Algorithm useful in evaluating COVID-19 reinfection cases in Singapore

A study from Singapore demonstrated the usefulness of a systematic programme using combined microbiologic, serologic, and genomic criteria to assess possible COVID-19 reinfection cases.

“Following a surge of cases attrib­uted to the Delta variant [in Singapore in 2021], continued rostered routine testing (RRT) often detected polymerase chain reaction (PCR) positives in persons with a history of COVID-19 infec­tion,” said the researchers.

This prompted the development of an algorithm to evaluate suspected COVID-19 reinfection cases by using clinical features, epidemiologic and serologic data, cycle threshold (Ct) values from molecular amplification methods, and genetic sequencing to ascertain reinfections.

The team conducted a retrospective case-control analysis including all COVID-19 cases evaluated for possible reinfection under the local COVID-19 reinfection evaluation programme between 1 June 2020 and 30 June 2021. Microbiologic and serologic criteria were used to evaluate reinfection, while whole genome sequencing (WGS) was used to confirm reinfection. [BMC Infect Dis 2023;doi.org/10.1186/s12879-023-08056-8]

Seventy-four possible reinfection cases were evaluated (median age 34 years, 96 percent male) using the programme, 32 of which were eventually verified as reinfection while the rest were deemed as non-reinfection cases. “[The] persistent positive PCR results in non-reinfection cases were attributed to prolonged viral shedding of likely nonviable virus,” the researchers noted.

Mean Ct value of the initial PCR test performed at the 2nd episode was significantly lower in confirmed reinfection vs non-reinfection cases (mean 23 vs 34; p<0.001).

Of the 11 individuals with available sequential serology findings, eight were considered as reinfection, and all had either about a twofold increase in anti-S/N titre after 48 hours or an anti-S titre of >1,000 U/mL on initial reading.

Confirmatory WGS results were available for 31 individuals and 26 reinfection cases. Delta and Beta variants were most commonly identified. These, according to the researchers, signify reinfection as these were not ‘wild-type’ and were not circulating at the time of the initial infection.

“[In sum, with] the RRT programme … we found that reinfections were often evidenced by a repeat PCR positive with low Ct value … accompanied by a ‘boosted’ serologic response (eg, anti-S antibody titres), and could be corroborated by WGS showing a new variant (in this case, Delta), compared with the prevailing SARS-CoV-2 strain (wild-type) during the index infection,” they explained.

A challenge in case reporting

“The degree to which previously infected individuals are immune to reinfections pose a challenge to public health professionals in case reporting,” said the researchers. Diagnosing reinfection is further confounded by the inconsistent duration of viral shedding, wherein viral ribonucleic acid is detected in respiratory specimens even after the acute illness has resolved. [Lancet Microbe 2021;2:e13-e22; Infect Control Hosp Epidemiol 2021;42:659-668]

“A rigorous system of RRT and surveillance enabled prompt detection of possible reinfection cases, allowing a coherent and standardized method of evaluation across different public health institutions,” the investigators said.

“This systematic programme of assessing reinfection cases was key to understanding the true magnitude of each wave and the downstream clinical outcomes observed in reinfection cases … [It] can provide a framework for evaluation that may be modified for future similar situations,” they concluded.

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