What if Covid-19 immunity doesn’t last?
In the last of a series about the assumptions underlying our Covid-19 strategy, Nikki Macdonald looks at what happens if there’s no lasting immunity, or no vaccine.
The human immune system is a wondrous thing, fighting off invaders of all spots and stripes. But it’s also a little mysterious.
Right now, the shape of life in the future depends upon understanding how the body responds to the virus that has brought the world to its knees.
Governments are stuck in a terrible pick-a-path novel, in which no-one knows the ending. If we could understand whether those who have had Covid-19 are protected from reinfection, and how long that immunity lasts, that could light the way to the safest exit path.
What if there’s no natural immunity, and you can get the disease repeatedly?
The bad news for Covid-19 immunity is that the virus that causes the disease – Sars-CoV-2 – is in the same family of coronaviruses as two of those that cause the common cold.
As any miserable, sniffling cold sufferer knows, you don’t just get a cold once and then find you’re protected for life. Immunity from those two viruses wanes within a year.
Sars-CoV-2’s closest genetic relative is Sars-CoV-1, the virus that caused the devastating Sars epidemic, which infected 8098 people and killed 774. Immunity from that lasted about two years.
The reason it matters is best summed up by Harvard University researchers reporting in the journal Science, who modelled what the future might look like, depending how long immunity lasts. If protection is short-lived, Covid-19 and its sometimes deadly consequences will sit alongside ‘‘common cold’’ in our lexicon of seasonal afflictions. If it’s permanent, the disease could disappear altogether for five years.
Sweden’s controversial experiment brings the question into sharp focus. It’s banking on holding the virus back with herd immunity, rather than the lockdown favoured by most countries.
A concept usually associated with vaccination, herd immunity means enough of the population is protected against infection that the disease dies out because it struggles to find new victims.
In the context of Sweden, that means allowing most of the population to get the disease. The strategy has already cost almost 3000 deaths. But as infectious diseases specialist Dr Ayesha Verrall points out, if immunity doesn’t last, it will have been for nothing.
‘‘If you’ve gone through this outbreak and herd immunity hasn’t developed, well, that’s awful. You’ve had all of that suffering, for no gain, in terms of immunity.
‘‘I think this reflects the real difficulty facing world leaders when they choose – if they made a conscious choice – which path they go down. There’s a massive amount of assumption and uncertainty in both paths.’’
What we know about Covid-19 immunity so far is limited.
Usually when the immune system has fought off a disease, antibodies stay in the blood, providing an immune memory for the next fight.
Antibody tests exist for Covid19, but their accuracy has been patchy at best. An analysis of 14 tests by 50 scientists found only three were consistently reliable.
A Lancet paper entitled What policy makers need to know about Covid-19 protective immunity notes that 10-20 per cent of people who have had symptomatic Covid-19 have few or no detectable antibodies.
And even if someone has an army of antibodies, it’s not yet clear if they’re immune. As Auckland University vaccinologist and associate professor Helen Petousis-Harris points out, antibodies might not be the key to preventing reinfection.
‘‘It might be something you’re not measuring – because the immune response can work in a different way.’’
Even for some long-standing diseases, such as whooping cough, a blood test can’t determine immunity, she says. ‘‘Some things, you look for antibodies in their blood, you can hardly find any, but they’re perfectly immune, it’s just that the immune memory is hiding out and you’re not measuring it.
‘‘There are lots of tricky things like that. It’s about getting to know the natural immune response to this and also knowing where to look. It could be hiding behind a rock.’’
The measure of protection is called the immune correlate, and that takes time to pin down, because you need to see infected people not being reinfected.
That said, the Lancet paper says a study of Sars survivors showed about 90 per cent had virus-neutralising antibodies and about half had strong responses from another immune system warrior – the T-cell. ‘‘These observations bolster confidence in a simple view that most survivors of severe Covid19 would be expected to have protective antibodies.’’
So it’s a case of watch, wait and hope, says Otago University professor of public health Michael Baker.
‘‘Natural immunity is the great unknown. Immunity is almost certainly real, but it’s likely to be short duration, if this virus is like the other circulating coronaviruses/respiratory viruses that cause the common cold, where immunity may be only a couple of years. So that’s going to be quite an important feature that is still unknown for this virus.’’
If natural immunity doesn’t last, what does that mean for a vaccine?
Researchers have called a Covid19 vaccine ‘‘our only clear exit strategy’’. In the meantime, we have to limit our lifestyles, and keep our borders closed, to keep the disease at bay.
But what if there’s no vaccine? Petousis-Harris says even if there’s no lasting natural immunity, that doesn’t mean scientists can’t develop an effective vaccine. An artificial immune boost can take a different approach to the body’s own fighting mechanisms.
‘‘You can build a vaccine that is better at making the immune system take notice and to make a long-lasting broad response . . . I think a vaccine more likely than not. Lots of things can go wrong, but we’ve got a lot of different angles of attack.’’
Otago University infectious diseases expert Professor David
Murdoch notes no-one has successfully taken a coronavirus vaccine to market. But that doesn’t mean it’s impossible.
Some vaccines produced to fight Sars and Mers showed signs of working in early human trials. But the impetus died off as the outbreaks waned. ‘‘I’m cautiously optimistic, but there’s no guarantee,’’ Murdoch says.
Auckland University School of Medicine Professor Des Gorman is confident the lure of dollar signs will get a vaccine across the line.
‘‘Does everyone get an antibody response, and do those antibody responses prove to be protective? In general with coronaviruses, the answer is ‘yes’ and ‘yes’. The economic prize for having a vaccine in place before the next northern hemisphere winter – I mean, goodness me.’’
But if a vaccine is not possible – and no antiviral treatments emerge to treat the most deadly symptoms – the future looks dire. If you knew now that no help was on the horizon, countries would have no choice but to follow Sweden’s model.
‘‘That’s a very grim scenario,’’ Petousis-Harris says. ‘‘You’d need to be damn sure you had enough intensive care beds and ventilators, and you’d want to be able to contact-trace . . . Because if you lose control, it could get pretty ugly.’’
What’s the risk that countries with bad outbreaks get to herd immunity and return to normal, while NZ is stuck waiting for a vaccine?
The biggest question is just how many people have been infected with Covid-19. That seemingly simple number is critical to knowing what to do next.
At present, all decisionmakers have to go on is the number of reported cases. But everyone agrees that will be a massive under-estimate.
Countries such as Britain were so overwhelmed they were only testing people who needed hospital treatment. And then there are all those who have had the disease without symptoms.
Why the penetration of the disease matters goes back to herd immunity. Taking into account Covid-19’s infectiousness (every infected person infects about 2.2 others), researchers have estimated at least 60 per cent of the population would need to be protected – either through natural immunity or vaccination – to stop subsequent deadly outbreaks.
Baker says real infection numbers are ‘‘probably the biggest question globally at the moment’’. ‘‘Those countries that have been through a pretty hellish experience are hoping that they have now built up a lot of immunity and that that is going to give them some protection, and this is still one of the great unknowns – knowing what level of immunity has been achieved, and how long is it going to last.’’
In theory, countries that have suffered massive outbreaks could reach herd immunity and resume relatively normal life, while New Zealand is stuck with no immunity, haemorrhaging jobs and money while we wait for a vaccine. As American news website The Hill bluntly put it in a headline: The pandemic winner: Will It be Sweden or New Zealand?
Initial antibody surveys in the United States have suggested up to 4 per cent of the population could have been infected. However, those studies have been criticised as not being representative, and inaccurate antibody tests can produce high numbers of false positive results.
Baker doubts any country is anywhere near herd immunity yet. But even if Sweden, the US or Britain got there, it’s unlikely they would return to normality, leaving us stuck in limbo.
The Harvard study predicts that, unless immunity is permanent, the virus is likely to keep circulating. If immunity lasts less than a year, outbreaks are likely every year. If it lasts two years, outbreaks could happen every two years.
Getting to herd immunity will also take time, Baker says. Covid-19 could take up to two years to filter through global populations, unlike history’s flu pandemics, which blast through in weeks.
‘‘It’s not at all clear that the rest of the world will be able to get on with life as usual, because they’re still going to have this circulating virus . . . The most likely scenarios are that it will come back as a recurring problem at irregular intervals for a number of years before the population develops a reasonable degree of immunity, if it ever does.
‘‘So the rest of the world is just as dependent on getting a vaccine, probably even more so.’’
As Petousis-Harris points out, herd immunity doesn’t mean no cases. New Zealand has up to 90 per cent immunity from measles, but we’ve still been caught out with a serious outbreak.
‘‘They’re unlikely to get rid of it. What you see with every other infectious disease that plagues us is you have epidemics.’’
Scientists generally agree what happens in Sweden could be a gamechanger for policymakers. Gorman says the massive hit to New Zealand’s economy also has a huge social cost, so if evidence emerges that countries can control the disease with less draconian measures, that should be considered.
‘‘I think people have drawn a dichotomy between the public health issue and the economic issue. There is no dichotomy.
‘‘Most disease is socially determined and disease has social outcomes . . . The living experiment of Sweden is an important one to watch. It’s going to have a high mortality.
It’s very brave – some people would describe it as reckless.’’
Murdoch agrees we should keep a close eye on Sweden.
‘‘If there was conclusive evidence that there was development of herd immunity without stringent measures, it would make you think again. But there are so many moving parts at the moment. There’s no clear right or wrong. If another country was able to show that there was a natural peaking with evidence of herd immunity, you would have to think about a change of strategy.’’
Baker says New Zealand should be looking for evidence the virus is getting more benign, or for treatments that make it less deadly. But thinking a nice clean wave will immunise everyone overseas is ‘‘fantasy’’.
‘‘I know where I’d rather be for the next year.’’
‘‘Sweden [far left] is an important [experiment] . . . It’s very brave – some would describe it as reckless.’’