Testing has dominated the public debate about coronavirus, ever since the Secretary General of the World Health Organisation emphatically urged countries to “test, test, test” for the new coronavirus. The test being referred to is the PCR test, that indicates the presence of the SARS-CoV-2 virus. It can tell whether or not you are are currently infected.

But what is the logic behind the calls for mass testing, and is it effective? Why is it believed to be such a desirable strategy when trying to combat the new coronavirus?

The situation is far from straightforward. Scientists and public health officials are learning as they go, studying the conflicting international evidence. In Britain, a country that was initially slow to get moving on testing, learning the lessons is vital if we are to be be better prepared for the next phase of the outbreak. Already there are some hopeful signs – suggestions from scientists that a suite of policies on testing and tracing using mobile apps – will help combat future peaks before a vaccine is available.

On paper, the experts explain, the importance of testing for the virus is obvious. It allows governments and health authorities to identify infected persons, trace their contacts, and disrupt the transmission of the virus. It is about stopping the virus from building up momentum by breaking chains of infection.

It also allows for “community surveillance”, enabling public health bodies to concentrate resources where they are most needed as well as understand precisely where the disease is spreading. This has the added benefit of empowering epidemiologists and others to calculate how infection is spreading and where it could still conceivably continue to do so.

This is the ideal – but in practice, many countries have come up against significant obstacles in wielding mass testing programmes to combat the virus.

One challenge for policymakers in Europe has been that the models for successful mass testing campaigns are largely in countries such as South Korea, Singapore and Hong Kong, who were prepared to respond much earlier.

Early action to promote testing campaigns seems to have formed an important part of these countries’ ability to suppress the numbers of coronavirus deaths and avoid exponential increases in underlying death rates.

By 6 April, the South Korean Centre for Disease Control had conducted around 9 tests per thousand people in the population. On 9 April, South Korea had only experienced 204 deaths.

Yet there are also European examples of similar successes. While in Italy, the coronavirus epidemic has unleashed a tragically high death toll in the country overall, there is one regional exception which proves the rule.

In Veneto, where the region’s governor and virologists implemented a widespread testing programme, including drive-through swab tests and testing at medical centres, the death toll has been suppressed far more effectively.

As of Saturday 4 April, the authorities in Veneto had conducted 133,289 tests on a population of just 4.9 million people. To put that into perspective, the United Kingdom, with a population of 66.7 million people, had tested 208,837 people by 6 April.

On an international level, countries such as Norway and Australia, who locked down early and have tested large proportions of their populations, are also showing signs of suppressing a sharp upward trajectory in the number of deaths taking place.

In all of these cases, processing large numbers of PCR tests appears to have gone hand-in-glove with dragging down the upward path of underlying death rates. The early actors on testing – whether regional or national – appear to be better in control of their epidemics.

But while it is one thing to draw attention to such correlations, it is quite another to assume that mass PCR testing is the solution by itself. In fact, it seems that there are a variety of factors in play when examining the success of any given country.

In South Korea and Taiwan, a crucial formative influence has been the terrible experience of SARS in 2003 and MERS in 2012.

Dr Michael Head, Senior Research Fellow in Global Health at the University of Southampton, told Reaction:  “A lack of preparedness is certainly part of the reason why the UK hasn’t been scaling up capacity in the same way as other countries”.

“Countries like South Korea have recent memory of SARS and MERS”, he said, adding that these countries had “clearly included testing capabilities and public health laboratory infrastructure in their preparedness planning.”

It is not in spite of their connections with China that these countries have been prepared and responded rapidly to a new respiratory disease, but because of these connections.

Societies learn from recent experiences, and the awful memories of previous respiratory disease outbreaks have shaped public policy for a long period of time – years if not decades.

In both Taiwan and South Korea, strategies have been developed over years alongside crisis infrastructure to cope with a future epidemic, which turned out to arrive in the form of SARS-CoV-2. Stockpiles of reagents and PCR tests have been gathered, testing laboratories have been optimised, and special centres for respiratory diseases have been set up.

And as Eva Moody reported from Taipei this week for Reaction, sophisticated methods of contact tracing and surveillance have been pioneered. In South Korea, it has been pointed out, the price for the effective management of the Covid-19 epidemic has been the acceptance of a greater level of surveillance technology, including CCTV and invasive tracking of bank and mobile phone usage.

It is not just that they have the laboratories and the stockpiles of test kits and reagents required to conduct large-scale testing – they also have effective strategies, based on experience, and manpower to go along with their testing. Their ability to keep the virus under control does not end with the power to perform tests. They are also able to rigorously follow up and enforce quarantine measures.

Taiwan is a curious case. For while Taiwan has very successfully contained the outbreak of SARS-CoV-2, it also appears to have done so while carrying out a low number of tests per thousand people in its population.

According to Our World in Data, Taiwan had performed just 1.56 tests per thousand people of the population. The United Kingdom, by contrast, has tested 3.96 people per thousand of its population.

Japan also appears to have carried out low levels of testing and had low numbers of deaths, so far, although experts say that the units of measurement and the data here are unclear.

Of course, the way in which the figures for Tawian are calculated and compared may provide only a skewed impression of the amount of testing going on in the country.

But if they are a true reflection, then it provides a potentially fascinating insight. The lesson here could be that a range of policies – including well targeted testing, early action, public engagement, and well-developed crisis infrastructure – have all played a crucial role. There is no single, magic bullet. Testing is one device among many in a successful battle against the global pandemic.

And beyond those countries with the memory of SARS and MERS outbreaks, the evidence strong suggests that it is those who have locked down relatively quickly – such as Australia, Austria, New Zealand and Norway – which are also succeeding in stifling sharp rises in death rates in their epidemics. Whether mass testing could have been successful in these countries without a strict lockdown, or vice versa, is perhaps a difficult question to answer.

Whereas, what is so striking about Taiwan and South Korea is the extent to which a combination of testing, public engagement, resilient infrastructure and crisis strategies – from pop-up, drive-thru testing to rigorous contact tracing efforts – have enabled these countries to avoid locking down society to the same extent as elsewhere.

The key lesson still appears to be that any measures taken – whether testing or otherwise – are far more effective the earlier they are introduced.

But can large-scale testing still play a role once SARS-CoV-2 has begun to spread significantly throughout a country? Perhaps Germany, where the number of deaths rose to comparatively high levels before the country’s federal states began locking down, could provide some clues.

Understandably, Germany has drawn a great deal of attention. A remarkable number of tests have been carried out, but it is not yet entirely clear whether this will significantly suppress the country’s upward trajectory in the increase in the country’s underlying death rate.

At the time of writing, the latest logarithmic calculations from outlets such as the Financial Times appear to show signs that the country has been successful in slightly pulling down the upward trajectory of the underlying death rate. The early indications look better for the Germans than for the populations of UK, Spain, and France. But it is ultimately too early to confirm this beyond all reasonable doubt.

On testing, Germany is the leader among its peers in Europe. The Germany Health Ministry recently said that it is testing an average of 300,000 citizens per week. Recent weeks have seen this number rise to around 350,000. The President of the Robert Koch Institute, Lothar Wieler, believes that the country’s capacity could theoretically be pushed to as many as 500,000 tests per week.

There is no single reason why Germany has been able to achieve this head start on testing. And as Wolfgang Münchau has warned in the Financial Times, there are many reasons – cultural, demographic, and statistical – to be cautious about declaring Germany’s campaign against Covid-19 an unqualified triumph at this stage.

But if their testing programme turns out to be successful in mitigating the impact of Covid-19, it will also be down to a similar combination of early preparedness – among the country’s scientists if not its politicians – and rigorous contact tracing.

What has helped is the readiness of Germany’s advisory body on epidemiology and infectious diseases – the Robert Koch Institute. This in no small part because Professor Christian Drosten, Deputy Coordinator of Emerging Infections at Berlin’s Charité University, a crucial part of the RKI’s wider network, is a specialist in the SARS ad MERS outbreaks.

The highly impressive Drosten was one of the co-discoverers of SARS in 2003 and was a part of a team which developed a diagnostics test for the disease. This background, and Drosten’s extensive research into respiratory diseases, is probably why the RKI and Germany’s experts were very quick in responding to the potential seriousness of news emerging from Wuhan in January 2020.

There are also some signs that Germany has been adopting some of the rigorous contact-tracing which has worked elsewhere. The Robert Koch Institute is hiring “Containment Scouts” from among medical school students to help busy public health authorities with tracing those who have been in contact with positive cases.

The swift reflex of Germany’s scientists appears to have mitigated the less assured response of the country’s politicians.

Yet even when a country such as Germany is able to test on a large scale, laboratory-based PCR testing inevitably takes up significant lab bandwidth and involves huge logistical efforts to sustain on a large scale.

As the German newspaper Der Spiegel reported last week, there are also many German virologists who fear that, for all the country’s testing capacity, they are in danger of wasting too many of their tests. Experts in the country are wary that, in a country of 83  million people, the capacity to conduct 500,000 tests per week would still make these tests a scarce resource.

Tests still need to be targeted wisely, even when they are carried out on a large scale. Germany’s resource base of reagents, testing kits, and laboratories is large and impressive, but it is not unlimited or inexhaustible. It is still essential that the most severely ill, as well as frontline healthcare staff are prioritised.

Then there are the technical difficulties involved with PCR testing itself – no diagnostic can guarantee 100% accuracy, and the sample used provides only one snapshot in time. It can tell you whether you are infected based upon the swab taken, that is then transported to a laboratory at a certain moment. There is no guarantee, without continuous testing, that someone who has been tested one day will not be infected the next.

While epidemiologists are able to outline with great clarity the ideal responses to the Covid-19 epidemic, the economics of public health introduces limitations to the possibilities which can be pursued by public health authorities. Real logistical constraints remain for countries seeking to increase testing capacity in the supply of essential reagents and other raw materials required for tests.

Whatever the shortcomings of PCR testing, if Germany proves successful in wresting back control from the upward trajectory of fatalities it will provide a crucial lesson for other countries.

At the moment, the countries that have been most successful in employing testing have done so by acting early and effectively suppressing the growth of death rates. If Germany is also able to get its epidemic under control, then it suggests that while mass testing is most effective when containing the virus, it can also play an important role later on in the course of an epidemic.

This also provides hope for the United Kingdom. The British arrived much later in the game when it came to testing, and it may be too late for testing to play much of a part in significantly combatting the peak of the infection. But there remains an opportunity for Britain’s government and health authorities to make a difference, if the UK can increase reagent supply, continue to build up PCR testing capacity, and bring a “game changer” antibody test into play.

Professor Martin Hibberd, Emeritus Professor of Emerging Infectious Disease at the London School of Hygiene and Tropical Medicine, explains:

“The successful strategies we have seen elsewhere involve very large-scale testing of the population and as much contact tracing as possible… While this strategy was difficult to achieve at the beginning of the outbreak, because of logistical problems in getting the testing done at such as large scale and our lack of experience at large scale contact tracing, we should now be able to overcome these problems.”

Looking ahead to a potential exit strategy from the UK’s lockdown, Hibberd says: “A combination of some social distancing measures… and extensive testing and automated contact tracing could allow both a more regular social activity and a significant control of the outbreak.”

He believes that employing contact-tracing technology which uses mobile phone apps could provide a solution.

If Britain can eventually mobilise greater testing capacity and more sophisticated contact tracing, while also ensuring the accuracy of these tests, it will not only be able to manage an exit strategy from national lockdown but also stifle any potential second wave of infection with a much greater epidemiological arsenal.

That such an exit strategy might eventually be within our grasp provides some much needed grounds for hope.