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Is the herd immunity dream dead?

Herd immunity has become one of the pandemic’s tortured terms. I have taken to grimacing when I read it.

What does it really mean? Most epidemiologists think of herd immunity as the level of immunity needed in a country so that a virus can no longer widely circulate. It’s not magic – the virus rarely just disappears – but it does drastically cut the number of new infections. Measles, mumps and polio, once common, are now exceedingly rare illnesses.

To calculate the herd immunity threshold, you start with the virus’s reproduction number (R0), the average number of new infections each infected person causes.

If you can vaccinate enough of the population so that number falls below 1, you will theoretically achieve herd immunity.

Masked market-goers in Wuhan, China. Authorities are trying to head off an outbreak of the scale seen in the central China city early last year.

Masked market-goers in Wuhan, China. Authorities are trying to head off an outbreak of the scale seen in the central China city early last year. Credit:Getty Images

Why is this? Because the vaccine is given in the arm, but the antibodies are needed in the nose.

“It will stop a systemic infection. But it’s hard to keep those antibodies up in the nose,” says Nobel laureate immunologist Professor Peter Doherty. “And you can’t keep high antibody levels in the nose with a vaccine you give in the arm.”

A word on this, before you worry too much: this data is just for catching the virus. The evidence we have suggests both Pfizer and AstraZeneca do a great job of protecting us from dying from the virus, which is what really matters.

To recap: Delta is more contagious and our vaccines are not 100 per cent capable of stopping it infecting us. That probably pushes us over the limit of realistically ever being able to achieve herd immunity, says Professor Russell.

“Going forward, it’s not going to be possible to eradicate the virus. It’s really quite impossible.”

As a case study, take Iceland, where the vaccination rate is over 85 per cent. The country is now experiencing its largest weekly caseload since the pandemic began, although 97 per cent of infected people have no or mild symptoms.

The Icelandic experience implies that the virus will be with us in future. There is no magic vaccine level where it disappears.

Iceland is experiencing its largest caseload of the pandemic so far.

Iceland is experiencing its largest caseload of the pandemic so far.Credit:AP

Not everyone agrees on herd immunity – or even thinks it’s the right question to ask. Modelling led by Professor Emma McBryde at James Cook University suggests we could get there if we can get a very large proportion of the population vaccinated.

Says University of Sydney epidemiologist Professor Alexandra Martiniuk: “Recent reports in as yet unpublished papers, suggest vaccination rates nearing or at 93 per cent of the total population including all children will be needed for ‘herd immunity’ if the concept even applies any more to COVID.”

Professor Robert Booy, senior professorial fellow at Australia’s National Centre for Immunisation Research and Surveillance. says he thinks the herd immunity question is out of order.

“Herd immunity is a wonderful place to aim for, but if you get close to it, you achieve what you want – and that is disease control, where any outbreak is short, small in number, easily dealt with, and does not cause death.”

Future foe scenarios

There are a lot of ideas for what happens next. Most of them are based on models and educated guesses, so we need to take them with a grain of salt at this stage.

We’ll take a look at three leading ones, which I’m calling: just another cold, COVID season, and escape mutants.

The “just another cold” scenario is the most optimistic by far. There are at least four coronaviruses circling at the moment that we don’t worry about because they cause the common cold. With time, COVID-19 might join their ranks.

The hope is that over time we will build stronger and stronger individual immunity to COVID-19. Part of this will be vaccines and regular boosters. Part of it will be catching COVID-19 when vaccinated, which should boost our immunity even higher.

“The endemic state, when people are exposed frequently, their immunity will be increasingly protective against disease from subsequent exposures, even in the face of viral evolution,” says Jennie Lavine, an infectious-disease researcher at Emory University in the US.

And part of it might be childhood infections; when kids fall sick, they develop a much stronger immune responses than adults do. Combine that with a vaccine and you might end up with very robust immunity.

Cautions Professor Booy, “We don’t know if that will happen, because COVID is so new.”

But history is promising. People infected as babies with influenza A during the 1918-19 Spanish Flu pandemic had strong immunity when the same virus started circulating in 2009 as H1N1, a paper led by Professor Booy shows. “Original immunity early in life can last a lifetime.”

Some hope COVID-19 might eventually evolve to be more contagious but less deadly. The University of Sydney’s Professor Edward Holmes, among the world’s foremost authorities on the evolution of COVID-19, says we simply cannot make that prediction yet.

“In reality, virulence can go up, down, or stay the same through time. Not at all true to say that it inevitably declines,” he says. “The most important thing I’ve learnt in this field is not to make predictions.”

COVID season

COVID-19 may become seasonal. It’s easy to believe it already is, with much of Australia again in lockdown during winter – but the evidence remains out as some countries experience surges in their summer. The virus has a whole pool of unprotected hosts it can spread to, so at this stage seasonality may not be obvious. It may take some time for it to settle into a rhythm.

If it does, COVID-19 would resemble the flu, rather than a cold. You might see an annual surge in infections and a corresponding public health campaign reminding everyone to get their boosters. You might see a constant circulation, and then in some years a particularly dangerous seasonal variant.

Prime Minister Scott Morrison is certainly hoping we’ll soon be able to treat the virus like the flu. But we probably treat the flu with a bit too little regard. On average, it kills 584 people a year.

Escape mutants

By far the darkest of the scenarios is the constant emergence of new variants that undermine our vaccine protection. There are several reasons to fear this scenario.

The virus is picking up about two new mutations a month, on average, with variants of concern evolving even faster. And SARS-CoV-2 has a spike protein that seems particularly plastic, capable of picking up new quirks that allow it to evade our vaccines.

Adding an extra wrinkle, the virus is probably able to recombine: two different variants can swap genes, leading to more diversity. These recombination events, which can also occur in animals, cause the virus to change dramatically. Influenza is a problem every year; when a recombination event produces a big change, like swine flu, we can get a pandemic.

And don’t forget the animals. SARS-CoV-2 can jump from humans into bats, cats, dogs, ferrets and hamsters, among others. In Denmark, it jumped into mink and picked up a new mutation, and then jumped back into humans, causing a fresh outbreak.

“The big unknown is whether we’re going to have a series of escape mutations,” says Professor Peter Doherty. “If we don’t get immune escape variants, we’re going to become more solidly immune.“

If COVID-19 remains out of control in any part of the world, it puts vaccinated nations at risk.

Professor Booy says there are “tens of thousands of chains of transmission happening in countries like Indonesia. They are a melting pot for finding the next better-transmissible virus. And Indonesia is a canoe ride away from Australia.”

Wrapping it up

Let’s shrug off this doom and gloom and return, for a moment, to Iceland. Iceland’s viral surge did not just happen. It happened after the country lifted domestic restrictions. This is the key point: we remain in control of the virus. We can allow it back in our community when enough people are vaccinated, when we are ready, and with some restrictions to stop the virus spreading so fast it overwhelms our healthcare system, as is now happening in Iceland.

None of these predictions suggest that COVID-19 will vanish. But we can learn to live with it, as we do many other viruses, says Professor Fiona Russell.

“We don’t stop the economy or go into lockdown for a flu epidemic,” she says.

“Once were vaccinated, and we’re minimising the death and morbidity, that’s when we need to start thinking about and responding to COVID in a similar way to other viruses that we have every year, and put it in perspective. We can’t keep living like this.”

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