How to screen for Covid-19 is still far from clear

Getting the right testing criteria for coronavirus presents an ongoing medical challenge

One of the medical challenges in beginning to loosen social distancing is whether any symptoms of Covid-19 can be utilised in a way that would permit a population to be screened for suitability to live and work under relaxed conditions.

Because we have only known about the novel coronavirus for such a brief period, scientific knowledge and certainty is in short supply. Witness the degree to which Covid-19 testing criteria have been altered as knowledge develops.

Take temperature screening of people returning to work as an example. Measuring the temperature of travellers in airports, as was mooted early in the pandemic, did not work. According to the World Health Organisation, it is not an effective way to stop the international spread of the virus. This is because infected individuals could be in the incubation period of the disease, or may be asymptomatic early in the illness, or indeed may have used medication to lower their temperature – all factors which render temperature measurement an ineffective screening tool for the viral infection. It may be possible to include temperature as part of a future algorithm alongside other measurements, but as a solo screening instrument its utility is limited.

An emerging factor in attempting to define return to work criteria is the finding that presymptomatic people are infectious. Recent reports suggest that patients with the coronavirus may be infectious one to three days before symptom onset and that up to 40-50 per cent of cases may be attributable to transmission from asymptomatic or presymptomatic people.


What about so-called immunity passports? These require an effective antibody test that reliably reveals who has had the virus. But the presence of an antibody to the virus does not tell us how well protected that person is, or for how long any protection will last. An antibody test confirms who has been exposed to the virus, but cannot tell us the level of protection the antibodies confer.

Loss of smell and loss of taste are relatively unusual presenting symptoms. Of particular interest is emerging evidence documenting a sudden loss of smell and/or taste as a primary symptom of Covid-19. It raises the possibility the twin symptoms could be included in a future screening system.

If these symptoms are present in many with asymptomatic Covid-19, asking about them could be a way to identify people with the virus in the absence of other symptoms. However, the WHO has yet to list loss of smell or taste as potential symptoms of Covid-19, technically called SARS-CoV-2.

Ear, nose and throat (ENT) specialists in Britain have advised doctors to have a high index of suspicion for the virus infection in patients with acute-onset loss of smell or taste. Of note, they say, the loss of taste and smell, in contrast to other respiratory infections, occurs without the person having nasal congestion. This and other features suggest the loss of senses in Covid-19 may be due to a different mechanism of action.

Author of a recently published study, Dr Carol H Yan, a head and neck surgeon at the University of California San Diego, says “if you have smell and taste loss, you are more than 10 times more likely to have Covid-19 infection than other causes of infection. The most common first sign of a Covid-19 infection remains fever, but fatigue and loss of smell and taste follow as other very common initial symptoms”.

Meanwhile, making medical decisions around novel coronavirus is most likely to continue on a trial and error basis using ongoing risk analysis.

Muiris Houston

Dr Muiris Houston

Dr Muiris Houston is medical journalist, health analyst and Irish Times contributor