COMMENT:

The Covid crisis has brought out some illogical behaviours in all of us, likely borne of fear or a lack of knowledge.

Daily I see people crossing the street to avoid a disease that spreads by tiny mucus droplets rather than evil vapours; people wearing rubber gloves for hours, inadvertently cross-contaminating everything they touch; businesses performing "deep cleans" as though Covid is radioactive dust that hides in crevices.

This is completely understandable from members of the public, because this is a new and frightening experience, dealing with a novel virus that in just a few months has killed well over 100,000 people worldwide.

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But what is less forgivable is when pundits and politicians with access to experts spread nonsensical ideas in public. Repeatedly.

In this case, the nonsense is Covid "herd immunity".

Some UK government leaders first used the term in March, suggesting that very strict social distancing measures weren't necessary. It was well-known by then that the vast majority of people with Covid would get only mild symptoms. They suggested the virus could be allowed to creep its way through the population, with the vulnerable hiding out for a while until herd immunity was achieved.

Focus: How to stop the spread of coronavirus. Video / AP / Mark Mitchell

Herd immunity would occur when there were enough recovered people in the community that Covid would have few targets left. It would burn itself out, like a grass fire reaching pavement. The vulnerable could then come out of hiding.

It sounded good in theory, but the concept was flawed to its core. It's not that herd immunity isn't effective. It can be, if we have an effective vaccine or widespread natural immunity.

Let's take measles as an example. We have an effective vaccine for measles. When 95 per cent of the population is immunised, measles loses its legs. Without lots of vulnerable people to infect, it dies off. Those who cannot be vaccinated, such as pregnant women, infants, or the immunocompromised, are protected by the rest of the community being measles-resistant.

Herd immunity can also develop naturally. For this to occur, the majority of people have to get infected, recover, and retain their immune response long-term. Herd immunity is an amazing thing, but it only happens under the right conditions.

And these aren't the right conditions.

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First off, as we all know, there is no vaccine for Covid. And Covid's a new virus, so any immune protection will have to come from the majority of people contracting the infection and surviving. Along the way, many chronically ill elderly will die, along with a few unlucky younger adults, and a sizeable number of healthcare workers.

Some pundits would see this as collateral damage. A necessary evil on the way to herd immunity.

The problem is herd immunity to Covid in 2020 was always a myth.

Even if we allowed the infection to ravage our communities and kill unfettered, there is still no guarantee that survivors would be immune. In fact, evidence from another coronavirus, Sars, showed the antibody response in survivors was short-lived, as little as one or two years. Survivors of Covid may never develop long-lasting immunity.

Without long-lasting natural immunity, or an effective vaccine, you simply cannot have herd immunity.

To suggest otherwise is to misunderstand or mislead.

And yet the phrase "herd immunity" keeps popping up in the public discourse - initially as a reason to avoid a lockdown, and lately as a reason to end effective social distancing measures prematurely.

To be clear: there may be good reasons to end a lockdown, but the wishful fiction of achieving Covid 'herd immunity' is not one of them.

• Dr Gary Payinda is an emergency doctor and Herald columnist. He thanks people for sacrificing their time, money, and independence to successfully blunt the Covid surge.
Covid19.govt.nz: The Government's official Covid-19 advisory website