It's hard to predict things in a pandemic. The situation changes so much on a daily basis that everything you thought you knew last week is wrong by the end of the day. Things are changing so fast that even the solid certainties that we thought we were sure of – the reproductive rate, the symptoms of the infection, the key to making a good quarantine – are suspect and need to be re-evaluated.

But among all this uncertainty, I can say for sure that there is one thing that I would never have seen coming: the discussion about herd immunity. It is so out of the blue that the first time a journalist asked my opinion on whether it was effective for the coronavirus, I literally laughed out loud because I assumed they were joking.

And yet, here we are. Countless articles and think-pieces on the COVID-19 virus are making the argument that, albeit potentially risky, achieving herd immunity could be one response to our crisis. Many of them frame herd immunity as a preventive strategy that may stall the tidal wave of disease so many are predicting.

All of this is simply nonsense. Herd immunity without a vaccine is by definition not a preventative measure.

Let me explain.

Herd immunity is an epidemiological concept that describes the state where a population – usually of people – is sufficiently immune to a disease that the infection will not spread within that group. In other words, enough people can't get the disease – either through vaccination or natural immunity – that the people who are vulnerable are protected.

For example, let's think about mumps. Mumps is a very infectious disease that, while relatively benign, is very uncomfortable and sometimes causes nasty life-long complications. It's also vaccine-preventable, with a highly effective vaccine that has made the disease incredibly rare in the modern age.

Mumps has a basic reproductive rate (R0) of 10-12, which means that in a population which is entirely susceptible – meaning no one is immune to the virus – every person who is infected will pass the disease on to 10-12 people.

This means that without vaccination roughly 95 percent of the population gets infected over time. But even with something that is this infectious, there are still some people – 5 percent of the population – who don't get sick, because once everyone else is immune there's no one to catch the disease from.

We can increase that number by vaccinating, because vaccination makes people immune to infection, but it also stops infected people passing on the disease to everyone that they otherwise would. If we can get enough people immune to the disease, then it will stop spreading in the population.

And that's herd immunity, in a nutshell.

For mumps, you need 92 percent of the population to be immune for the disease to stop spreading entirely. This is what's known as the herd immunity threshold. COVID-19 is, fortunately, much less infectious than mumps, with an estimated R0 of roughly 3.

With this number, the proportion of people who need to be infected is lower but still high, sitting at around 70 percent of the entire population.

Which brings us to why herd immunity could never be considered a preventative measure.

If 70 percent of your population is infected with a disease, it is by definition not prevention. How can it be? Most of the people in your country are sick! And the hopeful nonsense that you can reach that 70 percent by just infecting young people is simply absurd. If only young people are immune, you'd have clusters of older people with no immunity at all, making it incredibly risky for anyone over a certain age to leave their house lest they get infected, forever.

It's also worth thinking about the repercussions of this disastrous scenario – the best estimates put COVID-19 infection fatality rate at around 0.5-1 percent. If 70 percent of an entire population gets sick, that means that between 0.35-0.7 percent of everyone in a country could die, which is a catastrophic outcome.

With something like 10 percent of all infections needing to be hospitalised, you'd also see an enormous number of people very sick, which has huge implications for the country as well.

The sad fact is that herd immunity just isn't a solution to our pandemic woes. Yes, it may eventually happen anyway, but hoping that it will save us all is just not realistic. The time to discuss herd immunity is when we have a vaccine developed, and not one second earlier, because at that point we will be able to really stop the epidemic in its tracks.

Until we have a vaccine, anyone talking about herd immunity as a preventative strategy for COVID-19 is simply wrong. Fortunately, there are other ways of preventing infections from spreading, which all boil down to avoiding people who are sick.

So stay home, stay safe, and practice physical distancing as much as possible.

Gideon Meyerowitz-Katz is an epidemiologist working in chronic disease in Sydney, Australia. He writes a regular health blog covering science communication, public health, and what that new study you've read about actually means.

Opinions expressed in this article don't necessarily reflect the views of ScienceAlert editorial staff.