ABSTRACT
The Omicron variant of SARS-CoV-2 (B.1.1.529), first identified during November 2021, is rapidly spreading throughout the world, replacing the previously dominant Delta variant. Omicron has a high number of mutations in the spike gene, some of which are associated with greatly increased transmissibility and immune evasion. The BA.1 sublineage has been most prevalent but there is recent evidence that the BA.2 sublineage is increasing in proportion in many countries. Genome sequencing is the gold standard for Omicron identification but is relatively slow, resource intensive, of limited capacity and often unavailable. We therefore developed a simple, rapid reverse transcription PCR (RT-PCR) method for sensitive and specific detection of the Omicron variant, including both the BA.1 and BA.2 sublineages. The assay targets a total of 5 nucleotide mutations in the receptor binding domain of the spike gene that give rise to 4 amino acid substitutions at G339D, S371L, S373P and S375F. The forward primer was designed as a double-mismatch allele specific primer (DMAS) with an additional artificial mismatch located four nucleotides from the 3 end to enhance binding specificity. Assay specificity was confirmed by testing a wide range of previously-sequenced culture-derived viral isolates and clinical samples including the Alpha, Beta and Delta variants and wild type SARS-CoV-2. Respiratory syncytial virus and influenza A were also tested. The assay can be run in singleplex format, or alternatively as a multiplex RT-PCR to enable Omicron and Delta variants to be detected and distinguished within the same reaction by means of probes labelled with different fluorescent dyes. Sublineages BA.1 and BA.2 can be differentiated if required. The methods presented here can readily be established in any PCR laboratory and should provide valuable support for epidemiologic surveillance of Omicron infections, particularly in those regions that lack extensive sequencing facilities.
ABSTRACT
BackgroundLateral flow immunoassays (LFIAs) have the potential to deliver affordable, large scale antibody testing and provide rapid results without the support of central laboratories. As part of the development of the REACT programme extensive evaluation of LFIA performance was undertaken with individuals following natural infection. Here we assess the performance of the selected LFIA to detect antibody responses in individuals who have received at least one dose of SARS-CoV-2 vaccine. MethodsThis is a prospective diagnostic accuracy study. SettingSampling was carried out at renal outpatient clinic and healthcare worker testing sites at Imperial College London NHS Trust. Laboratory analyses were performed across Imperial College London sites and university facilities. ParticipantsTwo cohorts of patients were recruited; the first was a cohort of 108 renal transplant patients attending clinic following SARS-CoV-2 vaccine booster, the second cohort comprised 40 healthcare workers attending for first SARS-CoV-2 vaccination, and 21 day follow up. A total of 186 paired samples were collected. InterventionsDuring the participants visit, capillary blood samples were analysed on LFIA device, while paired venous sampling was sent for serological assessment of antibodies to the spike protein (anti-S) antibodies. Anti-S IgG were detected using the Abbott Architect SARS-CoV-2 IgG Quant II CMIA. Main outcome measuresThe accuracy of Fortress LFIA in detecting IgG antibodies to SARS-CoV-2 compared to anti-spike protein detection on Abbott Assay. ResultsUsing the threshold value for positivity on serological testing of [≥]7.10 BAU/ml, the overall performance of the test produces an estimate of sensitivity of 91.94% (95% CI 85.67% to 96.06%) and specificity of 93.55% (95% CI 84.30% to 98.21%) using the Abbott assay as reference standard. ConclusionsFortress LFIA performs well in the detection of antibody responses for intended purpose of population level surveys, but does not meet criteria for individual testing.