COVID-19 research briefs: Antibody testing yields variable results; population study provides rich data on infection rates

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Published: 2020-08-16 © 2020 John Wiley & Sons, Inc.

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Clinical question
How effective is antibody testing for diagnosing COVID-19? What can a population-based study that tested 86% of the population in an Italian town tell us about infection rates?

Bottom line
The accuracy of antibody tests for COVID-19 are highly variable. The results from a study that tested an entire town in Italy during the peak of the epidemic suggest an infectious period between 3.6 days to 6.5 days, with infectiousness peaking on the day of symptom onset. (LOE = 2c)

Reference
Bastos ML, Tavaziva G, Abidi SK, et al. Diagnostic accuracy of serological tests for covid-19: systematic review and meta-analysis. BMJ 020;370:m2516. doi: http://dx.doi.org/10.1136 bmj.m2516

Study design: Not applicable

Funding source: Unknown/not stated

Setting: Other

Synopsis
Research Brief #45: This meta-analysis identified 40 studies of serologic tests for IgG or IgM antibodies to SARS-CoV-2. The authors searched standard databases, as well as preprint servers. Studies were evaluated in parallel by at least 2 researchers, and study quality was assessed using the QUADAS-2 tool. Most of the studies were done in China, used a case-control design, and studied inpatients. To estimate specificity, half the studies used samples collected before the pandemic (good), while half used samples from patients not suspected to have COVID-19 (bad, as many cases are asymptomatic). The risk of bias for patient selection was high for all but one study due to their case-control designs, and high or unclear for the index test due to the lack of masking and an uncertainty about how the cutoff for an abnormal test result was determined. There was a mix of commercial and home-grown tests. The chemiluminescent test had the best sensitivity by far and good specificity. For all the tests, sensitivity was unacceptably low for both IgG and IgM until at least 3 weeks after symptom onset. Given the high risk of bias and the lack of evaluation in outpatients, these tests should be used with caution and are not useful for acute diagnosis. An interactive calculator can be found here: https://www.bmj.com/content/369/bmj.m1808.full. Bastos ML, Tavaziva G, Abidi SK, et al. Diagnostic accuracy of serological tests for covid-19: systematic review and meta-analysis. BMJ 020;370:m2516. doi: http://dx.doi.org/10.1136 bmj.m2516 Research Brief #46: This is one of the only whole-population studies of COVID-19 to have been performed, and is likely more representative of the real world than similar studies of cruise ships, aircraft carriers, prisons, and maternity wards. The Italian town of Vo' was one of the first affected, and was entirely locked down early in the pandemic. Researchers tested everyone they could, which was 86% of the population between February 21 and 29, 2020, and 72% on March 7, 2020. These are the highlights from this very rich set of data. First, in late February, 2.6% of all samples were positive. Notably, the researchers found that 42.5% of those infected were asymptomatic at the time of testing and never became symptomatic. The time to clear the infection for those with a positive result on the first test and negative result on the second test ranged from 8 days to 13 days, with a mean of 9.3 days. The viral load was similar for symptomatic and asymptomatic individuals. Through contact tracing, the researchers determined that asymptomatic individuals can transmit the infection. The lockdown reduced the effective reproductive number from 2.49 to 0.41. Finally, the authors estimate an infectious period between 3.6 days to 6.5 days, with infectiousness peaking on the day of symptom onset. Lavezzo E, Franchin E, Ciavarella C, et al. Suppression of a SARS-CoV-2 outbreak in the Italian municipality of Vo'. Nature 2020; https://doi.org/10.1038/s41586-020-2488-1 Research brief #46: This is one of the only 'whole population' studies of COVID-19 to have been performed, and is likely more representative of the real world than similar studies on cruise ships, aircraft carriers, prisons, and maternity wards. The Italian town of Vo' was one of the first affected, and was entirely locked down earlier in the pandemic. Researchers tested everyone they could, 86% of the population between February 21 and 29, and 72% on March 7, 2020. This is a very rich dataset: here are the highlights. First, in late February, 2.6% of all samples were positive. Importantly, they found that 42.5% of those infected were asymptomatic at the time of testing, and never became symptomatic. The time to clear the infection for those positive on the first survey and negative on the second ranged from 8 to 13 days, with a mean of 9.3 days. The viral load was similar for symptomatic and asymptomatic individuals, and through contact tracing they determined that asymptomatic individuals can transmit the infection. The lockdown reduced the effective reproductive number from 2.49 to 0.41. Finally, they estimate an infectious period between 3.6 to 6.5 days, with infectiousness peaking on the day of symptom onset. Lavezzo E, Franchin E, Ciavarella C, et al. Suppression of a SARS-CoV-2 outbreak in the Italian municipality of Vo'. Lancet 2020; https:// doi.org/10.1038/s41586-020-2488-1.

Mark H. Ebell, MD, MS
Professor
University of Georgia
Athens, GA

Copyright © 2020 John Wiley & Sons, Inc.