Viral interference cannot be concluded from datasets containing only symptomatic patients – Authors' reply
Authors: Anchi Wu, Valia T Mihaylova, Marie L Landry, Ellen F Foxman
Journal: The Lancet Microbe (2021)

We appreciate the important point raised by Rickard Nordén and colleagues, that when using co-detection analysis to evaluate viral interference, if the virus detection rates in the study population do not match those in the larger community, then a mathematical bias could be introduced. However, if the virus detection rates in the study population do approximate those in the community, co-detection analysis is a useful approach. Based on the available data, the proportion of individuals testing positive for a virus in our study1Wu A Mihaylova VT Landry ML Foxman EF Interference between rhinovirus and influenza A virus: a clinical data analysis and experimental infection study.Lancet Microbe. 2020; 1: e254-e262Summary Full Text Full Text PDF PubMed Scopus (37) Google Scholar was similar to that in the community. Our analysis included 13 707 samples from adults tested with the respiratory virus PCR panel. Of these, 3218 (23·5%) were positive for a respiratory virus; 977 (7·2%) for rhinovirus only, 910 (6·7%) for influenza A only, and 12 (0·1%) for both viruses.1Wu A Mihaylova VT Landry ML Foxman EF Interference between rhinovirus and influenza A virus: a clinical data analysis and experimental infection study.Lancet Microbe. 2020; 1: e254-e262Summary Full Text Full Text PDF PubMed Scopus (37) Google Scholar Although we do not have community testing data that specifically match our study population and time frame (Nov 1 to March 1, 2016–17, 2017–18, and 2018–19), one year-round study of 137 adults and 77 children found that 737 (17·5%) of 4215 samples were virus-positive, regardless of symptoms,2Galanti M Birger R Ud-Dean M et al.Longitudinal active sampling for respiratory viral infections across age groups.Influenza Other Respir Viruses. 2019; 13: 226-232Crossref PubMed Scopus (25) Google Scholar and another study showed that 33 (8·3%) of 400 asymptomatic adults were rhinovirus-positive.3Granados A Goodall EC Luinstra K Smieja M Mahony J Comparison of asymptomatic and symptomatic rhinovirus infections in university students: incidence, species diversity, and viral load.Diagn Microbiol Infect Dis. 2015; 82: 292-296Crossref PubMed Scopus (28) Google Scholar These detection rates are similar to those observed in our study. Even if community detection rates were actually half of those observed in our study population, the numbers would still indicate a lower than expected odds of rhinovirus and influenza A co-detection (ie, 489 [3·6%] of 13 707 positive for rhinovirus only, 455 [3·3%] positive for influenza A only, and six [<0·1%] co-detections; odds ratio 0·344 [95% CI 0·153–0·774]), with observed co-detections significantly lower than expected by chance (χ2 p=0·0090). The theoretical analysis by Nordén and colleagues assumes that virus positivity is 12·5 times higher in the study population than in the community; an assumption that is not consistent with the virus positivity rates in our study.1Wu A Mihaylova VT Landry ML Foxman EF Interference between rhinovirus and influenza A virus: a clinical data analysis and experimental infection study.Lancet Microbe. 2020; 1: e254-e262Summary Full Text Full Text PDF PubMed Scopus (37) Google Scholar PCR panel results might match the community surveillance data better than other test modalities, such as rapid influenza testing, because patients receive this test for many reasons, including for non-specific symptoms. The 8284 tests mentioned in Nordén and colleagues' supplementary figure were the year-round rhinovirus-positive or influenza-positive tests from all testing modalities. These results were used only to establish seasonality, and were not used for co-detection analysis. Nordén and colleagues also cite interesting data by Essaidi-Laziosi and colleagues,4Essaidi-Laziosi M Geiser J Huang S Constant S Kaiser L Tapparel C Author correction: interferon-dependent and respiratory virus-specific interference in dual infections of airway epithelia.Sci Rep. 2020; 1012523Crossref Scopus (2) Google Scholar showing that some respiratory virus infections interfered with others, but that pre-infection with rhinovirus did not block influenza infection. While the second result might appear to contradict ours, the difference is not surprising, and highlights the importance of viral replication and host response kinetics. As stated in a published erratum,4Essaidi-Laziosi M Geiser J Huang S Constant S Kaiser L Tapparel C Author correction: interferon-dependent and respiratory virus-specific interference in dual infections of airway epithelia.Sci Rep. 2020; 1012523Crossref Scopus (2) Google Scholar this study used a much lower infectious dose of rhinovirus (multiplicity of infection of 0·001 vs 0·1 in our study), as well as a shorter interval between infections and a lower epithelial cell culture incubation temperature than our study.1Wu A Mihaylova VT Landry ML Foxman EF Interference between rhinovirus and influenza A virus: a clinical data analysis and experimental infection study.Lancet Microbe. 2020; 1: e254-e262Summary Full Text Full Text PDF PubMed Scopus (37) Google Scholar All of these conditions would delay and reduce interferon responses,5Foxman EF Storer JA Fitzgerald ME et al.Temperature-dependent innate defense against the common cold virus limits viral replication at warm temperature in mouse airway cells.Proc Natl Acad Sci USA. 2015; 112: 827-832Crossref PubMed Scopus (109) Google Scholar which are the basis of viral interference. Notably, Essaidi-Laziosi and colleagues4Essaidi-Laziosi M Geiser J Huang S Constant S Kaiser L Tapparel C Author correction: interferon-dependent and respiratory virus-specific interference in dual infections of airway epithelia.Sci Rep. 2020; 1012523Crossref Scopus (2) Google Scholar did show interferon-dependent viral interference for some sequential infections (eg, respiratory syncytial virus followed by rhinovirus), supporting the general concept that activation of host antiviral defences can lead to viral interference among respiratory viruses. We declare no competing interests. Interference between rhinovirus and influenza A virus: a clinical data analysis and experimental infection studyThese findings show that one respiratory virus can block infection with another through stimulation of antiviral defences in the airway mucosa, supporting the idea that interference from rhinovirus disrupted the 2009 IAV pandemic in Europe. These results indicate that viral interference can potentially affect the course of an epidemic, and this possibility should be considered when designing interventions for seasonal influenza epidemics and the ongoing COVID-19 pandemic. Full-Text PDF Open AccessViral interference cannot be concluded from datasets containing only symptomatic patientsIn the study by Anchi Wu and colleagues,1 the authors claim that rhinovirus reduces susceptibility to influenza A virus through a mechanism known as viral interference, basing their conclusions on viral detection rates in clinical samples and in-vitro experiments. We agree that viral interference might contribute to a reduction in the number of co-infections. However, this conclusion cannot be drawn from their data, as calculating the expected number of co-infections in a sample of non-randomly selected patients is not statistically accurate. Full-Text PDF Open Access