A large, multi-centre prospective cohort study the SIREN study was published in The Lancet on April 9th 2021 by Hall and others.

The study aimed to investigate whether antibodies against SARS-CoV-2 were associated with a decreased risk of symptomatic and asymptomatic infection.  

This large, multi-centre, prospective cohort study was done using participants recruited from publicly funded hospitals in all regions of England. 

Participants were excluded if they had no PCR test after enrolment, or had insufficient PCR and antibody data for cohort assignment.  

At enrolment, the participants were assigned either to the positive cohort i.e. antibody positive or previous positive PCR or antibody test, or the negative cohort antibody negative, no previous positive PCR or antibody test.  

The primary outcome was a reinfection in the positive cohort or a primary infection in the negative cohort and this was determined by PCR tests.  

Potential re-infections were clinically reviewed and classified according to case definitions confirmed, probable or possible symptom status depending on the hierarchy of evidence. 

Primary infections in the negative cohort were defined as a first positive PCR test and zero conversions were excluded when not associated with a positive PCR test.  

30,625 participants were enrolled into the study. Out of these participants, 25,661 were included in the analysis.  

Results showed that in the baseline positive cohort of 8,278 participants, there were 155 infections, so called re-infections.  This compared with 1704 new PCR positive infections in the negative cohort group of 17,383 participants.  

This revealed an incidence density of 7.6 re-infections for 100,000 person days in the positive cohort group compared with 57.3 primary infections per 100,000 person days in the negative cohort group.  

The results showed that a previous history of SARS-CoV-2 infection was associated with an 84% lower risk of infection with the median protective effect observed seven months following primary infection.  

The researchers concluded that previous infection with SARS-CoV-2 induces effective immunity to future infections in most individuals.  

They went on to conclude that this was similar to the level of prevention of symptomatic infection as those vaccinated within the working age.  

They also showed that immunity from previous infection was protective against reinfection with the B1.1.1.7 variant.  

Primary infection also reduced the risk of asymptomatic infection and thus onward transmission.  

This is particularly important as healthcare had been considered a potential driver for ongoing community transmission during the first wave in the United Kingdom. 

Their findings increased the likelihood that this protection could also be attained by vaccine induced immunity, which is a separate research analysis within the SIREN cohort had demonstrated. 

This clearly is very reassuring for those patients who have been infected and those who are vaccinated.  

The London General Practice commends the Government on its vaccination programme and encourages all those who are eligible to take up the Government’s offer of vaccination.

Dr Paul Ettlinger

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