We do not know for certain what proportion of people infected with coronavirus have no symptoms of COVID-19 at any one time. This is in part because the time period from exposure to displaying symptoms is not yet known.

The time between catching the virus and developing symptoms (the incubation period) is estimated at between one and 14 days. But people are not thought to be infectious for all this time. Advice from trusted sources (WHO and ECDC) suggests that people are infectious for up to about two days before symptoms.

Reports from China say that about 70 to 80 in 100 people with the virus show no symptoms. However, it is not clear how people were selected for testing, so we do not know how accurate this estimate is, but other studies also show surprisingly high numbers.

These high estimates support the importance of contract tracing, where the asymptomatic or symptomatic contacts of people who have tested positive are rapidly identified, tested and isolated.

Where did the story come from?

The BMJ reported the Chinese figures in a news story, based on figures published in China from April 1. The news story said: “A total of 130 of 166 new infections (78%) identified in the 24 hours to the afternoon of Wednesday 1st April were asymptomatic, said China’s National Health Commission.”

What is the basis for the claim?

The BMJ story is based on figures published by China’s National Health Commission. We don’t have information about how people were selected for testing, so we don’t know whether they were a representative sample of the population.

The Oxford Centre for Evidence-Based Medicine looked at 21 reports from around the world, with differing estimates of how many SARS-Cov-2-infected people had no symptoms. The study found wide estimates, from less than 10% identified in the early days of the pandemic, to the 70 to 80% reported in China on April 1st. This wide range is partly because early testing was mostly confined to people with symptoms or those they came into close contact with; Later testing included more people, including those without symptoms.

The reports analysed by the Oxford Centre included one from a US nursing home, where all residents were offered tests after a staff member became sick with COVID-19. In total, 48 of 76 residents tested positive for coronavirus, but only about 50% (27 people) had symptoms. However, almost all of them developed symptoms in the next two to four days.

Another from a cruise ship where many passengers were infected found almost 20% of those testing positive had no symptoms, while mass testing undertaken in an Italian village found 50 to 80% of those testing positive had no symptoms at the time of testing.

What do trusted sources say?

The Oxford Centre for Evidence-Based Medicine researchers concluded: “between 5% and 80% of people testing positive for SARS-CoV-2 may be asymptomatic” and that symptom-based screening will miss many people.

They added: “It is likely we will only learn the true extent [of this] once population-based antibody testing is undertaken.”

The World Health Organization says: “Some reports have indicated that people with no symptoms can transmit the virus. It is not yet known how often it happens.”

Analysis by EIU Healthcare, supported by Reckitt Benckiser

Citation

1.     Day Michael. Covid-19: four fifths of cases are asymptomatic, China figures indicate. BMJ 2020; 369 :m1375 https://www.bmj.com/content/369/bmj.m1375

Reading list

1.     Carl Heneghan, Jon Brassey, Tom Jefferson. COVID-19: What proportion are asymptomatic? Covid-19 Evidence, Oxford Centre for Evidence-Based Medicine, April 6, 2020. https://www.cebm.net/covid-19/covid-19-what-proportion-are-asymptomatic/ (Accessed 19 May 2020)

2.     European Centre for Disease Prevention and Control (ECDC). Q and A What is the mode of transmission? How (easily) does it spread? https://www.ecdc.europa.eu/en/covid-19/questions-answers