The new antibody testing study also found that more than a quarter who did fall ill didn’t have the three core signs of the disease: persistent cough, fever and loss of smell (anosmia).
This is the first UK-based pre-print study linking detailed ongoing symptom collection data with antibody testing, and highlights the likely extent of COVID-19 infection across the region.
To understand the true picture of the disease, researchers say that it is crucial to understand the extent to which people have previously been exposed to the SARS-CoV-2 coronavirus, as revealed by testing for antibodies in the blood.
The virus is difficult to study as some people become seriously or fatally ill, others will have a wide range of less severe symptoms, while some will have very mild or no symptoms at all (asymptomatic).
The researchers carried out antibody testing on blood samples from 432 adults aged 18-89 in the ongoing TwinsUK study living in Greater London and South East England, 382 of whom had also been regularly logging their health over three months using the COVID Symptom Study app. Participants were also given a swab PCR test to check whether they were currently infected with coronavirus, and asked if they had ever previously been tested.
The team discovered that one in 8 (51, 12%) were positive for viral antibodies. Focusing on 48 people with a positive antibody result who had also been regularly logging their health, the researchers found that nearly one in five (9, 19%) never experienced any COVID-19 symptoms throughout the duration of the study.
More than one in four (16, 27%) who had antibodies and experienced symptoms didn’t have the 3 core NHS symptoms of persistent cough, fever and anosmia. However, loss of smell alone was still highly specific for having COVID-19, compared with the combination of all three symptoms, highlighting its importance as a key early warning sign of the disease.
Additionally, around half of the app users who reported symptoms that were highly predictive of COVID-19 did not have antibodies against the virus, confirming other studies showing that current antibody testing misses a substantial fraction of cases.
Further work by the team is looking into whether alternative aspects of immunity, such as T cells, may also play a part in the immune response to the virus.
Study lead and consultant geriatrician from King’s College London Dr Claire Steves said, “Our findings highlight the fact that a significant proportion of people who get infected with coronavirus don’t have any obvious symptoms but may still pass the disease on to others. It’s essential that we all take steps to protect the health of everyone by sticking to social distancing guidelines, wearing face coverings in public and following good hand hygiene practices.
“Testing is much more available now. So if you have symptoms you can’t explain - get a swab or saliva test, as that way you will know to protect yourself and the people around you.”
Professor Tim Spector from King’s College London said: “These results suggest that both swab and antibody testing significantly underestimate the extent of coronavirus infections in the population. We need to combine testing approaches together with getting as many people as possible logging their daily health through the COVID Symptom Study app to really understand the spread of the virus and control it over the months ahead.”
The findings are available online as a pre-print, non-peer reviewed study: Estimates of the rate of infection and asymptomatic COVID-19 disease in a population sample from SE England https://www.medrxiv.org/content/10.1101/2020.07.29.20162701v1