People with a second bout of Covid-19 should be considered cases of reinfection if their first bout was at least 102 days prior to their latest positive test result and they had at least one negative test in the period between, the Indian Council of Medical Research (ICMR) suggested after studying 1,300 people, 4.5% of whom appeared to have been infected by the virus twice.
Published online by the Cambirdge Epidemiology and Infection journal as an accepted manuscript, the study is based on telephone interviews with people selected from ICMR’s laboratory database of at least 80 million individuals tested between January 22 and October 7, 2020. The government has been working on a definition for ‘reinfection’ of cases since October.
The 4.5% who were found infected were people infected in the first wave. “This is an averaged out figure over a period of about eight months— January to October, 2020—, which will vary with behaviour and intensity of epidemic,” said Dr Balram Bhargava, director general, ICMR.
According to the study, the authors said: “Taking available evidence into consideration, re-infection with Sars-CoV-2 in our study was defined as any individual who tested positive for SARS-CoV-2 on two separate occasions by either molecular tests or rapid antigen test at an interval of at least 102 days with one negative molecular test in between. In this archive based, telephonic survey, 58 out of 1300 individuals (4.5%) fulfilled the above mentioned definition”.
Interestingly, the study did not look at a genomic analysis of the first and second infections, the only sure way of proving a case of reinfection.
The study, the authors added, was meant to “develop an epidemiological case definition of possible Sars-CoV-2 re-infection and assess its magnitude in India… The epidemiological case definition… was developed from literature review of data on viral kinetics… While Sars-CoV-2 re-infection is still a rare phenomenon, there is a need for epidemiological definition of re-infection for establishing surveillance systems and this study contributes to such goal..,” the paper said, adding that “obtaining genomic evidence is resource intensive and not always feasible”.
The globally accepted benchmark for reinfection is less than 1%.
The US Centers for Disease Control and Prevention considers the duration of 90 days between two positive Sars-Cov-2 infections, along with genomic evidence of reinfection as an investigative criterion to understand the phenomenon of reinfection.
To develop working epidemiological definition, ICMR researchers collated prospective cohort studies from across the globe for in-depth examination.
“A meta-analysis of 43 investigations (including cohort studies and randomized controlled trials) probed into the dynamics of viral shedding and reported the mean duration of detection of SARS CoV-2 viral RNA from upper respiratory tract to be 17 days… with the maximum period of viral shedding being 83 days. Persistent RT-PCR positivity for Sars-CoV-2 RNA has been reported up to 104 days in a solitary case-report. Taking all these available data into consideration, reinfection with SARS CoV-2 in our study was defined…,” said the researchers in the paper.
Among those eligible, 91,592 (13.2%) individuals were tested at a gap of 102 or more days, and therefore satisfied the criteria for being included in the investigation frame.
Cases of reinfection may be rare but researchers warn against being lax with taking precautions.
“Just because the percentage is small doesn’t mean one should go easy on taking preventive measures such as wearing a mask, maintaining physical distancing and hand hygiene, and as far as possible avoiding being part of mass gatherings,” said one of the study researchers who did not wish to be named.