Covid-19: Do many people have pre-existing immunity?
BMJ 2020; 370 doi: https://doi.org/10.1136/bmj.m3563 (Published 17 September 2020) Cite this as: BMJ 2020;370:m3563
Peter Doshi, associate editor
It seemed a truth universally acknowledged that the human population had no pre-existing immunity to SARS-CoV-2, but is that actually the case? Peter Doshi explores the emerging research on immunological responses
Even in local areas that have experienced some of the greatest rises in excess deaths during the covid-19 pandemic, serological surveys since the peak indicate that at most only around a fifth of people have antibodies to SARS-CoV-2: 23% in New York, 18% in London, 11% in Madrid.123 Among the general population the numbers are substantially lower, with many national surveys reporting in single digits.
With public health responses around the world predicated on the assumption that the virus entered the human population with no pre-existing immunity before the pandemic,4 serosurvey data are leading many to conclude that the virus has, as Mike Ryan, WHO’s head of emergencies, put it, “a long way to burn.”
Yet a stream of studies that have documented SARS-CoV-2 reactive T cells in people without exposure to the virus are raising questions about just how new the pandemic virus really is, with many implications.
Not so novel coronavirus?
In a study of donor blood specimens obtained in the US between 2015 and 2018, 50% displayed various forms of T cell reactivity to SARS-CoV-2.511 A similar study that used specimens from the Netherlands reported T cell reactivity in two of 10 people who had not been exposed to the virus.7
In Germany reactive T cells were detected in a third of SARS-CoV-2 seronegative healthy donors (23 of 68). In Singapore a team analysed specimens taken from people with no contact or personal history of SARS or covid-19; 12 of 26 specimens taken before July 2019 showed reactivity to SARS-CoV-2, as did seven of 11 from people who were seronegative against the virus.8 Reactivity was also discovered in the UK and Sweden.6910
Though these studies are small and do not yet provide precise estimates of pre-existing immunological responses to SARS-CoV-2, they are hard to dismiss, with several being published in Cell and Nature. Alessandro Sette, an immunologist from La Jolla Institute for Immunology in California and an author of several of the studies (box 1), told The BMJ, “At this point there are a number of studies that are seeing this reactivity in different continents, different labs. As a scientist you know that is a hallmark of something that has a very strong footing.”
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