ABSTRACT
BackgroundAntigen lateral flow devices (LFDs) have been widely used to control SARS-CoV-2. Changes in LFD sensitivity and detection of infectious individuals during the pandemic with successive variants, vaccination, and changes in LFD use are incompletely understood. MethodsPaired LFD and PCR tests were collected from asymptomatic and symptomatic participants, across multiple settings in the UK between 04-November-2020 and 21-March-2022. Multivariable logistic regression was used to analyse LFD sensitivity and specificity, adjusting for viral load, LFD manufacturer, setting, age, sex, assistance, symptoms, vaccination, and variant. National contact tracing data were used to estimate the proportion of transmitting index cases (with [≥]1 PCR/LFD-positive contact) potentially detectable by LFDs over time, accounting for viral load, variant, and symptom status. Findings4131/75,382 (5.5%) participants were PCR-positive. Sensitivity vs. PCR was 63.2% (95%CI 61.7-64.6%) and specificity 99.71% (99.66-99.74%). Increased viral load was independently associated with being LFD-positive. There was no evidence LFD sensitivity differed between Delta vs. Alpha/pre-Alpha infections, but Omicron infections were more likely to be LFD positive. Sensitivity was higher in symptomatic participants, 68.7% (66.9-70.4%) than in asymptomatic participants, 52.8% (50.1-55.4%). 79.4% (68.6-81.3%) of index cases resulting in probable onward transmission with were estimated to have been detectable using LFDs, this proportion was relatively stable over time/variants, but lower in asymptomatic vs. symptomatic cases. InterpretationLFDs remained able to detect most SARS-CoV-2 infections throughout vaccine roll-out and different variants. LFDs can potentially detect most infections that transmit to others and reduce risks. However, performance is lower in asymptomatic compared to symptomatic individuals. FundingUK Government. Research in contextO_ST_ABSEvidence before this studyC_ST_ABSLateral flow devices (LFDs; i.e. rapid antigen detection devices) have been widely used for SARS-CoV-2 testing. However, due to their imperfect sensitivity when compared to PCR and a lack of a widely available gold standard proxy for infectiousness, the performance and use of LFDs has been a source of debate. We conducted a literature review in PubMed and bioRxiv/medRxiv for all studies examining the performance of lateral flow devices between 01 January 2020 and 31 October 2022. We used the search terms SARS-CoV-2/COVID-19 and antigen/lateral flow test/lateral flow device. Multiple studies have examined the sensitivity and specificity of LFDs, including several systematic reviews. However, the majority of the studies are based on pre-Alpha infections. Large studies examining the test accuracy for different variants, including Delta and Omicron, and following vaccination are limited. Added value of this studyIn this large national LFD evaluation programme, we compared the performance of three different LFDs relative to PCR in various settings. Compared to PCR testing, sensitivity was 63.2% (95%CI 61.7-64.6%) overall, and 71.6% (95%CI 69.8-73.4%) in unselected communitybased testing. Specificity was 99.71% (99.66-99.74%). LFDs were more likely to be positive as viral loads increased. LFD sensitivity was similar during Alpha/pre-Alpha and Delta periods but increased during the Omicron period. There was no association between sensitivity and vaccination status. Sensitivity was higher in symptomatic participants, 68.7% (66.9-70.4%) than in asymptomatic participants, 52.8% (50.1-55.4%). Using national contact tracing data, we estimated that 79.4% (68.6-81.3%) of index cases resulting in probable onward transmission (i.e. with [≥]1 PCR/LFD-positive contact) were detectable using LFDs. Symptomatic index cases were more likely to be detected than asymptomatic index cases due to higher viral loads and better LFD performance at a given viral load. The proportion of index cases detected remained relatively stable over time and with successive variants, with a slight increase in the proportion of asymptomatic index cases detected during Omicron. Implications of all the available evidenceOur data show that LFDs detect most SARS-CoV-2 infections, with findings broadly similar to those summarised in previous meta-analyses. We show that LFD performance has been relatively consistent throughout different variant-dominant phases of the pandemic and following the roll-out of vaccination. LFDs can detect most infections that transmit to others and can therefore be used as part of a risk reduction strategy. However, performance is lower in asymptomatic compared to symptomatic individuals and this needs to be considered when designing testing programmes.
ABSTRACT
BackgroundPre-Delta, vaccination reduced SARS-CoV-2 transmission from individuals infected despite vaccination, potentially via reducing viral loads. While vaccination still lowers the risk of infection, similar viral loads in vaccinated and unvaccinated individuals infected with Delta question how much vaccination prevents transmission. MethodsWe performed a retrospective observational cohort study of adult contacts of SARS-CoV-2-infected adult index cases using English contact testing data. We used multivariable Poisson regression to investigate associations between transmission and index case and contact vaccination, and how these vary with Alpha and Delta variants (classified using S-gene detection/calendar trends) and time since second vaccination. Results54,667/146,243(37.4%) PCR-tested contacts of 108,498 index cases were PCR-positive. Two doses of BNT162b2 or ChAdOx1 vaccines in Alpha index cases were independently associated with reduced PCR-positivity in contacts (aRR, adjusted rate ratio vs. unvaccinated=0.32[95%CI 0.21-0.48] and 0.48[0.30-0.78] respectively). The Delta variant attenuated vaccine-associated reductions in transmission: two BNT162b2 doses reduced Delta transmission (aRR=0.50[0.39-0.65]), more than ChAdOx1 (aRR=0.76[0.70-0.82]). Variation in Ct values (indicative of viral load) explained 7-23% of vaccine-associated transmission reductions. Transmission reductions declined over time post-second vaccination, for Delta reaching similar levels to unvaccinated individuals by 12 weeks for ChAdOx1 and attenuating substantially for BNT162b2. Protection in contacts also declined in the 3 months post-second vaccination. ConclusionsVaccination reduces transmission of Delta, but by less than the Alpha variant. The impact of vaccination decreased over time. Factors other than PCR Ct values at diagnosis are important in understanding vaccine-associated transmission reductions. Booster vaccinations may help control transmission together with preventing infections.
ABSTRACT
BackgroundSchool-based COVID-19 contacts in England are asked to self-isolate at home. However, this has led to large numbers of missed school days. Therefore, we trialled daily testing of contacts as an alternative, to investigate if it would affect transmission in schools. MethodsWe performed an open-label cluster randomised controlled trial in students and staff from secondary schools and further education colleges in England (ISRCTN18100261). Schools were randomised to self-isolation of COVID-19 contacts for 10 days (control) or to voluntary daily lateral flow device (LFD) testing for school contacts with LFD-negative contacts remaining at school (intervention). Household contacts were excluded from participation. Co-primary outcomes in all students and staff were symptomatic COVID-19, adjusted for community case rates, to estimate within-school transmission (non-inferiority margin: <50% relative increase), and COVID-19-related school absence. Analyses were performed on an intention to treat (ITT) basis using quasi-Poisson regression, also estimating complier average causal effects (CACE). Secondary outcomes included participation rates, PCR results in contacts and performance characteristics of LFDs vs. PCR. FindingsOf 99 control and 102 intervention schools, 76 and 86 actively participated (19-April-2021 to 27-June-2021); additional national data allowed most non-participating schools to be included in the co-primary outcomes. 2432/5763(42.4%) intervention arm contacts participated. There were 657 symptomatic PCR-confirmed infections during 7,782,537 days-at-risk (59.1/100k/week) and 740 during 8,379,749 days-at-risk (61.8/100k/week) in the control and intervention arms respectively (ITT adjusted incidence rate ratio, aIRR=0.96 [95%CI 0.75-1.22;p=0.72]) (CACE-aIRR=0.86 [0.55-1.34]). There were 55,718 COVID-related absences during 3,092,515 person-school-days (1.8%) and 48,609 during 3,305,403 person-school-days(1.5%) in the control and intervention arms (ITT-aIRR=0.80 [95%CI 0.53-1.21;p=0.29]) (CACE-aIRR 0.61 [0.30-1.23]). 14/886(1.6%) control contacts providing an asymptomatic PCR sample tested positive compared to 44/2981(1.5%) intervention contacts (adjusted odds ratio, aOR=0.73 [95%CI 0.33-1.61;p=0.44]). Rates of symptomatic infection in contacts were 44/4665(0.9%) and 79/5955(1.3%), respectively (aOR=1.21 [0.82-1.79;p=0.34]). InterpretationDaily contact testing of school-based contacts was non-inferior to self-isolation for control of COVID-19 transmission. COVID-19 rates in school-based contacts in both intervention and control groups were <2%. Daily contact testing is a safe alternative to home isolation following school-based exposures.
