Previous COVID-19 Infection Reduces Reinfection By 84%, Finds Lancet Study
Previous SARS-CoV2 infection conferss a high degree of protection, according to a new study.

Previous infection with SARS-CoV2 induces effective immunity against future infections by 84% – but reinfection rate is still 16%, found a study published in The Lancet on Friday. This suggests that infection-induced immunity is similar to, or greater than, vaccine-associated immunity, said the authors. 

The SARS-CoV2 Immunity and Reinfection Evaluation (SIREN) study involved 25,661 health workers in the UK and took place between June 2020 and January 2021. 

It was conducted by researchers at Public Health England, and the Universities of Oxford, Bristol and Cambridge.

Participants were separated into a positive cohort – based on antibody positive or previous positive PCR tests – and a negative cohort – antibody negative or no previous positive PCR test.

Questionnaires on symptoms were sent to participants and diagnostic testing was conducted every two weeks, and antibody testing took place every four weeks. Reinfection was defined by the researchers as a participant with two positive PCR tests 90 or more days apart or an antibody-positive participant with a positive PCR test. 

Some 155 reinfections were detected in the positive cohort of 8,278 participants and 1,704 new infections were identified in the negative cohort of 17,383 participants. The interim results from the study showed that previous infection reduced reinfection by at least 84%.


Weekly frequency of study participants with a positive PCR test result by cohort assignment, from March 2020, to January 2021.


Approximately 50.3% of the reinfections were symptomatic, with 32.3% of those having typical COVID-19 symptoms, which include cough, fever, and loss of taste or smell. The average interval between primary infection and reinfection among participants was 201 days.

The lowest level of protection against reinfection was provided to asymptomatic infection, with 76 of the 155 participants with reinfection having asymptomatic reinfection.

Vaccines and Variants

During the study period, 52.2% of the participants were vaccinated, however, the authors said that the findings on the durability of protection following their previous infection were independent of the vaccine effect. 

The researcher recommended that future studies examine the protective effect of both previous infections and vaccine efficacy. 

In addition, the B.1.1.7 variant had spread rapidly during the study period causing over 50% of the infections among participants. Despite the circulation of the more transmissible variant, the study found no evidence that the spread of the variant adversely impacted reinfection rates. 

This shows that immunity from a previous infection from a different SARS-CoV2 strain is still protective against the variant. 

Comparable Protection from Infection and Vaccines, Say the Authors

“Our findings…show equal or higher protection from natural infection, both for symptomatic and asymptomatic infection [compared to vaccines],” said the authors. 

The protection against asymptomatic reinfection is particularly important to reduce the risk of onward transmission.

Although the Pfizer/BioNTech and Moderna COVID-19 vaccines were 90% effective against PCR-confirmed infection, according to a study conducted by the US Centers for Disease Control and Prevention (CDC) in early April, natural infection induces a wider range of immune responses. 

Antibodies induced by infection are often lower in concentration compared to antibody responses induced from vaccinations, but can include responses beyond the spike protein, which is the target of current vaccines, said Florian Krammer, Professor of Microbiology at the Icahn School of Medicine at Mount Sinai in New York, in a comment to the study. 

This study is “valuable to understand the nature and duration of protective immunity,” said Soumya Swaminathan, WHO Chief Scientist, on Twitter

Further studies on the longevity of antibody responses, reinfection with the new SARS-CoV2 variants, and the impact of the existing vaccines on reinfection are reportedly underway.

Image Credits: Flickr – International Monetary Fund, The Lancet.

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