Mycobacterium tuberculosis infection is a dynamic continuum. Clinical outcomes reflect complex host-pathogen interactions. Epidemiological and animal studies have suggested influenza coinfection as a risk factor for progression from contained infection to active disease, but human studies have been lacking. Using a whole blood luminescent mycobacterial growth inhibition assay within a human influenza challenge study, we show that influenza infection reduces immunological control of mycobacterial growth. Transcriptome-wide RNA sequencing, cytokine and cellular analyses of subjects' blood before and after influenza infection reveal that innate immune pathways, including type 1 interferon signalling, are activated by influenza but their subsequent responsiveness to mycobacteria is reduced, with multiple genes' responses to BCG lux infection repressed by influenza coinfection. Our data suggest that influenza infection impairs immune mechanisms that contain mycobacterial growth and may be a risk factor for tuberculosis (TB) disease. Influenza vaccination might offer high risk, high prevalence populations protection against TB disease.

Introduction: Abdominal aortic aneurysms (AAA) are strongly correlated with lower extremity aneurysms (LEA), with AAA present in 50-90% of patients with femoral aneurysms and up to 50% in those with popliteal aneurysms. Conversely, around 14% of patients with AAA have concurrent popliteal aneurysms. While the genetic architecture of AAA has been well characterized, the genetic basis of lower extremity aneurysms (LEA) comprising those in the femoral and popliteal arteries remains unknown. Methods: We conducted a genome-wide association study for LEA in the Million Veterans Program. Veterans were divided into European, African and Hispanic cohorts based on genetically similarity to 1000 Genomes reference populations. Association testing was conducted for each respective population and results were combined with inverse variance weighted fixed effects meta-analysis. Genetic correlation between LEA, AAA, TAAD, and peripheral artery disease (PAD) were estimated using linkage disequilibrium score regression (LDSC). The top 5 lead variants were then examined for colocalization with AAA. Results: After quality control measures, approximately 25 million variants tested for association with LEA in 3,094 Veterans with LEA and 594,497 Veterans without LEA (Figure 1). We identified 63 genome-wide significant variants across three loci (p < 5 × 10 -8 ) (Table 1). Additionally, 1,473 variants spanning 406 loci reached suggestive significance (p < 5 × 10 -5 ). Genetic correlation analysis using LDSC demonstrated a strong shared genetic architecture between LEA and AAA (rg = 0.882, p = 1 × 10 -7 ), a moderate correlation with PAD (rg = 0.687, p = 4 × 10 -6 ), and weaker correlation with TAAD (rg = 0.391, p = 1 × 10 -2 ) (Figure 2). Pairwise colocalization analysis revealed significant overlap between LEA and AAA at three of the top five lead variants, with a greater than 80% likelihood of the same causal genetic signal observed at the LPA , CDKN2B-AS1 , and AGPAT4 loci. Conclusion: This study identifies 3 novel genome wide significant loci associated with LEA. Furthermore, we provide evidence for genetic contribution to the relationship between LEA and AAA – particularly at the LPA , CDKN2B-AS1 and AGAPT4 loci. These findings provide a foundation for further investigation into the genetic etiology of LEA.

The 2025 World Allergy Organization (WAO) Guidelines for the Classification, Diagnosis, and Treatment of Hereditary Angioedema (HAE) with Consideration of Worldwide Disparities provide a comprehensive, evidence-informed, and globally applicable framework for the care of this rare and potentially life-threatening disorder. HAE is a genetic disease characterized by recurrent episodes of subcutaneous and submucosal swelling, most commonly mediated by bradykinin, and is associated with substantial morbidity, impaired quality of life, and a lifelong risk of fatal laryngeal edema.The Guidelines were developed by an international panel of 40 experts from 22 countries, with representation from all world regions, reflecting the commitment of WAO to geographic diversity, inclusiveness, and global relevance. The development process for these guidelines followed a structured and transparent methodology that integrated systematic literature review, appraisal of real-world evidence, and application of the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) framework adapted for rare diseases, complemented by a formal Delphi consensus process. This approach was specifically designed to address the limitations of conventional evidence hierarchies in rare disorders, while ensuring clinical applicability across heterogeneous healthcare systems and resource settings.A central element of the guidelines is an updated classification of HAE based on underlying pathophysiology and disease endotypes. The traditional distinction between HAE types 1 and 2 is unified under the term HAE with C1 inhibitor deficiency (HAE-C1-INH), reflecting shared biological mechanisms and management principles. The guidelines also recognize an expanding spectrum of HAE with normal C1 inhibitor (HAE-nC1-INH), including forms associated with pathogenic variants in F12, PLG, ANGPT1, KNG1, MYOF, HS3ST6, CPN1, and DAB2IP, as well as cases with currently unidentified genetic causes.The diagnostic strategy emphasizes early clinical recognition based on characteristic features, including recurrent angioedema without urticaria, abdominal or laryngeal involvement, early symptom onset, and family history. A simplified diagnostic algorithm is proposed, prioritizing the C1 inhibitor functional assay as the preferred initial test when performed in a reliable specialized laboratory. Alternative diagnostic pathways are outlined for settings with limited access to specialized testing, including pragmatic combinations of biochemical assays and selective use of genetic testing, particularly relevant for HAE-nC1-INH and family screening.Management recommendations address on-demand treatment of acute attacks, short-term prophylaxis, and individualized long-term prophylaxis. Universal access to on-demand therapy is emphasized for all patients with confirmed HAE, including those who are asymptomatic, given the unpredictable nature of attacks and lifelong risk. Long-term prophylaxis is addressed within a treat-to-target framework aimed at achieving complete disease control and sustained improvement in health-related quality of life, with regular reassessment and shared decision-making. Empowering patients and caregivers through structured education, access to appropriate medications, and integration with specialized referral centers is associated with earlier treatment, reduced healthcare utilization, and improved equity of care and reduced avoidable morbidity and mortality worldwide.The 2025 WAO Guidelines for Hereditary Angioedema establish an evidence-informed, patient-centered, and forward-looking framework for the classification, diagnosis, and management of HAE. By integrating advances in pathophysiology, diagnostics, and therapeutics with global expert consensus and real-world considerations, the guidelines aim to support consistent, equitable, and high-quality care for patients with HAE across regions and healthcare systems.

Introduction Young children and children living with HIV are at high risk of progressing to tuberculosis (TB) disease following Mycobacterium tuberculosis (Mtb) exposure and infection, and also of developing severe forms of disease and TB-related mortality. Identifying children who have very early (sub-clinical) TB disease, prior to progression to clinically apparent TB, would mean that TB preventive treatment (TPT) could be more efficiently targeted to this group. Identifying biomarker changes on drug therapy in children with Mtb infection or very early disease could pave the way for the development of tests that can identify which children have viable bacilli and are therefore at increased risk of disease progression.Methods and analysis The INTREPID study will use already collected samples taken from well-phenotyped paediatric cohorts in three clinical studies conducted in South Africa in children <5 years, including a drug-resistant TPT trial (TB-CHAMP), an observational household contact study (interferon-gamma release assay studies) and a prospective diagnostic study (Umoya), all conducted in a setting with a high burden of TB and HIV. We will employ transcriptomic, proteomic, metabolomic and serology approaches to analyse changes in host blood profiles at every stage along the TB continuum, from Mtb exposure to disease and from children treated for Mtb infection and early TB disease, as well as targeted Mtb antibody analysis. Data on viral co-infections and relevant clinical and epidemiological parameters will be integrated and evaluated to identify the optimal biosignatures that can predict future progression to clinically overt disease in children below 5 years of age, including those living with HIV.Ethics and dissemination The study protocol received ethical approval from the Stellenbosch University Health Research Ethics Committee (N23/03/025). The study findings will be disseminated through peer-reviewed publications, scientific conferences and formal presentations to healthcare professionals and to local communities, in collaboration with the Desmond Tutu TB Centre Community Advisory Board.

Vascular injury and coagulopathy are key drivers of mortality in bacterial sepsis. In Neisseria meningitidis infection, endothelial adhesion and thrombosis cause the characteristic petechial rash and, in the most severe cases, purpura fulminans. Although antibiotics rapidly kill bacteria, inflammation and vascular injury often persist or worsen after bacterial clearance, suggesting ongoing toxicity from released bacterial components. Here we identify bacterial histone-like proteins (HLPs), small positively charged DNA-binding proteins conserved across bacterial species, as previously unrecognized mediators of vascular damage. In vitro HLPs are released following antibiotic exposure, disrupting endothelial integrity. In patients with severe sepsis, they are detectable in plasma and tissue, colocalising with areas of vascular leak and coagulopathy. Non-anticoagulant heparins and anti-HLP antibodies neutralize HLP-induced endothelial disruption and toxicity in vitro and in vivo. These findings reveal HLPs as antibiotic-released bacterial toxins and suggest new therapeutic strategies to prevent vascular injury in sepsis.

Transcriptomic analyses reveal the status of cells, tissues, or organisms, across states of health and disease. RNA velocity adds a temporal dimension to single cell analyses, predicting future transcriptomic and phenotypic states, based on the current spliced and unspliced mRNA of each cell. We hypothesized that RNA velocity could be adapted to predict future clinical state of individuals with acute and chronic illnesses, using their whole-blood transcriptomes. We developed VeloCD, a method for quantitative prediction of transitions in clinical state from a single time-point RNA sample. This predicts transcriptomic trajectories and future infection status in influenza A and SARS-CoV-2 controlled human infection studies, which are consistent with trajectories in naturally acquired infections. In HIV-TB coinfected individuals, VeloCD predicts the onset of immune reconstitution inflammatory syndrome. In individuals receiving biological therapy for inflammatory bowel disease, whole blood RNA velocity after the first dose of treatment indicates whether remission will be achieved by the end of the treatment course. In a multinational observational study of acutely unwell febrile children, VeloCD predicts those with greatest medical care requirements. Our results demonstrate proof-of-concept for the use of RNA velocity to predict trajectories of human diseases.

Importance:Dilated cardiomyopathy (DCM) is a major cause of heart failure that disproportionately affects individuals of African genetic ancestry (AFR), among whom familial clustering of disease is also more pronounced relative to those of European ancestry (EUR). However, established monogenic DCM genes, identified primarily in EUR populations, explain a smaller proportion of DCM cases in AFR populations. A recent study identified a common AFR-specific nonsense variant in CD36 that accounts for a substantial burden of DCM in AFR. How the risk and population impact of this variant compare with those of established genetic causes of DCM is unknown. Objective:To compare the contribution of a CD36 nonsense variant to DCM risk with that of truncating variants in TTN and pathogenic or likely pathogenic (P/LP) variants in other established DCM genes. Design Setting and Participants:Multicohort genetic association study including AFR and EUR participants with exome or genome sequence and DCM case status from four datasets: All of Us, Million Veteran Program, Penn Medicine Biobank, and the DCM Precision Medicine Study. Exposure:Carrier status for TTN truncating variants, P/LP variants in 11 high confidence DCM genes, and the CD36 nonsense variant (Y325*; 0, 1, or 2 copies). Main Outcomes and Measures:Odds of DCM; prevalence of risk-variant carriers among DCM cases; and population attributable fraction (PAF) for DCM. Results:Among 82,623 AFR individuals across four studies, the mean age was 53.4 years and 1,625 had DCM. CD36 Y325* risk-allele homozygotes had 4.8-fold (95% CI, 3.1-7.3) increased odds of DCM, and CD36 Y325* heterozygotes had 1.4-fold (95% CI, 1.2-1.7) increased odds. TTN truncating variants also conferred elevated risk of DCM in AFR participants (OR, 8.46; 95% CI, 5.3-12.3). Among AFR DCM cases, 2.5% were CD36 homozygotes, second only to TTN truncating variants (4.3%) and exceeding all other high-confidence DCM genes combined (1.5%). In population-level analyses incorporating both heterozygous and homozygous CD36 Y325* carriers, the population-attributable fraction for CD36 (9.0%) surpassed that of TTN truncating variants (3.6%). Conclusions and Relevance:An ancestry-specific CD36 variant contributes more to DCM burden in AFR ancestry than established DCM genes, including TTN truncating variants, typically considered the most common genetic cause of DCM. These findings reshape the known genetic architecture of DCM in individuals of African ancestry and highlight the importance of representation in genomic research. Key Points:Question: To what extent does a common, African ancestry-specific nonsense variant in CD36 contribute to the genetic architecture of dilated cardiomyopathy (DCM) in individuals of African ancestry in the United States? Findings: In an analysis of African ancestry individuals from multiple U.S.-based cohorts, a CD36 nonsense variant accounted for a greater population burden of DCM than TTN truncating variants and pathogenic or likely pathogenic variants in other established DCM genes. Meaning: A single ancestry-specific CD36 variant substantially alters current understanding of DCM genetic architecture in individuals of African ancestry and underscores the importance of including ancestral diversity in all genetic studies.

Background: Identifying the cause of infection is important for clinical management and public health decisions, including vaccination strategies. In low-resource settings, causes of fever are often not identified. In this study, molecular testing panels were used to identify the causes of pediatric fever in the Kathmandu Valley, Nepal. A dengue fever outbreak facilitated the investigation of dengue diagnostics. Methods: Children under 14 years of age were recruited to this prospective cohort study at Patan Hospital, Nepal. Clinical data and routine diagnostics were used to classify cases, including nonstructural protein 1 (NS1) antigen testing for dengue. Additional molecular diagnostics were performed on blood (12 viral, 26 bacterial and 6 fungal targets) and respiratory samples (17 viral and 3 bacterial targets). Results: From September 1, 2021, to April 19, 2023, 565 children were enrolled, median age 3 (interquartile-range 1-7) years. Pathogens identified included dengue virus (n = 101), respiratory syncytial virus (n = 30), influenza (n = 25), typhoidal Salmonella spp. (n = 7) and Neisseria meningitidis (n = 2). During the dengue outbreak, dengue polymerase chain reaction (PCR) and NS1 positivity rates were both high early in dengue disease, but if >3 days of symptoms, PCR positivity rates declined (10.3%) while NS1 positivity remained high into the second week of illness (80%). Conclusions: This prospective cohort study is the most comprehensive effort to date to describe the causes of pediatric fever in the Kathmandu Valley, Nepal. The United States Centers for Disease Control and Prevention recommends dengue PCR or NS1 antigen testing during the first 7 days of dengue fever. Our data indicate that PCR positivity declines after 3 days of symptoms, resulting in missed cases when relying solely on PCR.

Controlled human influenza infection studies can uniquely interrogate the early immune factors associated with clinical outcome. In this study, 27 healthy volunteers with low strain-specific serum neutralizing antibody levels were challenged with influenza A/H3N2 virus. Twenty-two became infected, with 18 developing mild-to-moderate symptoms and four remaining asymptomatic. Local and systemic immune profiling revealed innate pathways that engaged more rapidly and to a higher level in symptomatic participants. Earlier monocyte and dendritic cell activation correlated with higher symptom scores but also enhanced natural killer and CD8+ T cell activation thereafter. At baseline, peripheral blood mononuclear cells from symptomatic participants were more responsive to in vitro challenge, indicating a predisposition to divergent immunological outcomes at the time of virus exposure that was subsequently modulated by infection. These results show that human innate cell responsiveness is a predeterminant of both symptomatic disease and cellular immune responses known to promote viral clearance, suggesting potential targets for therapeutic intervention if decoupled.

Introduction: Estrogen-related receptors (ERRs) are nuclear receptors essential for postnatal cardiac maturation. Our recent studies have shown that ERRs are necessary for coordinated activation of cardiomyocyte metabolic and structural gene programs through interactions with PGC-1 coactivators and cardiogenic factors such as GATA4. In heart failure (HF), ERR transcriptional programs may revert to a fetal-like state, leading to metabolic inefficiency and energy starvation. Methods: To better understand ERRs’ role in HF, 176 SNPs from a recent HF GWAS were mapped onto the human cardiomyocyte cistrome to identify potential HF-associated variants proximal to ERRγ and/or ERRα binding regions. Overlaps were analyzed for ERR binding motifs with an 80% position-weight matrix threshold. Permutation testing was performed to determine if HF-associated variants were found more frequently than expected near ERR binding regions. Hypergeometric testing assessed whether HF-associated SNPs were enriched in ERR binding regions compared to genome-wide SNPs, with per-chromosome p-values combined using Fisher’s method. Stratified linkage disequilibrium score regression (S-LDSC) was performed to quantify whether SNPs within ERR binding regions disproportionately contributed to HF heritability compared to genome-wide averages. Results: Both permutation and hypergeometric testing showed significant enrichment of HF-associated variants proximal to ERR binding regions (p < 0.05). The number of overlaps increased with the size of the flanking region, ranging from 48 at 5 kb, to 135 at 50 kb. Consensus ERR motifs were identified in 6.4% of ERRγ overlaps and 4.0% of ERRα overlaps. S-LDSC demonstrated notable HF heritability enrichment within ERRγ binding regions (enrichment score: 8.8 ± 3.0, p = 0.010), whereas ERRα showed non-significant enrichment (3.8 ± 3.3, p = 0.426), and GATA4 showed negligible enrichment (−0.4 ± 5.27, p = 0.786). Conclusion: This computational analysis reveals significant enrichment of HF-associated SNPs in/near genomic regions containing ERR transcription factors, highlighting ERR-driven transcriptional networks as potentially important mechanisms underlying heart failure development. These results establish a platform for future functional experimental studies to determine the impact of HF-associated variants on ERR-mediated transcriptional regulatory function and additional computational analyses to clarify the precise biological roles of ERRs in HF progression.

BACKGROUND:White blood cell count (WBC) is a widely used marker for the prediction of serious bacterial infection (SBI); however, previous research has shown poor performance. This study aims to assess the value of WBC in the prediction of SBI in children at the emergency department (ED) and compare its value with C reactive protein (CRP) and absolute neutrophil count (ANC). METHODS:This study is an observational multicentre study including febrile children aged 0-18 years attending 1 of 12 EDs in 8 European countries. The association between WBC and SBI was assessed by multivariable logistic regression, adjusting for age, CRP and duration of fever. Additionally, diagnostic performance was assessed by sensitivity and specificity. Results were compared with CRP and ANC. RESULTS:We included 17 082 children with WBC measurements, of which 1854 (10.9%) had an SBI. WBC >15 had an adjusted OR of 1.9 (95% CI 1.7 to 2.1) for prediction of SBI, after adjusting for confounders. Sensitivity and specificity were 0.56 (95% CI 0.54 to 0.58) and 0.74 (0.73 to 0.75) for WBC >15, and 0.32 (0.30 to 0.34) and 0.91 (0.91 to 0.91) for WBC >20, respectively. In comparison, CRP >20 mg/L had a sensitivity of 0.87 (95% CI 0.85 to 0.88) and a specificity of 0.59 (0.58 to 059). For CRP >80 mg/L, the sensitivity was 0.55 (95% CI 0.52 to 057) and the specificity was 0.91 (0.90 to 0.91). Additionally, for ANC >10, the sensitivity was 0.55 (95% CI 0.53 to 0.58) and the specificity was 0.75 (0.75 to 0.76). The combination of WBC and CRP did not improve performance compared with CRP alone. CONCLUSION:WBC does not have diagnostic benefit in identifying children with an SBI compared with CRP and should only be measured for specific indications.

Mycoplasma pneumoniae causes atypical pneumonia in children and young adults. Its lack of a cell wall makes it resistant to beta-lactams, which are the first-line treatment for typical pneumonia. Current diagnostic tests are time-consuming and have low specificity, leading clinicians to administer empirical antibiotics. Using a LASSO regression simulation approach and blood microarray data from 107 children with pneumonia (including 30 M. pneumoniae) we identify eight different transcriptomic signatures, ranging from 3-10 transcripts, that differentiate mycoplasma pneumonia from other bacterial/viral pneumonias with high accuracy (AUC: 0.84–0.95). Additionally, we demonstrate that existing signatures for broadly distinguishing viral/bacterial infections and viral/bacterial pneumonias are ineffective in distinguishing M. pneumoniae from viral pneumonia. The new signatures are successfully validated in an independent RNAseq cohort of children with pneumonia, demonstrating their robustness. The high sensibility of these signatures presents a valuable opportunity to guide the treatment and management of M. pneumoniae pneumonia patients. Using blood microarray data from 107 children with pneumonia, the authors here identify eight transcriptomic signatures that distinguish Mycoplasma pneumoniae pneumonia from other viral and bacterial pneumonias, paving the way for precise diagnosis and targeted treatment guidance.

Invasive meningococcal disease (IMD) imposes a heavy burden of mortality and life-long sequelae on infected individuals and has devastating impacts on their family members. International data show that meningococcal vaccination programs have reduced IMD incidence and changed the serogroup distribution of the disease. Furthermore, newer data show that although the public health measures in response to the coronavirus disease 2019 (COVID-19) pandemic temporarily reduced the incidence of IMD, there has been a resurgence in the years since. In the Asia-Pacific (APAC) region, many countries do not include meningococcal vaccines in their routine vaccination programs, and approaches to IMD surveillance are inconsistent. This review summarizes recent data and consensus statements from a group of experts from selected APAC countries on the burden of IMD in the region, evidence for vaccination, and how barriers to IMD vaccination may be addressed.

Background: Blood lipid levels are among the leading causal risk factors for cardiovascular disease. While genome-wide association studies (GWAS) have identified numerous loci associated with mean lipid levels, genetic contributions to blood lipid variability remain underexplored. To address this, we conducted a multi-population genome-wide variance quantitative trait loci (vQTL) analysis to identify single nucleotide polymorphisms (SNPs) influencing the variability of blood lipid traits. Methods: vQTL analyses were performed on five circulating lipid measures: high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), total cholesterol (TC), triglycerides (TG), and lipoprotein(a) (Lp(a)) in the Penn Medicine Biobank (PMBB). Analyses were implemented using QUAIL, which transforms the phenotype into a quantile integrated rank score and employs quantile regression to assess genetic effects on trait variance. Analyses were performed for each population, adjusting for age, sex, and genetic principal components. Genome-wide significance was set at p < 5e-8. Results: vQTL analysis identified 306 genome-wide significant loci associated with lipid variability. Nearby genes identified included CETP, CELSR2, APOA5, LPL, LPA, and PPM1H. Several loci overlapped with known GWAS loci for lipid levels, while others were novel, suggesting unexplored genetic mechanisms regulating variance. Conclusions: These findings provide new insights into the genetic regulation of lipid variability, which may improve risk stratification for cardiovascular disease and inform precision medicine approaches. Future analyses will replicate this work in the other cohorts (MVP, AoU, and UKB) to validate findings and further elucidate the genetic architecture of lipid variability.

Background: Variants at the APOH genomic locus have been associated with lipid traits, Lp(a), coagulation traits, coronary artery disease, and fatty liver disease. The mechanisms by which the ApoH protein influences these cardiometabolic phenotypes are not understood. An APOH missense variant C325G (rs1801689) has a minor allele frequency of 2.8% and is predicted to impact protein structure, making it a potential genetic tool for understanding ApoH biology. Hypothesis: We hypothesize that the association of APOH C325G with clinical phenotypes and circulating proteins and metabolites will provide new insights into the relationship between APOH and cardiometabolic traits. Methods: We accessed internal data from the Penn Medicine BioBank (PMBB) and publicly available data from the UK Biobank, the Million Veteran Program, and the All of Us cohort for these analyses. In addition, siRNA was used to knock down APOH in HuH-7 hepatocytes. Results: PheWAS of APOH C325G in the PMBB identified association with a number of cardiometabolic conditions, several of which were replicated in the other cohorts. Analyses of lab data indicated that APOH C325G was associated with lower triglyceride (TG) levels and higher LDL-C, apoB, and Lp(a) levels (p < 0.01). Analysis of NMR-derived plasma metabolites in UKB showed that APOH C325G is associated with highly significant reductions in the larger VLDL particle subclasses, increases in the small VLDL particle subclasses, and increases in all LDL subclasses. Analysis of UKB proteomic data revealed that APOH C325G is associated with significantly reduced plasma levels of the ApoH protein and changes in other circulating proteins that are related to lipid metabolism and cardiometabolic traits (p < 5 x 10 -8 ). Mendelian randomization analysis suggested that genetically determined lower ApoH plasma levels causally decrease plasma TG levels (p < 5 x 10 -8 ). Finally, siRNA silencing of APOH in human Huh7 cells led to reduced TG secretion (two-sample t-test, p < 0.05). Conclusions: The APOH missense variant C325G is associated with reduced TGs, large VLDLs, and increased LDL, apoB, Lp(a), and cardiometabolic conditions. It may serve as a useful genetic tool to understand the biology of APOH . Silencing of APOH in hepatocytes reduces TG production. Recall-by-genotype deep phenotyping of APOH C325G carriers in the PMBB is underway.

Importance:The upsurge in invasive disease caused by Streptococcus pyogenes among children reported in several European countries during 2022 to 2023 has not been fully explained. Objective:To evaluate whether changes in the circulation of common respiratory pathogens associated with the introduction of nonpharmaceutical interventions (NPIs) during the COVID-19 pandemic were associated with acquisition of immunity to S pyogenes and common respiratory viruses. Design, Setting, and Participants:This cross-sectional study recruited children with suspected infection and afebrile control participants at hospitals in 10 European countries. Data were collected before (September 2016 to March 2020) and after (April 2020 to July 2023) the introduction of NPIs. Main Outcomes and Measures:Molecular detection of bacterial and viral pathogens on throat swabs and age-stratified total serum immunoglobin G (IgG) reactivity to S pyogenes cell wall extract from 2 strains, respiratory syncytial virus (RSV), 5 influenza viruses, 4 common cold coronaviruses, and SARS-CoV-2, measured by immunoassay. Results:Throat swabs from 1942 children aged 0 to 4 years were tested for respiratory pathogens (1449 recruited before introduction of NPIs [median (IQR) age, 19.7 (8.2-38.1) months; 798 (55.1%) male]; 493 recruited after [median (IQR) age, 20.7 (9.7-38.1) months; 269 (54.7%) male]). A decrease in detection of S pyogenes, RSV, common cold coronaviruses, and influenza viruses was observed between March 2020 to July 2021, corresponding to the maximal period of NPIs. Antibodies to S pyogenes were measured in 252 children recruited before NPIs and 200 thereafter. Antibodies to viral antigens were measured in 230 children before NPIs and 92 thereafter. Total IgG to S pyogenes and RSV was significantly lower in children aged 3 to 4 years recruited after NPI introduction compared with those recruited before (S pyogenes emm1 strain: after, 67 participants; median [IQR] 0.13 [0.44-0.44] relative units [RU]; before, 87 participants; median [IQR] 0.35 [0.10-0.65] RU; P = .007. RSV: after, 30 participants; median [IQR] 49.6 [31.1-120.7] mesoscale units [MU]/1000; before, 76 participants; median [IQR] 141.8 [78.1-423.1] MU/1000; P < .001). No such differences were observed for children aged 0 to 2 years or for individual influenza viruses or SARS-CoV-2. Conclusions and Relevance:In this cross-sectional study, there was a significant reduction in serum antibodies to S pyogenes and RSV in children aged 3 to 4 years after introduction of NPIs. Equivalent to approximately a 1-year delay in acquisition of immunity, these data suggest a putative biological basis for the 2022 to 2023 upsurge in severe S pyogenes infections in this age group.