The Study: SARS-CoV-2 infection rates of antibody-positive compared with antibody-negative health-care workers in England: a large, multicentre, prospective cohort study (SIREN)

 

 Overview: This is a large, prospective study to investigate whether antibodies from previous SARS-CoV-2 infection(s) were associated with a decreased risk of reinfection.

 

Key Points: 

·         A total of 25,661 participants with linked in data on antibody and PCR testing were recruited from publicly funded hospitals in all regions of England. 

·         Participants were tested for antibodies and completed questionnaires regarding any symptoms or exposures every 2-4 weeks for one year. 

·         Once accepted into the study, participants were assigned to one of two cohorts: a) the positive cohort (those with prior positive PCR or antibody test) or b) the negative cohort (those with no previous positive PCR or antibody test). 

·         The primary outcome being studied was a confirmed reinfection in the positive cohort or a primary infection in the negative cohort, as determined by PCR testing.  

·         Potential reinfections in the positive cohort were clinically reviewed according to the “hierarchy of evidence” and classified as either “confirmed, probable, or possible”. The patient’s symptom status (or lack thereof) was also considered.  

·         Infections in the negative cohort were defined as a first positive PCR test. 

·         Incidence rate ratios (IRR) were calculated to compare infection rates in the two cohorts. 

·         155 reinfections were detected out of the 8278 participants in the positive cohort (those with prior history of infection). 

·         1704 new PCR positive infections were detected in the 17,383 participants in the negative cohort(those with no prior history)..  

·         Previous infection reduced the incidence of reinfection by “at least 90%” when the authors defined a positive case as those having BOTH symptoms and a positive PCR test 

·         Even when they included participants classified as only “possible” or “probable” re infections”, a prior history of SARS-CoV-2 infection “was associated with an 84% lower risk of infection”. They believe this 84% figure to be the minimum beneficial effect of prior infection. 

·         The authors accounted for the fact that COVID-19 vaccines were introduced to some members of their patient cohorts from Dec 8, 2020, onwards but the effect was determined to be modest. 

 

Author’s Conclusions: “This study shows that previous infection with SARS-CoV-2 induces effective immunity to future infections in most individuals.”

 

Reviewer's Comments:  

Most of the re infections discovered were only classified as either probable or possible however some are still undergoing further serological testing.  At the time of publication no cases of re infection which met the most stringent case definitions have been confirmed.   None.  Zero. 

It’s also important to note that this study covered the time period when the B.1.1.7 line of variants were emerging and spreading across the U.K.  The authors concluded that previous infection also appears to be protective against the new strains.  

The results of this study “show equal or higher protection from natural infection, both for symptomatic and asymptomatic infection” compared to what has been demonstrated in vaccination studies.  

Primary infection also appears to reduce asymptomatic infections and the associated risk of unwitting transmission to others.  

Finally, this study is reviewed here for your consideration but should not be taken as definitive advice as to whether or not those with a prior history of infection should ultimately undergo vaccination.

 

 Reviewer:  Mark R. Payne DC 

 

Reference:  Hall V, Foulkes S, Charlett A, et.al.  SARS-CoV-2 infection rates of antibody-positive compared with antibody-negative health-care workers in England: a large, multicentre, prospective cohort study (SIREN). Lancet. 2021 Apr 17;397(10283):1459-1469. doi: 10.1016/S0140-6736(21)00675-9. Epub 2021 Apr 9.

 Link to Full Text: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8040523/pdf/main.pdf

 

* All BOLD emphasis in this review was added by the author and not included in the original manuscript.