The Office for National Statistics (ONS) have released the latest report from the COVID-19 infection survey, looking at characteristics of people testing positive for COVID-19.
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“ONS have released one of their bulletins reporting on some specific characteristics of people who have tested positive for the virus that can cause Covid-19 in their infection survey. This time it covers just one set of characteristics – the percentages of people who report various symptoms of Covid-19, out of those who had what ONS describe as a ‘strong’ positive test result when they were swabbed in the survey. These are people who had a relatively high viral load, as measured by the cycle threshold (Ct) value for their test. The reason for looking only at such people is that it excludes those with a low viral load, who are generally less likely to be infectious and who may be in the very early or late stages of an infection, which could be a reason for not showing symptoms.
“The results show that, in March this year, fewer than half (47%) of the people with these strong positive results reported any symptoms at all, when visited by the survey staff within 35 days of their first positive test. As well as data for March, ONS report data for the three previous months. The results do vary a bit from month to month, but across all four months, only about half show any symptoms at all. The most commonly reported symptoms were fatigue, headache and cough. Of those specifically listed, the least common were stomach and gut symptoms (nausea, vomiting, abdominal pain, diarrhoea). But the only specific symptoms that can be reported on are the 12 that are included in the infection survey questionnaire, and they don’t cover (for example) things like palpitations of difficulty concentrating. (ONS do ask about those, but only in relation to ‘long Covid’, so they aren’t considered separately in these data.) The data in this bulletin on whether someone has symptoms aren’t just based on these specific 12 symptoms though – there is a question on the questionnaire asking whether the person believes they have any symptoms at all “consistent with COVID-19 infection”, so people might have reported that they had a symptom that wasn’t on the list of 12. But if they did that, there’s no way to know what that symptom was.
The lower the Ct value, the higher the level of virus on the swab of the person being tested. In this bulletin ONS define a strong positive result as being one with a Ct value less than 30. There is no standard definition for a strong positive result in terms of Ct values, and some other studies have used different limits (e.g. treating a Ct value less than 25 as indicating the really infectious individuals in some studies of rapid tests). I don’t have sufficient expertise on PCR testing to be able to comment on what would be the most appropriate limit of Ct to use, but the median Ct level in positive tests in the ONS survey is 29.2 according to a preprint* by the ONS’s academic partners for the survey, so the people with a ‘strong’ positive result in this report make up around half of the people who test positive in the survey. These particular ONS bulletins now come out once a fortnight, but they don’t cover the same characteristics each time.
All our previous output on this subject can be seen at this weblink:
Prof Kevin McConway: “I am a Trustee of the SMC and a member of its Advisory Committee. I am also a member of the Public Data Advisory Group, which provides expert advice to the Cabinet Office on aspects of public understanding of data during the pandemic. My quote above is in my capacity as an independent professional statistician.”