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expert reaction to study looking at risk of reinfection up to 10 months after COVID-19 infection

A study as published in Lancet Healthy Longevity looks at risk of being reinfected with SARS-CoV-2 up to 10 months following a first infection.


Prof Rowland Kao, the Sir Timothy O’Shea Professor of Veterinary Epidemiology and Data Science, University of Edinburgh, said:

“By comparing individuals who had previously tested positive for COVID-19 vs. those who had not, this study shows substantial levels of protection is retained over the duration of the study – i.e. for those initially infected between June and November 2020, how many become re-infected between October 2020 and Feb 2021 compared to those who were not infected in the initial period. Long lasting natural immunity is likely to be an important component of keeping total infection numbers down, as it means that there is some additional long lasting protection for those who have been infected but who have not yet been vaccinated, even for a substantial period after initial infection. The study also has similar results to an earlier study in Denmark with a similar design, which showed good protection for most people for up to six months after infection. While the Danish study shows reduced protection for the elderly, the evidence here is that protection remains high, however the numbers of subjects involved in the study would indicate some caution in generalising. Also, the study does not allow us to infer anything about whether others, for example with pre-existing health conditions, those from higher risk ethnic groups, or those who have had long covid, for example, are similarly protected. Thus vigilance in protecting those most vulnerable must continue to be a key feature of our approach to COVID-19 control measures for the foreseeable future. An additional cautionary note is that because the delta variant only became prominent after the end of this study, the study does not inform directly the protection against reinfection with this variant, which is currently dominant in the UK.”


Dr Alexander Edwards, Associate Professor in Biomedical Technology, University of Reading, said:

“This high quality, study is vital as it provides more information about how well COVID-19 infection protects you for getting infected again. We do expect the immune system to be effectively trained by this type of viral infection, and remember the virus to protect us much faster every time we encounter the virus again. What we do know from other diseases is the level of protection can vary quite a lot. Some infections provide very strong, life-long immunity. The concern for COVID-19 is that we might not see complete protection or that protection might fall with time.

It’s therefore great news to see that in the two groups studied here, reinfection wasn’t seen very often, confirming that recovering from COVID-19 infection does give some protection. However, the authors conclude “previous infection reduced the risk of reinfection by approximately 85% in residents and 60% in staff members” so it’s important to also confirm that reinfection does occur- so protection is not complete. We still that expect natural infection should protect against more severe infection, but we still don’t have enough data to know this.

“What is perhaps surprising is how uncertain we still are about the level of protection following infection. The authors use the word “approximately” because even for this excellent, detailed study, relatively small numbers of people can be used to calculate protection. It turns out it’s harder to answer this critical question than you might expect, and indeed harder than testing vaccines. With vaccines, you plan carefully a large group of people, and use placebos and controls to be able to precisely count how many people get infected. With natural infection, you are monitoring people who happened to get infected, so there becomes much more uncertainty. Indeed the success of vaccination programs brings this type of study to an end, because when people become vaccinated you can no longer follow protection after natural infection.

“Measuring the level of immune protection provided by prior infection is vital, for public health planning and for predicting rates of spread or hospitalisation, and if this level changes from 60% or 85% or 90% (either over time or with variants) it would make a very big difference to the rate of spread. It’s therefore important to remember how hard it is to pin this number down accurately.”


Prof Ian Jones, Professor of Virology, University of Reading, said:

“The finding of long lived protection in previously infected care home staff and occupants adds more support to recent experimental data showing that bone marrow plasma cells, those cells that produce protective antibodies, can be found in people for up to a year following infection. There were concerns early in the pandemic that protection would be short lived but this data suggests otherwise and it is particularly encouraging that this applied as much to those over 65 as those under. If the same is true of vaccinated individuals, which seems likely, boosters may not be needed as frequently as has been supposed. The few people that did become reinfected all had low virus levels suggesting that virus circulation will remain suppressed in immune populations which in turn will limit the occurrence of variants.”



‘Incidence of SARS-CoV-2 infection according to baseline antibody status in staff and residents of 100 long-term care facilities (VIVALDI): a prospective cohort study’ by Maria Krutikov et al was published in Lancet Healthy Longevity at 23:30, Thursday 3 June 2021.



All our previous output on this subject can be seen at this weblink:



Declared interests

Prof Ian Jones: “No conflicts.”

None others received.

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