year, or perhaps no hip replacements for the next two years. One of the absolutely critical things is the need to monitor long-term care facilities like a hawk – these facilities are at high risk."
Can such a future be avoided?
"The only solution to this is a vaccine," Petrovsky says. "There is no other solution."
"Infectious disease experts have been warning about the next pandemic for decades," Baxter says. "Well now we are living it, and the next pandemic [which will come because a different organism – perhaps coronavirus, perhaps influenza, perhaps something else] may be even worse."
Funding such research has never been popular with politicians: "Preventing something is hard to campaign on and easy to cut funding to," she says. "But this is what we need."
Exactly whether or not Covid-19 becomes a close companion in our future depends on several poorly understood aspects of its nature.
Exactly how contagious is it? How fast does it mutate?
The Harvard study is built upon what we know about its closest family members: Sars and Mers, and two less terrifying members bundled into the catch-all "common cold" category.
SARS-CoV-1 has a fatality rate of about 9 per cent. Mers is much higher, at 36 per cent. But both are much less contagious than Covid-19. And immunity appears to last a long time.
Family members HCoV-OC43 and HCoV-HKU1 (together the second most common cause of the common cold) are also highly contagious. But they are generally only a risk to those already very ill. But there is no long-term immunity after an infection: within a year, people are susceptible again.
"Current Covid-19 case fatality rates are estimated to lie between 0.6 per cent and 3.5 per cent", the study notes. But Covid-19 is highly contagious early in its infectious cycle, often before the onset of symptoms. And the characteristics of any immunity its sufferers gain are uncertain.
Petrovsky says much can be inferred from the known mechanics of other human coronaviruses. And that's very different to influenza.
Influenza doesn't stop an immune system from creating antibodies. Instead, it rapidly mutates so that it's unrecognisable the following year.
"Covid-19 has a totally different mechanism," Petrovsky says. "So rather than bothering to mutate and jump around old antibodies, it suppresses the immune response. It makes sure that any antibodies you make are very short-lived."
That means that after maybe 10 months or so - how long remains uncertain - you'll be susceptible to contracting the disease again.
But that, Petrovsky says, may also be Covid-19's Achilles heel.
"We don't know how seasonal this is, and we don't know how prolonged the immunity is," Baxter says.
"However we do need to all be prepared that we will not be going back to normal after this period of social distancing starts to relax.
"And for Australia, if the second wave of Covid-19 is not controlled with testing, contract-tracing, and isolation, we may need another prolonged period when we have to shelter at home with businesses shut down. The second peak could be higher than the first."
How do we keep on top of Covid-19?
According to the Harvard paper, that depends on the availability of new therapies, vaccines, aggressive contact-tracing and quarantine measures.
"In the absence of such interventions, surveillance and intermittent distancing [or sustained distancing if it is highly effective] may need to be maintained into 2022, which would present a substantial social and economic burden," the study argues.
Alternatively, hospital capacity to treat sufferers would need to expand dramatically.
But this doesn't always have to be how things are, Petrovsky says.
"This paper is not telling us the future. It is modelling scenarios. These outline several probable paths that can only become clearer once we invest in detailed research."
"Although it doesn't model the impact of a vaccine, I think they clearly infer that a vaccine is the only way out because a vaccine will create the herd immunity to stop the spread of the virus."
Covid-19 has not evolved all that much in the past four months, he says. And that's significant given an entire "generation" of the virus is just a few hours.
In the case of flu, you're susceptible to reinfection after 12 months because your antibodies have stopped working. In the case of Covid-19, it's because you no longer have antibodies.
"So it's a difference in terms of the behaviour of the virus," Petrovsky says. "From what we've seen Covid-19 hasn't mutated much over the last four or five months. It's been minimal. So it's a very stable virus. And that's promising because it may mean that we can protect people with a vaccine for far longer than the natural infection does."
Given the likely crucial role of a vaccine, Petrovsky says he cannot understand the lack of government or investor interest.
"It's been very modest relative to what they're throwing into economic stimulus packages," he says.
"And yet, you know, we need that research to better understand the virus in order to defeat it."
• Covid19.govt.nz: The Government's official Covid-19 advisory website
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