Combined Ag/Ab rapid testing boosts case-finding in COVID-19

Elvira Manzano
29 Jul 2021

A rapid and affordable test for diagnosing COVID-19 outside the walls of the healthcare units could improve case-finding, contact tracing, and infection control, says an expert during a pre-ECCMID 2021 session.

“If we can identify people who might unknowingly transmit the virus, we can isolate and manage them immediately. This will potentially reduce virus transmission and cases requiring hospitalization,” said Professor Orlando Ferreira from the Federal University of Rio de Janeiro in Brazil.

As of July 27, COVID-19 cases in Brazil have reached 19,707,662 million. Over half a million have died. Experts described the COVID-19 carnage in the country as “the largest human tragedy in Brazilian history.”

Emergence of new variants

Experts detected 92 coronavirus variants in Brazil, including the P.1 variant that is 2.5 times more contagious than the original strain and more resistant to antibodies. The P.1 variant emerged in Amazonas state in November 2020, spreading quickly in the state capital Manaus, and accounted for 73 percent of cases by January 2021, said Ferreira.

“Following that, we sequenced 180 complete genomes collected between April and November 2020 from Rio de Janeiro, and identified 37 samples with a new variant lineage,” he continued. A new lineage was classified as P.2 by Pangolin (COG-UK), a descendant of the B.1.1.28 strain, and distinguished by five lineage-defining mutations, including  S E484K. [J Virol 2021; 95(10):e00119-21]

“The phylogenetic reconstruction indicates that 86.8 percent of Brazilian genomes fall within three clades – clade I comprise B.1.1.33 strains, while clades II and III comprise B.1.1.28 strains. The P.2, which emerged in July 2020, falls within clade II,” Ferreira said.

The rapid spread of the virus in a partially immune population, either by infection or vaccination, as in the case of Brazil, set the stage for the emergence of new variants, he explained. “This was true for the P.1 variant in the Amazonas and the P.2 in Rio de Janeiro.”

“RT-PCR* remains the gold standard test for COVID-19 diagnosis. However, it is restricted to most locations globally. If a test is not accessible to the patient, it has no role in diagnosis,” said Ferreira.

The timing of sampling is also important. If a sample is not collected timely, viral concentrations can below and may yield negative results, he continued. “Also, RT-PCR has a high turnaround time which is a problem in Brazil. This impacts isolation strategy and patient management.”

RT-PCR is also costly and unaffordable to resource-poor countries. Hence, there’s a window of opportunity for combining antigen (Ag) and antibody (Ab) rapid tests for increased mass testing and surveillance, Ferreira added.

Best of three strategies

His team compared three diagnostic testing strategies – RT-PCR, rapid Ag, and the combination of Ag and Ab rapid tests, in 1,399 adult individuals suspected of having COVID-19. All patients received an RT-PCR test, the Panbio COVID-19 Ag Rapid Test, and the Panbio COVID-19 IgG/IgM Rapid Test.

At 0-12 days following symptom onset, 431 cases were detected using the Ag rapid test. More cases were detected using the Ag/Ab combined test vs RT-PCR (494 vs 478 cases, or a 3.3 percent increase in positives). When the data were split to 0-7 days after symptom onset, the impact of the Ag/Ab test became more evident (425 cases were detected vs 407 for RT-PCR, or a 4.4 percent increase in positives).

“The huge number of patients in need of diagnosis and the increasing demand for speeding up test results require new diagnostic alternatives,” said Ferreira. “The use of combined Ag/Ab rapid test allows for massive and reliable testing of COVID-19 in minutes and reduces testing cost.”

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