A neonatal mouse model characterizes transmissibility of SARS-CoV-2 variants and reveals a role for ORF8
Rodriguez-Rodriguez, Bruno A; Ciabattoni, Grace O; Valero-Jimenez, Ana M; Crosse, Keaton M; Schinlever, Austin R; Galvan, Joaquin J Rodriguez; Duerr, Ralf; Yeung, Stephen T; McGrath, Marisa E; Loomis, Cynthia; Khanna, Kamal M; Desvignes, Ludovic; Frieman, Matthew F; Ortigoza, Mila B; Dittmann, Meike
Small animal models have been a challenge for the study of SARS-CoV-2 transmission, with most investigators using golden hamsters or ferrets 1,2 . Mice have the advantages of low cost, wide availability, less regulatory and husbandry challenges, and the existence of a versatile reagent and genetic toolbox. However, adult mice do not transmit SARS-CoV-2 3 . Here we establish a model based on neonatal mice that allows for transmission of clinical SARS-CoV-2 isolates. We characterize tropism, respiratory tract replication and transmission of ancestral WA-1 compared to variants alpha (B.1.1.7), beta (B.1.351), gamma (P.1), delta (B.1.617.2) and omicron (B.1.1.529). We identify inter-variant differences in timing and magnitude of infectious particle shedding from index mice, both of which shape transmission to contact mice. Furthermore, we characterize two recombinant SARS-CoV-2 lacking either the ORF6 or ORF8 host antagonists. The removal of ORF8 shifts viral replication towards the lower respiratory tract, resulting in significantly delayed and reduced transmission. Our results demonstrate the potential of our neonatal mouse model to characterize viral and host determinants of SARS-CoV-2 transmission, while revealing for the first time a role for an accessory protein this context.
Delta-Omicron recombinant escapes therapeutic antibody neutralization
Duerr, Ralf; Zhou, Hao; Tada, Takuya; Dimartino, Dacia; Marier, Christian; Zappile, Paul; Wang, Guiqing; Plitnick, Jonathan; Griesemer, Sara B.; Girardin, Roxanne; Machowski, Jessica; Bialosuknia, Sean; Lasek-Nesselquist, Erica; Hong, Samuel L.; Baele, Guy; Dittmann, Meike; Ortigoza, Mila B.; Prasad, Prithiv J.; McDonough, Kathleen; Landau, Nathaniel R.; St George, Kirsten; Heguy, Adriana
The emergence of recombinant viruses is a threat to public health, as recombination may integrate variant-specific features that together result in escape from treatment or immunity. The selective advantages of recombinant SARS-CoV-2 isolates over their parental lineages remain unknown. We identified a Delta-Omicron (AY.45-BA.1) recombinant in an immunosuppressed transplant recipient treated with monoclonal antibody Sotrovimab. The single recombination breakpoint is located in the spike N-terminal domain adjacent to the Sotrovimab binding site. While Delta and BA.1 are sensitive to Sotrovimab neutralization, the Delta-Omicron recombinant is highly resistant. To our knowledge, this is the first described instance of recombination between circulating SARS-CoV-2 variants as a functional mechanism of resistance to treatment and immune escape.
Molecularly distinct memory CD4+ T cells are induced by SARS-CoV-2 infection and mRNA vaccination
Gray-Gaillard, Sophie L; Solis, Sabrina; Monteiro, Clarice; Chen, Han M; Ciabattoni, Grace; Samanovic, Marie I; Cornelius, Amber R; Williams, Tijaana; Geesey, Emilie; Rodriguez, Miguel; Ortigoza, Mila Brum; Ivanova, Ellie N; Koralov, Sergei B; Mulligan, Mark J; Herati, Ramin Sedaghat
UNLABELLED:Adaptive immune responses are induced by vaccination and infection, yet little is known about how CD4+ T cell memory differs between these two contexts. Notable differences in humoral and cellular immune responses to primary mRNA vaccination were observed and associated with prior COVID-19 history, including in the establishment and recall of Spike-specific CD4+ T cells. It was unclear whether CD4+ T cell memory established by infection or mRNA vaccination as the first exposure to Spike was qualitatively similar. To assess whether the mechanism of initial memory T cell priming affected subsequent responses to Spike protein, 14 people who were receiving a third mRNA vaccination, referenced here as the booster, were stratified based on whether the first exposure to Spike protein was by viral infection or immunization (infection-primed or vaccine-primed). Using multimodal scRNA-seq of activation-induced marker (AIM)-reactive Spike-specific CD4+ T cells, we identified 220 differentially expressed genes between infection- and vaccine-primed patients at the post-booster time point. Infection-primed participants had greater expression of genes related to cytotoxicity and interferon signaling. Gene set enrichment analysis (GSEA) revealed enrichment for Interferon Alpha, Interferon Gamma, and Inflammatory response gene sets in Spike-specific CD4+ T cells from infection-primed individuals, whereas Spike-specific CD4+ T cells from vaccine-primed individuals had strong enrichment for proliferative pathways by GSEA. Finally, SARS-CoV-2 breakthrough infection in vaccine-primed participants resulted in subtle changes in the transcriptional landscape of Spike-specific memory CD4+ T cells relative to pre-breakthrough samples but did not recapitulate the transcriptional profile of infection-primed Spike-specific CD4+ T cells. Together, these data suggest that CD4+ T cell memory is durably imprinted by the inflammatory context of SARS-CoV-2 infection, which has implications for personalization of vaccination based on prior infection history. ONE SENTENCE SUMMARY/UNASSIGNED:SARS-CoV-2 infection and mRNA vaccination prime transcriptionally distinct CD4+ T cell memory landscapes which are sustained with subsequent doses of vaccine.
Delta-Omicron recombinant SARS-CoV-2 in a transplant patient treated with Sotrovimab [PrePrint]
Duerr, Ralf; Dimartino, Dacia; Marier, Christian; Zappile, Paul; Wang, Guiqing; Plitnick, Jonathan; Griesemer, Sara B; Lasek-Nesselquist, Erica; Dittmann, Meike; Ortigoza, Mila B; Prasad, Prithiv J; St George, Kirsten; Heguy, Adriana
We identified a Delta-Omicron SARS-CoV-2 recombinant in an unvaccinated, immunosuppressed kidney transplant recipient who had positive COVID-19 tests in December 2021 and February 2022 and was initially treated with Sotrovimab. Viral sequencing in February 2022 revealed a 5' Delta AY.45 portion and a 3' Omicron BA.1 portion with a recombination breakpoint in the spike N-terminal domain, adjacent to the Sotrovimab quaternary binding site. The recombinant virus induced cytopathic effects with characteristics of both Delta (large cells) and Omicron (cell rounding/detachment). Monitoring of immunosuppressed COVID-19 patients treated with antiviral monoclonal antibodies is crucial to detect potential selection of recombinant variants.
Efficacy and Safety of COVID-19 Convalescent Plasma in Hospitalized Patients: A Randomized Clinical Trial
Ortigoza, Mila B; Yoon, Hyunah; Goldfeld, Keith S; Troxel, Andrea B; Daily, Johanna P; Wu, Yinxiang; Li, Yi; Wu, Danni; Cobb, Gia F; Baptiste, Gillian; O'Keeffe, Mary; Corpuz, Marilou O; Ostrosky-Zeichner, Luis; Amin, Amee; Zacharioudakis, Ioannis M; Jayaweera, Dushyantha T; Wu, Yanyun; Philley, Julie V; Devine, Megan S; Desruisseaux, Mahalia S; Santin, Alessandro D; Anjan, Shweta; Mathew, Reeba; Patel, Bela; Nigo, Masayuki; Upadhyay, Rabi; Kupferman, Tania; Dentino, Andrew N; Nanchal, Rahul; Merlo, Christian A; Hager, David N; Chandran, Kartik; Lai, Jonathan R; Rivera, Johanna; Bikash, Chowdhury R; Lasso, Gorka; Hilbert, Timothy P; Paroder, Monika; Asencio, Andrea A; Liu, Mengling; Petkova, Eva; Bragat, Alexander; Shaker, Reza; McPherson, David D; Sacco, Ralph L; Keller, Marla J; Grudzen, Corita R; Hochman, Judith S; Pirofski, Liise-Anne; Parameswaran, Lalitha; Corcoran, Anthony T; Rohatgi, Abhinav; Wronska, Marta W; Wu, Xinyuan; Srinivasan, Ranjini; Deng, Fang-Ming; Filardo, Thomas D; Pendse, Jay; Blaser, Simone B; Whyte, Olga; Gallagher, Jacqueline M; Thomas, Ololade E; Ramos, Danibel; Sturm-Reganato, Caroline L; Fong, Charlotte C; Daus, Ivy M; Payoen, Arianne Gisselle; Chiofolo, Joseph T; Friedman, Mark T; Wu, Ding Wen; Jacobson, Jessica L; Schneider, Jeffrey G; Sarwar, Uzma N; Wang, Henry E; Huebinger, Ryan M; Dronavalli, Goutham; Bai, Yu; Grimes, Carolyn Z; Eldin, Karen W; Umana, Virginia E; Martin, Jessica G; Heath, Timothy R; Bello, Fatimah O; Ransford, Daru Lane; Laurent-Rolle, Maudry; Shenoi, Sheela V; Akide-Ndunge, Oscar Bate; Thapa, Bipin; Peterson, Jennifer L; Knauf, Kelly; Patel, Shivani U; Cheney, Laura L; Tormey, Christopher A; Hendrickson, Jeanne E
Importance/UNASSIGNED:There is clinical equipoise for COVID-19 convalescent plasma (CCP) use in patients hospitalized with COVID-19. Objective/UNASSIGNED:To determine the safety and efficacy of CCP compared with placebo in hospitalized patients with COVID-19 receiving noninvasive supplemental oxygen. Design, Setting, and Participants/UNASSIGNED:CONTAIN COVID-19, a randomized, double-blind, placebo-controlled trial of CCP in hospitalized adults with COVID-19, was conducted at 21 US hospitals from April 17, 2020, to March 15, 2021. The trial enrolled 941 participants who were hospitalized for 3 or less days or presented 7 or less days after symptom onset and required noninvasive oxygen supplementation. Interventions/UNASSIGNED:A unit of approximately 250 mL of CCP or equivalent volume of placebo (normal saline). Main Outcomes and Measures/UNASSIGNED:The primary outcome was participant scores on the 11-point World Health Organization (WHO) Ordinal Scale for Clinical Improvement on day 14 after randomization; the secondary outcome was WHO scores determined on day 28. Subgroups were analyzed with respect to age, baseline WHO score, concomitant medications, symptom duration, CCP SARS-CoV-2 titer, baseline SARS-CoV-2 serostatus, and enrollment quarter. Outcomes were analyzed using a bayesian proportional cumulative odds model. Efficacy of CCP was defined as a cumulative adjusted odds ratio (cOR) less than 1 and a clinically meaningful effect as cOR less than 0.8. Results/UNASSIGNED:Of 941 participants randomized (473 to placebo and 468 to CCP), 556 were men (59.1%); median age was 63 years (IQR, 52-73); 373 (39.6%) were Hispanic and 132 (14.0%) were non-Hispanic Black. The cOR for the primary outcome adjusted for site, baseline risk, WHO score, age, sex, and symptom duration was 0.94 (95% credible interval [CrI], 0.75-1.18) with posterior probability (P[cOR<1]â€‰=â€‰72%); the cOR for the secondary adjusted outcome was 0.92 (95% CrI, 0.74-1.16; P[cOR<1]â€‰=â€‰76%). Exploratory subgroup analyses suggested heterogeneity of treatment effect: at day 28, cORs were 0.72 (95% CrI, 0.46-1.13; P[cOR<1]â€‰=â€‰93%) for participants enrolled in April-June 2020 and 0.65 (95% CrI, 0.41 to 1.02; P[cOR<1]â€‰=â€‰97%) for those not receiving remdesivir and not receiving corticosteroids at randomization. Median CCP SARS-CoV-2 neutralizing titer used in April to June 2020 was 1:175 (IQR, 76-379). Any adverse events (excluding transfusion reactions) were reported for 39 (8.2%) placebo recipients and 44 (9.4%) CCP recipients (Pâ€‰=â€‰.57). Transfusion reactions occurred in 2 (0.4) placebo recipients and 8 (1.7) CCP recipients (Pâ€‰=â€‰.06). Conclusions and Relevance/UNASSIGNED:In this trial, CCP did not meet the prespecified primary and secondary outcomes for CCP efficacy. However, high-titer CCP may have benefited participants early in the pandemic when remdesivir and corticosteroids were not in use. Trial Registration/UNASSIGNED:ClinicalTrials.gov Identifier: NCT04364737.
Development and Validation of a Treatment Benefit Index to Identify Hospitalized Patients With COVID-19 Who May Benefit From Convalescent Plasma
Park, Hyung; Tarpey, Thaddeus; Liu, Mengling; Goldfeld, Keith; Wu, Yinxiang; Wu, Danni; Li, Yi; Zhang, Jinchun; Ganguly, Dipyaman; Ray, Yogiraj; Paul, Shekhar Ranjan; Bhattacharya, Prasun; Belov, Artur; Huang, Yin; Villa, Carlos; Forshee, Richard; Verdun, Nicole C; Yoon, Hyun Ah; Agarwal, Anup; Simonovich, Ventura Alejandro; Scibona, Paula; Burgos Pratx, Leandro; Belloso, Waldo; Avendaño-SolÃ¡, Cristina; Bar, Katharine J; Duarte, Rafael F; Hsue, Priscilla Y; Luetkemeyer, Anne F; Meyfroidt, Geert; Nicola, André M; Mukherjee, Aparna; Ortigoza, Mila B; Pirofski, Liise-Anne; Rijnders, Bart J A; Troxel, Andrea; Antman, Elliott M; Petkova, Eva
Importance:Identifying which patients with COVID-19 are likely to benefit from COVID-19 convalescent plasma (CCP) treatment may have a large public health impact. Objective:To develop an index for predicting the expected relative treatment benefit from CCP compared with treatment without CCP for patients hospitalized for COVID-19 using patients' baseline characteristics. Design, Setting, and Participants:This prognostic study used data from the COMPILE study, ie, a meta-analysis of pooled individual patient data from 8 randomized clinical trials (RCTs) evaluating CCP vs control in adults hospitalized for COVID-19 who were not receiving mechanical ventilation at randomization. A combination of baseline characteristics, termed the treatment benefit index (TBI), was developed based on 2287 patients in COMPILE using a proportional odds model, with baseline characteristics selected via cross-validation. The TBI was externally validated on 4 external data sets: the Expanded Access Program (1896 participants), a study conducted under Emergency Use Authorization (210 participants), and 2 RCTs (with 80 and 309 participants). Exposure:Receipt of CCP. Main Outcomes and Measures:World Health Organization (WHO) 11-point ordinal COVID-19 clinical status scale and 2 derivatives of it (ie, WHO score of 7-10, indicating mechanical ventilation to death, and WHO score of 10, indicating death) at day 14 and day 28 after randomization. Day 14 WHO 11-point ordinal scale was used as the primary outcome to develop the TBI. Results:A total of 2287 patients were included in the derivation cohort, with a mean (SD) age of 60.3 (15.2) years and 815 (35.6%) women. The TBI provided a continuous gradation of benefit, and, for clinical utility, it was operationalized into groups of expected large clinical benefit (B1; 629 participants in the derivation cohort [27.5%]), moderate benefit (B2; 953 [41.7%]), and potential harm or no benefit (B3; 705 [30.8%]). Patients with preexisting conditions (diabetes, cardiovascular and pulmonary diseases), with blood type A or AB, and at an early COVID-19 stage (low baseline WHO scores) were expected to benefit most, while those without preexisting conditions and at more advanced stages of COVID-19 could potentially be harmed. In the derivation cohort, odds ratios for worse outcome, where smaller odds ratios indicate larger benefit from CCP, were 0.69 (95% credible interval [CrI], 0.48-1.06) for B1, 0.82 (95% CrI, 0.61-1.11) for B2, and 1.58 (95% CrI, 1.14-2.17) for B3. Testing on 4 external datasets supported the validation of the derived TBIs. Conclusions and Relevance:The findings of this study suggest that the CCP TBI is a simple tool that can quantify the relative benefit from CCP treatment for an individual patient hospitalized with COVID-19 that can be used to guide treatment recommendations. The TBI precision medicine approach could be especially helpful in a pandemic.
Association of Convalescent Plasma Treatment With Clinical Status in Patients Hospitalized With COVID-19: A Meta-analysis
Troxel, Andrea B; Petkova, Eva; Goldfeld, Keith; Liu, Mengling; Tarpey, Thaddeus; Wu, Yinxiang; Wu, Danni; Agarwal, Anup; Avendaño-SolÃ¡, Cristina; Bainbridge, Emma; Bar, Katherine J; Devos, Timothy; Duarte, Rafael F; Gharbharan, Arvind; Hsue, Priscilla Y; Kumar, Gunjan; Luetkemeyer, Annie F; Meyfroidt, Geert; Nicola, André M; Mukherjee, Aparna; Ortigoza, Mila B; Pirofski, Liise-Anne; Rijnders, Bart J A; Rokx, Casper; Sancho-Lopez, Arantxa; Shaw, Pamela; Tebas, Pablo; Yoon, Hyun-Ah; Grudzen, Corita; Hochman, Judith; Antman, Elliott M
Importance:COVID-19 convalescent plasma (CCP) is a potentially beneficial treatment for COVID-19 that requires rigorous testing. Objective:To compile individual patient data from randomized clinical trials of CCP and to monitor the data until completion or until accumulated evidence enables reliable conclusions regarding the clinical outcomes associated with CCP. Data Sources:From May to August 2020, a systematic search was performed for trials of CCP in the literature, clinical trial registry sites, and medRxiv. Domain experts at local, national, and international organizations were consulted regularly. Study Selection:Eligible trials enrolled hospitalized patients with confirmed COVID-19, not receiving mechanical ventilation, and randomized them to CCP or control. The administered CCP was required to have measurable antibodies assessed locally. Data Extraction and Synthesis:A minimal data set was submitted regularly via a secure portal, analyzed using a prespecified bayesian statistical plan, and reviewed frequently by a collective data and safety monitoring board. Main Outcomes and Measures:Prespecified coprimary end points-the World Health Organization (WHO) 11-point ordinal scale analyzed using a proportional odds model and a binary indicator of WHO score of 7 or higher capturing the most severe outcomes including mechanical ventilation through death and analyzed using a logistic model-were assessed clinically at 14 days after randomization. Results:Eight international trials collectively enrolled 2369 participants (1138 randomized to control and 1231 randomized to CCP). A total of 2341 participants (median [IQR] age, 60 [50-72] years; 845 women [35.7%]) had primary outcome data as of April 2021. The median (IQR) of the ordinal WHO scale was 3 (3-6); the cumulative OR was 0.94 (95% credible interval [CrI], 0.74-1.19; posterior probability of OR <1 of 71%). A total of 352 patients (15%) had WHO score greater than or equal to 7; the OR was 0.94 (95% CrI, 0.69-1.30; posterior probability of OR <1 of 65%). Adjusted for baseline covariates, the ORs for mortality were 0.88 at day 14 (95% CrI, 0.61-1.26; posterior probability of OR <1 of 77%) and 0.85 at day 28 (95% CrI, 0.62-1.18; posterior probability of OR <1 of 84%). Heterogeneity of treatment effect sizes was observed across an array of baseline characteristics. Conclusions and Relevance:This meta-analysis found no association of CCP with better clinical outcomes for the typical patient. These findings suggest that real-time individual patient data pooling and meta-analysis during a pandemic are feasible, offering a model for future research and providing a rich data resource.
Neuraminidase B controls neuraminidase A-dependent mucus production and evasion
Hammond, Alexandria J; Binsker, Ulrike; Aggarwal, Surya D; Ortigoza, Mila Brum; Loomis, Cynthia; Weiser, Jeffrey N
Binding of Streptococcus pneumoniae (Spn) to nasal mucus leads to entrapment and clearance via mucociliary activity during colonization. To identify Spn factors allowing for evasion of mucus binding, we used a solid-phase adherence assay with immobilized mucus of human and murine origin. Spn bound large mucus particles through interactions with carbohydrate moieties. Mutants lacking neuraminidase A (nanA) or neuraminidase B (nanB) showed increased mucus binding that correlated with diminished removal of terminal sialic acid residues on bound mucus. The non-additive activity of the two enzymes raised the question why Spn expresses two neuraminidases and suggested they function in the same pathway. Transcriptional analysis demonstrated expression of nanA depends on the enzymatic function of NanB. As transcription of nanA is increased in the presence of sialic acid, our findings suggest that sialic acid liberated from host glycoconjugates by the secreted enzyme NanB induces the expression of the cell-associated enzyme NanA. The absence of detectable mucus desialylation in the nanA mutant, in which NanB is still expressed, suggests that NanA is responsible for the bulk of the modification of host glycoconjugates. Thus, our studies describe a functional role for NanB in sialic acid sensing in the host. The contribution of the neuraminidases in vivo was then assessed in a murine model of colonization. Although mucus-binding mutants showed an early advantage, this was only observed in a competitive infection, suggesting a complex role of neuraminidases. Histologic examination of the upper respiratory tract demonstrated that Spn stimulates mucus production in a neuraminidase-dependent manner. Thus, an increase production of mucus containing secretions appears to be balanced, in vivo, by decreased mucus binding. We postulate that through the combined activity of its neuraminidases, Spn evades mucus binding and mucociliary clearance, which is needed to counter neuraminidase-mediated stimulation of mucus secretions.
Type I Interferon Signaling Is a Common Factor Driving Streptococcus pneumoniae and Influenza A Virus Shedding and Transmission
Zangari, Tonia; Ortigoza, Mila B; Lokken-Toyli, Kristen L; Weiser, Jeffrey N
The dynamics underlying respiratory contagion (the transmission of infectious agents from the airways) are poorly understood. We investigated host factors involved in the transmission of the leading respiratory pathogen Streptococcus pneumoniae Using an infant mouse model, we examined whether S. pneumoniae triggers inflammatory pathways shared by influenza A virus (IAV) to promote nasal secretions and shedding from the upper respiratory tract to facilitate transit to new hosts. Here, we show that amplification of the type I interferon (IFN-I) response is a critical host factor in this process, as shedding and transmission by both IAV and S. pneumoniae were decreased in pups lacking the common IFN-I receptor (Ifnar1-/- mice). Additionally, providing exogenous recombinant IFN-I to S. pneumoniae-infected pups was sufficient to increase bacterial shedding. The expression of IFN-stimulated genes (ISGs) was upregulated in S. pneumoniae-infected wild-type (WT) but not Ifnar1-/- mice, including genes involved in mucin type O-glycan biosynthesis; this correlated with an increase in secretions in S. pneumoniae- and IAV-infected WT compared to Ifnar1-/- pups. S. pneumoniae stimulation of ISGs was largely dependent on its pore-forming toxin, pneumolysin, and coinfection with IAV and S. pneumoniae resulted in synergistic increases in ISG expression. We conclude that the induction of IFN-I signaling appears to be a common factor driving viral and bacterial respiratory contagion.IMPORTANCE Respiratory tract infections are a leading cause of childhood mortality and, globally, Streptococcus pneumoniae is the leading cause of mortality due to pneumonia. Transmission of S. pneumoniae primarily occurs through direct contact with respiratory secretions, although the host and bacterial factors underlying transmission are poorly understood. We examined transmission dynamics of S. pneumoniae in an infant mouse model and here show that S. pneumoniae colonization of the upper respiratory tract stimulates host inflammatory pathways commonly associated with viral infections. Amplification of this response was shown to be a critical host factor driving shedding and transmission of both S. pneumoniae and influenza A virus, with infection stimulating expression of a wide variety of genes, including those involved in the biosynthesis of mucin, a major component of respiratory secretions. Our findings suggest a mechanism facilitating S. pneumoniae contagion that is shared by viral infection.
An Infant Mouse Model of Influenza Virus Transmission Demonstrates the Role of Virus-Specific Shedding, Humoral Immunity, and Sialidase Expression by Colonizing Streptococcus pneumoniae
Ortigoza, Mila Brum; Blaser, Simone B; Zafar, M Ammar; Hammond, Alexandria J; Weiser, Jeffrey N
The pandemic potential of influenza A viruses (IAV) depends on the infectivity of the host, transmissibility of the virus, and susceptibility of the recipient. While virus traits supporting IAV transmission have been studied in detail using ferret and guinea pig models, there is limited understanding of host traits determining transmissibility and susceptibility because current animal models of transmission are not sufficiently tractable. Although mice remain the primary model to study IAV immunity and pathogenesis, the efficiency of IAV transmission in adult mice has been inconsistent. Here we describe an infant mouse model that supports efficient transmission of IAV. We demonstrate that transmission in this model requires young age, close contact, shedding of virus particles from the upper respiratory tract (URT) of infected pups, the use of a transmissible virus strain, and a susceptible recipient. We characterize shedding as a marker of infectiousness that predicts the efficiency of transmission among different influenza virus strains. We also demonstrate that transmissibility and susceptibility to IAV can be inhibited by humoral immunity via maternal-infant transfer of IAV-specific immunoglobulins and modifications to the URT milieu, via sialidase activity of colonizing Streptococcus pneumoniae Due to its simplicity and efficiency, this model can be used to dissect the host's contribution to IAV transmission and explore new methods to limit contagion.IMPORTANCE This study provides insight into the role of the virus strain, age, immunity, and URT flora on IAV shedding and transmission efficiency. Using the infant mouse model, we found that (i) differences in viral shedding of various IAV strains are dependent on specific hemagglutinin (HA) and/or neuraminidase (NA) proteins, (ii) host age plays a key role in the efficiency of IAV transmission, (iii) levels of IAV-specific immunoglobulins are necessary to limit infectiousness, transmission, and susceptibility to IAV, and (iv) expression of sialidases by colonizing S. pneumoniae antagonizes transmission by limiting the acquisition of IAV in recipient hosts. Our findings highlight the need for strategies that limit IAV shedding and the importance of understanding the function of the URT bacterial composition in IAV transmission. This work reinforces the significance of a tractable animal model to study both viral and host traits affecting IAV contagion and its potential for optimizing vaccines and therapeutics that target disease spread.