There has been a lot of speculation that the level of asymptomatic Covid -19 cases is surprisingly high. If it is it would have a big impact on all the modelling work, estimation of he R value etc.
By its nature it is difficult to estimate.
It also suggests that a significant proportion of the population may have some immune resilience. The human immune response it a lot more complicated than Owl thought.
This article was published a week or so ago.
More than two thirds of those who tested positive for coronavirus had no symptoms, in the first nationally representative sample.
The Office for National Statistics (ONS) said the figure underlined the importance of social distancing to avoid catching the virus from those who felt well, amid warnings that the scale of infection without symptoms could make the NHS contact tracing system much less effective.
However, other experts cautioned that many of the test results could be false positives, caused by the inherent difficulties of checking people at random for a disease that fewer than one in 400 people has at present.
The results also show that only one in 15 people had antibodies, indicating that they had recovered from corona–virus, dealing another blow to hopes that herd immunity would end the epidemic without the need for a vaccine or treatments.
Weekly figures from the ONS show that just under 8,000 people a day are becoming infected with coronavirus and about 133,000 in England have the virus. These are “relatively stable” compared with previous weeks. Sir Patrick Vallance, the government’s chief scientific adviser, said that 54,000 people being infected each week was “not a low number”, adding: “It’s worth remembering that we still have a significant burden of infection. We are still seeing new infections every day at quite a significant rate. There’s not a lot of room to do things, and things need to be done cautiously.”
Seventy-nine per cent of those who tested positive reported no symptoms on the day, and 70 per cent reported no symptoms at all in the weeks before and after being swabbed. Peter Benton, of the ONS, said: “If 70 per cent of people are asymptomatic that probably means there are people who are infectious and don’t know it, and therefore continuing with social distancing is important.”
The study cannot tell if these people were infectious but Mr Benton said: “I could be positive and not know and I don’t want to pass it on to others. If I was asymptomatic I may not be very infectious but I don’t want to take the chance. We don’t know for sure what’s going on but I would rather be cautious.”
Only 87 people out of 19,000 tested positive overall and Mr Benton acknowledged that the results were preliminary. Even with tests that are more than 95 per cent accurate, testing at random is known to be likely to produce false positives when few people have the disease, which is why the NHS is cautious about screening for ilnesses such as dementia and cancer.
Sarah Walker of Oxford University, who worked with the ONS on the survey, argued that even with so many people tested “we’ve only had 87 who have ever tested positive. So that does give some confidence that the test is pretty good on the false positives.”
She said the findings were consistent with studies from other countries, with a sample in pregnant women in New York finding that 89 per cent testing positive had no symptoms and 81 per cent on a cruise ship in Uruguay. Professor Walker aims to see whether those who tested positive without symptoms go on to develop antibodies, which would be a sign that they really had the disease.
The ONS study found that 6.8 per cent of 885 people tested were positive for antibodies, suggesting they had recovered from the disease, in line with estimates elsewhere. Professor Walker said: “If you were seeing 40 or 50 per cent of people who had had it you would be very excited about the prospect of herd immunity. But at 6.7 per cent that’s not worth talking about.”
Sir Patrick added: “The vast majority of us have not had the infection. And this is a virus to which all of us are susceptible. So this set of figures urges caution in terms of the measures we take.”
Carl Heneghan, of Oxford University, said of the asymptomatic figures: “If this is a true phenomenon it is hugely important. But you have to ask, is it a false positive problem? When someone says ‘I didn’t have any symptoms’ is that true or is that a false positive? Right now they are both equally likely.”
He said that if most infected people did not have symptoms “this could be hugely important in the test-and-trace strategy. The asymptomatic spread is the most significant thing about this virus.”