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  • Richard Cree

Immunity

Updated: Jun 28

Saturday 28th June 2020

Figures for 27th June

UK COVID Deaths 149 / Total 43,230

James Cook Hospital – Total COVID deaths – 254

All COVID cases within South Tees Hospitals Trust – 8

James Cook Critical Care COVID cases – 3 (3 ventilated)

James Cook Critical Care non-COVID cases – 41

We have admitted another COVID patient to the ICU. The patient has been admitted with traumatic injuries following a fall and is coincidentally COVID positive, rather than the virus being the reason for their admission. Otherwise, the total number of COVID patients in the Trust has fallen slightly in the last 24 hours.

Nicky and I have both had the results of our antibody tests yesterday. The antibody tests are offered to all healthcare workers at our hospital. They detect the presence of antibodies in your blood confirming that you have been infected with the virus in the past. The antibodies are produced in the body by white blood cells called B-Cells. Whether the presence of such antibodies provides immunity and if so, for how long, is extremely contentious.

Nicky and I have both tested negative. This means we haven’t had the virus, right? Well, maybe, or maybe not…

A negative test could indeed imply that you have not been exposed to the coronavirus and have not been infected. It could however, imply that you have only been infected recently and haven’t had time to produce antibodies. It’s believed it can take 1-3 weeks after an infection to produce antibodies. Some people make take even longer and some people might not ever produce antibodies despite being infected.

I know a few people who became ill, tested positive for the coronavirus at that time but subsequently did not test positive for antibodies. Studies have shown that anywhere between 10 to 20 per cent of people who have been infected have little or no detectable antibodies in their blood some weeks later.

It’s starting to look like a mild infection is less likely to produce an antibody response than a severe one. Presumably, the immune system isn’t as strongly stimulated, resulting in the failure to form a significant, lasting antibody response.

Of course, no test is 100% accurate. The home sampling antibody kits sent in the post are particularly unreliable. The two laboratory tests that are available (and used in hospitals) are manufactured by Roche and Abbott. The media have often reported that these tests are 100% accurate but this is not the case. The Roche test detected antibodies in 84% of positive patients, so 16% were missed. The Abbott test missed 6% of positive patients. These rates will increase a little if the sample is taken 20 days or more after the infection. These figures demonstrate the sensitivity of the test; the missed positive results are called ‘false negatives’.

The other measure of accuracy is the specificity of the test. In this situation, this refers to how many of the people who tested positive for antibodies didn’t actually have them in reality (these are called ‘false positives’). Fortunately, in this regard, both tests are nearly 100% accurate meaning that if your test detects antibodies, they are nearly always present.

This is all very well and good but true ‘immunity’ is complex and cannot be measured in this way. We currently do not know whether these antibodies offer full or partial protection against future infections or for how long. Other parts of the immune system cannot be measured by such blood tests. In particular, some white blood cells, called T-cells are an important line of defence. They can trigger infected cells to self-destruct, stopping progression of the virus. They also help the B-cells develop antibodies. Some T-cells can remain after an infection to help protect during any subsequent exposure to the virus. In HIV, one of the reasons the virus is so dangerous is that it destroys T-cells, leading to a failure of your immune system.

In other coronavirus infections, including SARS-CoV (Severe Acute Respiratory Syndrome Coronavirus) and MERS-CoV (Middle Eastern Respiratory Syndrome Coronavirus), T-cells play an important role. It’s likely that the same will be true for this coronavirus. It is difficult to test the body’s T-cell response and so it’s unlikely that there will be a test available to measure this aspect of immunity.


Of course, it’s worth pointing out that there is yet to be any reliable confirmation of anyone contracting the virus twice.

So where does this leave Nicky and I? Both of us believe that PPE offers protection from infected patients. We assumed that the risk of contracting the virus was more likely from working alongside potentially infected members of staff. However, we have to assume that despite working alongside colleagues who became unwell and tested positive for COVID-19 we have not been infected. Or, if we were infected, we were asymptomatic and failed to generate an antibody response. Of course, we may still have some ‘immunity’ if this is the case.

The bottom line is that we resigned ourselves some time ago to becoming infected at work at some point. So this result, whilst not reassuring, does not cause us too much anxiety. We will continue to be careful and cross our fingers. It’s all any of us can do.

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