One question which is key to understanding how coronavirus works and spreads through a population is about how many people have the disease but never show symptoms.
It’s widely agreed that some people can be infected with SARS-COV-2, the virus which causes the COVID-19 disease, but stay “asymptomatic”. But how many there are is still poorly understood, despite figures often being cited in the media.
A number of studies have attempted to answer the question. But in 21 of these different publications, the suggested proportion of asymptomatic cases ranged from 5 to 80 percent. That’s according to analysis this week by Dr Tom Jefferson, who is an Honorary Research Fellow at the Centre for Evidence-Based Medicine at the University of Oxford.
Dr Jefferson believes that none of the studies are reliable. He cites problems including that symptom-based screenings used in some studies are not always accurate, and the possibility that cases are recorded as asymptomatic when symptoms have merely not begun yet.
Another problem is the samples taken. Speaking to FactCheck, Dr Jefferson said that, for example, some of the studies he surveyed were carried out in the hardest hit areas of Italy, where the number of coronavirus cases were especially high. These shouldn’t be thought of as representative of the whole population, he said.
But why is a clear answer so difficult to find?
Dr Jefferson told FactCheck that for a representative study you would need two things: reliable testing of both who has the virus now and who has had it in the past, and a good sample of tests which cover a range of situations and risk categories.
“I would want to take samples from different levels of exposure, and different levels of risk,” Dr Jefferson said. These could include samples taken from places with both only a few known cases and others with many cases, and from sections of the population that are both at high and low risk of contracting COVID-19, for example old and young people.
But as with many aspects of the unfolding pandemic, an answer for asymptomatic COVID-19 cases in the UK may continue to be elusive until reliable testing systems can be widely rolled out here.
To find out who has had the virus, something called an antibody test is needed. This detects whether the immune system has reacted to the presence of the coronavirus in the past.
The UK government says it has placed orders for massive numbers of these antibody tests, but they’ve so far not been signed off by officials and put into general use in Britain – largely due to reliability issues.