There is a two-stage optimal strategy for dealing with Covid-19, and it is being implemented in a number of countries around the world. The first stage is a lockdown to stop the spread of the virus, implemented as quickly as possible. A lockdown gives a country time to build up its test, trace and isolate (TTI) infrastructure.

The lockdown needs to continue until three conditions are met. The first is that the number of new cases is very low. The second is that the TTI infrastructure is tested and ready to go. The third is that travellers from overseas are effectively quarantined for two weeks.

The second stage involves ending the lockdown step by step – and making sure that enough time has passed to ensure that the TTI regime can still cope before moving on to the next one. If at any stage the TTI regime cannot cope, that element of the lockdown has to be reinstated and some other relaxation tried, or the TTI regime has to be improved. The experience of a number of east Asian countries, and in particular South Korea, suggests that if the TTI regime works well most elements of lockdown can be removed.

Once people have confidence that the number of cases are very low and well controlled, they will leave their homes, they will happily send their children to school and they will travel to work. The economy can almost fully recover, although it may still be necessary to ban large social gatherings. South Korea’s TTI regime is so good that it decreased the number of cases without the need for the first-stage lockdown, but countries such as the UK, with less experience of TTI, should not be so ambitious.

A well functioning TTI regime, together with restrictions on overseas travel, is therefore the solution to how we get from here to when a vaccine is developed with as few deaths as possible, and with as little damage to the economy as possible. This is why there is no conflict between opening up the economy and saving lives.

What about those demanding a quicker end to the lockdown to “save the economy”? They seem to be making a simple error. Without a government-imposed lockdown the economy would not return to normal. With minimal measures to contain the pandemic and therefore many new cases each day, most people will stay at home and keep their children at home out of choice. Nearly all academic economists understand that you cannot restart the economy without getting the virus under control.

I can be confident about this because of work I helped to produce on the economic impact of a flu pandemic about 10 years ago. That included a severe case not unlike the current pandemic, and our estimate was that economic output would initially decline by 30%. This fall in GDP is very similar to the Bank of England’s best guess at the initial impact of this pandemic, yet in our study this fall in GDP was mainly the outcome of voluntary decisions by consumers rather than any action by the government. If lots of people are still dying from the virus most people will stay at home out of choice, whatever the government does. Lockdown is designed for the irresponsible minority, and to avoid intimidation by employers.

It has taken far too long for the UK government to understand this. Probably the most important task for any inquiry into our government’s handling of the pandemic is to find out why scientists advising the government appeared to discount the possibility of a TTI solution. It is a standard method for dealing with a deadly infectious disease, and in early March it was clear that South Korea had managed to get its outbreak under control by employing it. Despite this, together with advice from WHO and the actions of other major countries, our government convinced itself that herd immunity was the only way forward, as long as it was managed so the NHS could cope.

It was a mistake that probably cost most of the 50,000 or so excess deaths we have seen so far as a result of the pandemic. The failure to consider the obvious alternative of a TTI regime and instead to go for managed herd immunity was not just a failure of the scientific advice: I still find it incredulous that a prime minister can be told of a strategy that will see tens of thousands die and not demand that alternatives are investigated.

The result of their initial failure is not just tens of thousands of lives unnecessarily lost, but also a longer lockdown with greater damage to the economy. The longer a country takes to go into lockdown, the longer the lockdown must last to bring cases down to a level that TTI can manage. Herd immunity will be remembered as one of the most costly mistakes a UK government ever made in peacetime.

We can only hope that the government now has the right strategy and it can implement it successfully. There are worrying signs. Effective messaging has been ditched for something dangerously ambiguous because of needless fears that people have become addicted to staying at home. Most people will stay at home as long as the pandemic is uncontrolled. Significant voices in cabinet are calling for premature lockdown easing. As this government has messed up almost every aspect of responding to the pandemic, from testing to care homes, and from inadequate PPE stockpiles to confused messaging, hope is pretty well all we have left.