Antibody tests showing that you have had Covid-19 infection will be rolled out to the NHS and caregivers starting next week. So what happens when you test positive? Continue as before – and I should know.
Part of the work of a medical correspondent is involved. This means volunteering for medical trials, tests, etc. I forget the number of times I rolled up my sleeve to donate blood to illustrate a story, or went to an MRI scanner to image my brain. This is what we call “show and say” in the television business. So when the home antibody tests first appeared in the news, I decided to show how they worked.
The tests all vary a bit in how you do them. All you need is a drop or two of blood, which you squeeze into a hole, add a little chemical, and in a few minutes you get your result.
A positive result occurs, as with a pregnancy test, if you get two lines across the sample window. I did the finger prick test on camera and I was surprised and happy to see that I was positive for antibodies. I have since made other reports on antibody tests and have had the same positive result each time. You can see the photo – excuse the blood – of three positive results, although one of them has a weak line.
Imperial College London is testing these antibody tests at home for accuracy and ease of use. One of the teams there calculated that my repeated positive tests made it incredibly unlikely that I would continually produce a false result. In other words, it looks like I certainly had a coronavirus.
Fergus on Covid
As a BBC medical correspondent, since 2004, I have been reporting on threats from global diseases such as avian flu, swine flu, Sars and Mers – the two coronaviruses – and Ebola. You could say that I have waited a large part of my career for a global pandemic. And yet, when Covid-19 arrived, the world was not as ready as it could have been. Now this is where I, like everyone else, want it to go. Unfortunately, we may have to live with the coronavirus indefinitely. In this section, I will reflect on this new reality.
So when was it? I have had no symptoms in recent months. I am rarely sick, but I had a pneumonia attack in early January. I fell ill for about 10 days and had a cough and high temperature. I couldn’t get rid of it. My general practitioner in Windsor diagnosed a bacterial infection and gave me antibiotics. It helped me a little, but at the end of January I needed another antibiotic treatment. They seem to have done the trick. Was it really Covid-19?
I do not think so. The first confirmed case of coronavirus in the UK occurred in late January when two people from China fell ill in York. It was not until a month later that the first cases of domestic transmission occurred. Note that although I reported on the epidemic in China in mid-January, the most distant I had been in recent months was Christmas in Brussels.
So I don’t think I missed a story here – the first case of coronavirus in the UK was not me. But after that, I had no symptoms. Not a cough, not high temperature, normal smell and taste, and no aches and pains, headache, diarrhea, conjunctivitis, rash or any other possible warning signs listed by the World Health Organization.
So when I pricked my finger, I didn’t expect a positive result. To be honest, I was amazed. The test I did was positive for IgG antibodies – these are the ones that form at least two weeks AFTER an infection.
I can tell you that having a positive test has not changed my state of mind.
When I walk around, I always assume that everyone I meet has a coronavirus and that I have it. I don’t want to infect anyone and I don’t want them to infect me.
I’m still an obsessive hand basin – after talking to the UK audience over and over about the importance of social distancing and hygiene, I think I should lead by example. Every toilet I go to involves a demonstration of what I hope will be a perfect hand wash. I don’t really sing Happy Birthday twice out loud, but I hum it in my head.
At the start of the UK coronavirus epidemic, there was a lot of talk about how antibody tests could possibly help us break the deadlock. In March, the government spoke with confidence that the antibody tests were game-changing because it would help indicate who had previously been infected with Covid-19 and was therefore protected.
Unfortunately, it was not that simple.
The government bought 3.5 million finger prick antibody tests, but when evaluated by scientists at Oxford, they said none of the ones tested were accurate enough.
Things have changed a lot since then. There are now several laboratory antibody tests that seem fairly reliable.
Public Health England has evaluated Roche and Abbott’s antibody tests. Both require a blood test, so they are not home tests. The sample must be sent to a laboratory for analysis.
You can now also purchase home antibody tests that you must send to a laboratory. These are fairly accurate, but you don’t get an instant result.
The accuracy of a test is based on its specificity and sensitivity.
With coronavirus, you want to make sure that an antibody test is very specific so you don’t get false positives. This could be dangerous, as it would mean that some people are made aware that they have antibodies when they don’t have them. They may be asleep in a false sense of security, taking fewer precautions to guard against infection.
The Roche and Abbott tests are both very specific, with an accuracy close to 100%. It’s very reassuring. Then we come to sensitivity. This is the probability that a test will give a false negative. The PHE assessment showed that the Roche test was 87% sensitive with samples taken 21 days after the onset of symptoms while the Abbott test was 93%. This means that some people who definitely had anti-coronavirus antibodies may get “false negative” results. It’s less important, but it’s still a mistake.
The two companies said they could supply 10 million tests in the UK and the government has announced that antibody testing will be rolled out to frontline health workers starting next week.
But to what end? If you have the Roche or Abbott test and get a positive result, you can be fairly confident that you have had Covid-19. This will be especially helpful for anyone who has not had the nose and back of the throat swab tested. Remember, this is for the current infection, that you have Covid-19 right now. The antibody test tells you about a past infection. In addition, you must wait several weeks for these antibodies to appear in a test.
It’s great then. A reliable positive antibody test means that you are safe from lockout, free to meet friends knowing that you will not be infectious and that the disease is unlikely to spread, right?
Not so fast. It is not so simple.
There is no consensus on what a positive antibody test means for an individual. Some virologists I have spoken to believe that it will offer you some protection against coronaviruses, and especially severe symptoms, but whether it will last for months or years is uncertain.
With Sars, antibody levels started to disappear after two or three years.
There is another complication. Current antibody tests do not distinguish between the presence of neutralizing antibodies, which would rule out any new infections, and non-neutralizing antibodies.
We also don’t know the importance of T-cell responses – another part of the immune system, which does not involve antibodies.
So having antibodies to the coronavirus may not be the pass you might have assumed.
Everyone I have spoken to has said that no one should change their behavior based on a positive antibody test.
So what’s the point of deploying antibody tests to healthcare professionals?
Firstly, it will really help to build a picture of the number of people in the UK who have had a coronavirus. Current estimates are rough and range from around 17% in London to 5% elsewhere in England. This is a huge difference from the 65% or so estimated necessary for collective immunity – IF it turns out that people with antibodies are effectively immunized.
It will also give us the first really accurate picture of the number of people who have unknowingly had a coronavirus, so-called asymptomatic cases – people, it seems, like me.
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