Ross Clark

How can ‘test and trace’ stop a virus spread by the asymptomatic?

How can ‘test and trace’ stop a virus spread by the asymptomatic?
Photo by KAZUHIRO NOGI/AFP via Getty Images
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The government has placed a lot of hope in its test and trace system, but even disregarding teething problems with the smartphone app and reports of some of the 25,000 contact tracers being left idle, is it even possible for it to achieve its objective? The problem with Covid-19 all along, and the reason it has managed to evade the efforts of containment which had worked with previous novel viruses such as SARS and avian flu, is the sheer number of people who seem to be infected but who show no symptoms. Some studies have shown that 80 percent of cases might fall into this category. Worse, there is plenty of evidence to suggest that these asymptomatic individuals are capable of passing the infection to others. Short of testing the entire population every few days, just how can anyone attempt track and trace cases of the disease when we have no idea who is infected?

A team from the Scripps Research Translational Institute in La Jolla, California, has now attempted to quantify this problem. It has analysed data from 16 well-studied closed environments where Covid-19 was able to run riot – including the Diamond Princess, the USS Theodore Roosevelt and a Boston homeless shelter. Its conclusion is that in at least 30 percent of cases – and more likely 40 to 45 percent – the virus was spread by people who were themselves asymptomatic.

In other words, even if the government’s test and trace system worked perfectly – i.e. it managed to trace every single contact of every single person who had gone down with Covid-19, and that all those contacts had obeyed the instruction to confine themselves to their homes for 14 days -- it would only succeed in preventing around 60 percent of infections. The rest -- silent infections caused by asymptomatic individuals -- would carry on apace. The only answer to this problem would be to test and re-test the entire population at regular intervals. But if we had the capacity to carry out testing to that degree then we wouldn’t need the contact tracing system anyway.

The Scripps study has uncovered a disturbing phenomenon about asymptomatic cases. It reveals a detail from the Diamond Princess which has not previously been widely broadcast. CT scans performed on 76 of the asymptomatic cases revealed that nearly half of them showed some degree of lung damage. These people had not reported any symptoms, were unaware that they had had the disease, and yet still it seemed to have left its physical mark. Whether they will suffer any long-term consequences from this damage it is too early to say.