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April 14, 2020 no comments

COVID19: Serology is harder than it looks

More than a month after the World Health Organisation declared COVID19 a global pandemic on 11th March we are still missing one crucial piece of data from our modelling: the fraction of people who have established productive immunity against the SARS-CoV2 virus. While lockdowns may slow the rate of viral transmission in the short term, our longer time battle with the virus will depend on how quickly the population develops sufficient immunity (whether through natural exposure or a vaccine, once one becomes available).

If enough people have become immune, and critically can neither suffer symptoms nor act as a carrier for the virus, then immunity will replace social distancing as the key breaker of transmission chains. This is the concept of “herd immunity” – whether it is the “goal” of government policies or not, it is the only endgame in town given that we cannot remain in lockdown forever.

Understanding the current status of the human herd, however, is harder than it looks. Current tests for SARS-CoV2 are exclusively measures of the viral nucleic acid, exploiting the exquisite sensitivity and specificity of PCR-based amplification strategies. This gives a clear indication of current (or very recent) infection, but it tells us nothing about the cumulative number of people exposed, nor the degree or nature of their immune response to the virus. Added to that, most countries have adopted a testing strategy focused on people with symptoms, giving us a relatively poor understanding of the degree of spread among people without symptoms.

If there has been extensive spread of SARS-CoV2 in people who experienced no symptoms, then the fraction of the population with protective immunity may be much higher than predicted from nucleic acid testing among those with symptoms. Without a better estimate of who has been exposed at any point since the virus jumped into the human population we have only a poor estimate of where we are on the journey to herd immunity.

The solution is a test for the immune response to the virus (often called a “serology test”). At a stroke, such a test will tell us how many people have ever been exposed to the virus as well as quantifying for us the strength and nature of their immune response to it. This serology test then needs to be applied to a truly random sample of people in each population.

If data from a serology test applied to a random sample of people is so crucial to understanding where we are, why haven’t we got one in widespread use already?

Because there is more to designing and validating a serology test than meets the eye. The team at RxCelerate have been creating such assays for more than twenty years, and …


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