How long can we go on like this and when will the money run out? This is the question I am being asked all the time. Whether I am out for a walk, in the supermarket (at a safe distance) or on the phone, it’s the same question over and over. How long can we survive?

Some people are concerned about the massive increase in Government spending via the wage subsidy. When will that money run out? Small business owners want to know how long this can go on before they default on loans.

The decision to close Bewley’s Cafe could be the first of many closures; as business owners in hospitality do the sums and incorporate social distancing, many may conclude they will never make the numbers work. For others, the how long can we go on question refers to their own job, and whether there will be a job to go back to. Everyone has an angle, and all are united by anxiety.

To answer this question, it’s essential to form a view of this pandemic.

These types of pandemic outbreaks – the 1347 Black Death, the 1918 Spanish flu and the 2009 Swine flu – are called “crowd diseases”. They love a crowd and are characterised by five major attributes.

First, they are highly contagious from healthy to sick, and outbreaks happen very quickly. Second, they are acute diseases as infection lasts for a short time, a number of weeks for each person. Third, the outcomes are binary: patients either survive or don’t. The vast majority recover and rarely suffer any lasting health legacies. Fourth, once infected, most people acquire a measure of immunity.

Fifth, the disease dies when the ratio of infected to susceptible people in the population hits an immunity tipping point. As more and more people contract the disease, more and more become immune and the number of susceptible people falls to a level where the virus can’t replicate. Therefore, it disappears.

In order to save lives, we slow this process down and this slowing down itself determines the length of the economic quarantine. Existing economic conditions determine how long the economy can survive such a quarantine.

By existing economic conditions, I mean factors such as the size of the service sector relative to manufacturing; the level of debt in the economy; the relative size of the hospitality sector; how many people are employed in highly exposed areas; the size and broadness of the tax base; the willingness of lenders to finance borrowing; and the inventiveness and alacrity of our policymakers. This list goes on, but you get the point.

Such factors will determine the economy’s resilience, but how long can we lock down? Let’s do some back-of-the-envelope calculations. I stress back of the envelope, but the following numbers give us a framework.

The strategy is to wait until we get a vaccine. We start to open up, then if there is another outbreak we close, we open again. We trace, isolate and control. The critical figure is ICU beds. The economic future is, therefore, a function of bed capacity, the progression of the virus and scientific breakthroughs regarding a vaccine.

Now let’s “park” the vaccine for a minute and calculate how many days or months it would take for us to reach the point where the disease dies by itself. What does such immunity look like?

The rate at which the population becomes immune varies for each virus depending on its rate of transmission from a low of 40 per cent to as high as 95 per cent for certain virulent diseases. Covid-19 is new, so no one really knows yet. Let’s assume, for argument’s sake, that we need somewhere in the region of 75 per cent of the population for widespread immunity – and it may be higher.

Like other infectious diseases, Covid-19 is likely to mutate over time. We have no idea how long immunity from re-infection will last. For two other coronaviruses that regularly circulate – OC43 and HKU1– which cause common colds, immunity is less than a year.

If we assume the immunity threshold is 75 per cent of the population, that means 3,691,125 people in Ireland. So far, we have a total of 21,176 confirmed cases in Ireland so that’s a shortfall of 3,669,949 people who would need to be infected to reach the immunity threshold.

Given rates of daily infection and the objective of keeping the number of deaths as low as possible, we can calculate how long this might take. From there, we can work backwards to calculate the impact on the economy. Only then can we answer the how long can we survive question.

Achieving immunity will take time, presumably involving a staggered restoration of normality, beginning with those least vulnerable (children, students, young adults) before relaxing restrictions on more vulnerable groups.

Key to understanding how long it will take to achieve immunity also involves ascertaining the proportion of the population that has already been infected. Here, estimates vary dramatically.

One Italian study estimated a more than 50-fold discrepancy between the number of known and actual cases in late March. There were 100,000 known cases (0.2 per cent of population) yet it is estimated 10 per cent of the population might have had it without symptoms.

Let’s assume that the current situation is sustainable, ie the daily new confirmed case numbers (265 as of 6th May, though it was more than 800 on certain days in April) won’t overwhelm the health system. Let’s also take the best-case scenario from Italy implying 10 per cent of the population have already had it. This means we can subtract 461,390 odd people from the total population.

Recall that the 75 per cent immunity threshold in Ireland equates to a total of 3,691,125 new infections minus that 461,390.

By any calculation, reaching that 75 per cent without a vaccine would take years. My worst case number crunching arrives at a figure of more than 30 years. Even the sunnier statistical estimates suggest that, without a vaccine, we could have 14 years of this “on/off economy”.

(To get these figures you simply divide the number of daily cases reported this week – which we know the ICUs can sustain – by the targeted threshold population. The figure of 30 years is achieving immunity for the entire population. The 14 years figure assumes that we exempt under-35s in our population – all 2,583,005 of them – because they, almost in their entirety, recover rapidly from the disease.)

No matter what figures you take or how you adjust them, it is clear that today’s service economy is completely dependent on science and finding a vaccine. With no vaccine, we have no recovery. This is why we must implement more assertive macroeconomic policy by crediting accounts, fixing zero interest rates for business loans and mortgage holders.

This means repricing the entire debt structure of the economy, which is something the ECB has given central banks permission to do. This must be accompanied by an understanding that as the private economy goes into freeze, the State goes into action.

The money will not run out unless the Central Bank decides not to provide it.