The past few months have been a hectic experience for all of us. From the growing worry in February, to the endless abyss that was March, our lives have been consumed by the realities of the coronavirus pandemic that is sweeping the globe. And despite some early hiccups, our response has been truly amazing – completely shuttering our lives in a way that seemed impossible only a few weeks prior, all to fend off this horrifying disease that is crippling the world.

But now, as we sit in our makeshift home offices, there is a light at the end of the tunnel. Case numbers are starting to fall – in Australia, we’ve seen a sustained decrease in the rate of new cases for nearly a week now – and we wonder: what next? How do we go from lockdown to life as usual?

Which brings us to herd immunity.

As the end of our Covid-19 lockdown has come into distant sight, many people have started arguing that the best way to end this social distancing purgatory is to let the virus run free, or at least partially free. They talk about something called herd immunity to bolster this idea, as if it was an easy way out rather than something horrifying that we want to avoid at all costs.

Why is it so bad? Let me explain.

Herd immunity is a simple epidemiological concept that describes the state where enough people are immune to a disease that it stops spreading in the population. Basically, if 80 people out of 100 can’t get the disease, then the 20 people who can get it will be protected because they are never actually exposed. Any outbreaks of the infection would quickly die out in this scenario.

All of this sounds rosy, but the problem with herd immunity is that it really hinges on vaccination. Without a vaccine, the only way to become immune to a disease is to get it and live through, which makes the strategy a whole lot more fatal.

If we pursue herd immunity, the best-case scenario has between 43,000 and 100,000 people in Australia dying

To see how much more fatal, let’s look at the numbers. Our initial estimates put the herd immunity threshold for Covid-19 – the proportion of people who need to be immune for the disease to stop spreading – at 60-70% of the population. So 60-70% of people need to be infected with Covid-19 for herd immunity to come into play at a minimum, although new data from the CDC indicates the number may be as high as 85%. And it’s important to note that you can’t really pick and choose these people, because unless you reach that threshold in every part of the country you’ll still see disease outbreaks. The idea that we can only infect young people is simply incorrect, firstly because that’s not how society functions, but also because it’ll create clusters of low immunity where the disease can still spread.

In Australia, that would mean roughly 15-17 million coronavirus infections. Now, we also have a very good idea of the death rate from this disease – while the case-fatality rate is quite high, the true infection-fatality rate given a health system that is not overwhelmed is likely to be around 0.3-0.6%. In other words, including all the asymptomatic cases (people who get the disease but never experience symptoms), around three to six out of every thousand infected will die. Multiplying those numbers, we can see that, if we pursue herd immunity, the best-case scenario has between 43,000 and 100,000 people in Australia dying.

That’s really quite a lot.

But remember, that’s if the health system is not overwhelmed. As many have pointed out, letting the virus run free comes with costs, one of which is that hospitals simply cannot treat everyone who is sick at the same time. For example, Sweden, which initially chose a laissez-faire attitude towards Covid-19, is now reportedly refusing ICU beds in some hospitals to people over the age of 80 or even those under 80 with comorbid conditions due to the impossibility of caring for everyone. If we take a more pessimistic view of the fatality rate, where our hospital system is struggling, we could see as many as 250,000 people dead before we reached the herd immunity threshold of 60-70%.

And this is just deaths. We still do not know what the long-term complications of Covid-19 are, but it’s fair to assume that there will be some. There’s also a huge cost to having a large proportion of your population very sick at the same time, from the difficulties of providing essential services to the nightmare of entire families falling sick simultaneously.

The simple fact is that herd immunity, without a vaccine, is not an easy way forward. Even if we do eventually reach that magical 60-70% threshold, it will have come at a staggering cost. Given the nature of the disease, the deaths and illness will be most focused on older and disadvantaged people, which is particularly worrying for anyone who cares about those over the age of 60.

Ultimately, what we do next will be more a political than a scientific decision, and Scott Morrison confirmed on Thursday that herd immunity wasn’t an approach Australia was following. But it’s worth remembering that every way forward comes with a cost. To some, herd immunity may still sound idyllic, but it requires us to sacrifice the vulnerable on the altar of the economy in truly vast numbers. I can’t speak for everyone, but personally I’d rather stay inside for a few more months and still see my parents next year.

• Gideon Meyerowitz-Katz is an epidemiologist working in chronic disease