In my paper Love in the time of Coronavirus I suggested that an option for managing Covid-19 was to sequester the vulnerable in isolation and allow the remainder of the population to achieve 'Natural Herd Immunity'.

Both the UK and Sweden announced that this was the strategy they preferred although the UK was soon equivocal.

The other option I suggested was isolation of every case with comprehensive contact tracing and testing; supported by closed borders to all but essential travellers and strict quarantine.   

New Zealand; South Korea; Taiwan; Vietnam and, with reservations, Australia opted for this course - along with several other countries, including China - accepting the economic and social costs involved in saving tens of thousands of lives as the lesser of two evils.  

Yet this is a gamble as these populations will remain totally vulnerable until a vaccine is available and distributed to sufficient people to confer 'Herd Immunity'.

In the event, every country in which the virus has taken hold has been obliged to implement some degree of social distancing to manage the number of deaths and has thus suffered the corresponding economic costs of jobs lost or suspended; rents unpaid; incomes lost; and as yet unquantified psychological injury.

Those countries that failed to act quickly to close their borders; quarantine travellers; and treat every case as a critical emergency (with full isolation and comprehensive contact tracing) now have so many infections that they are attempting to 'close the stable door after horse has bolted'.  It's too late to track down and isolate all those infected.

As with other viruses that plague humanity, the Covid-19 pandemic will continue until Herd Immunity is reached.

There are many viruses, already circulating at low levels, for which 'Natural Herd Immunity' has already been achieved. Some, like the four corona viruses responsible for the common cold, are relatively benign or asymptomatic and are already endemic in most human populations.

Nastier, very infectious viruses like measles; mumps; herpes; polio and so on have been brought under control by vaccines.  It was no longer necessary for people to catch many of these for the community to gain Herd Immunity after safe, effective, vaccines were developed and distributed to sufficient people.

A vaccine for Covid-19 is under development and should achieve a similar outcome - hopefully some time in the next two years. 

In the absence of a vaccine, many countries are now heading down the Natural Herd Immunity path. 

Take, for example, Spain, the country that, at the time of writing, in early June 2020, has the highest number of deaths per million people. Spain has a population of 47 million with 27,940 deaths. Spain has 239,638 reported cases of Covid-19, resulting in an apparent case fatality rate (CFR) of close to 12%

While different populations have differing levels of overall health and life expectancy and pre-existing conditions are the biggest factor influencing Covid-19 mortality it's  improbable that the actual CFR in Spain is anywhere near this level.  Where there has been universal testing of a contained population, as on cruise ships, the CFR has been less than 1% and in Australia where testing is widespread and new cases  are below 10 per day the CFR is 1.43%. Most experts put the CFR of the present varient at around 0.8 but it's mutating and this could change for the better or worse.

For a period Spain's health system was overwhelmed but even in countries where case numbers were manageable patients sick enough to be put on a ventilator frequently died so that's unlikely to be the reason for the extraordinary CFR.

So the very high reported CFR in Spain is most probably due to insufficient testing and unnoticed or unreported cases and it's unlikely that Spain has a real CFR much above 1%.

Why is this important?

Because it means that Spain has had many more cases than they know about and many more unnoticed or unreported cases still circulating than they have detected.  As at June1 Spain is still suffering around 2000 deaths a week. Thus even if the real CFR is say 1% (100 cases per death) Spain may have as many as a hundred thousand, presently infected, people wandering around feeling slightly ill or totally oblivious.

At the moment, strict social distancing has the infection under control but if Spain returns to anything like the social and economic practices of last February with: crowded public transport, bars & clubs, sporting events and crowded public transport and those late night street parties, the virus will inevitably begin to multiply rapidly again.  In February and March 2020 Spain experienced ten doublings in less than a month, rapidly overwhelming the health system and exacerbating the death rate.  So a 'second wave' is a real possibility.

In the meantime the economic, social and psychological costs of such a long lockdown could well cost many lives due to other causes and condemn the, already struggling, economy to crippling debt and the Spanish people to terrible poverty for decades.

So one option is to open the economy again regardless. 

As several commentators have remarked, particularly in Sweden, slowing the rate is simply deferring the inevitable. And delaying these deaths needs to be balanced against the economic, social and psychological costs suffered by the healthy, in addition to the longer term debts to be made good by future generations. 

How bad could the death toll be? Most experts expect that the virus will be like others that are now endemic and will stabilise at a low background rate when Herd Immunity is reached, as did the 1918-19 Spanish Influenza.

There's a simple sum: Covid-19 seems to kill just under 1% of people who get it. To reach Herd Immunity most pundits say around 60 to 70% need to get it and survive (the Herd Immunity rate for measles is 90%; for flu about 60%). Thus, in the absence of a vaccine, Herd Immunity will not be reached until at least 0.55% of any population (5,500 in a million) in which it becomes endemic (not just Spain) have died.

Yet, as we have seen above, the death rate in Spain is currently around one tenth of this. So Spain is nowhere near Herd Immunity and without a vaccine Herd Immunity will not be achieved until around 260,000 Spaniards have died. 

And here Spain's leaders, like those in many other countries in which the virus is only constrained by a lock-down, now face the unenviable choice between the Scylla and Charybdis that I referred to in Love in the time of Coronavirus.

The same scenario applies to any country where the virus has become widespread.  So many countries, including the US; the UK; and Sweden can expect many many more deaths unless an effective, safe vaccine can be developed and widely distributed very soon.