Rare immune-mediated cardiac tissue inflammation can occur after vaccination, including after SARS-CoV-2 mRNA vaccines. However, the underlying immune cellular and molecular mechanisms driving this pathology remain poorly understood. Here, we investigated a cohort of patients who developed myocarditis and/or pericarditis with elevated troponin, B-type natriuretic peptide, and C-reactive protein levels as well as cardiac imaging abnormalities shortly after SARS-CoV-2 mRNA vaccination. Contrary to early hypotheses, patients did not demonstrate features of hypersensitivity myocarditis, nor did they have exaggerated SARS-CoV-2–specific or neutralizing antibody responses consistent with a hyperimmune humoral mechanism. We additionally found no evidence of cardiac-targeted autoantibodies. Instead, unbiased systematic immune serum profiling revealed elevations in circulating interleukins (IL-1β, IL-1RA, and IL-15), chemokines (CCL4, CXCL1, and CXCL10), and matrix metalloproteases (MMP1, MMP8, MMP9, and TIMP1). Subsequent deep immune profiling using single-cell RNA and repertoire sequencing of peripheral blood mononuclear cells during acute disease revealed expansion of activated CXCR3 ⁺ cytotoxic T cells and NK cells, both phenotypically resembling cytokine-driven killer cells. In addition, patients displayed signatures of inflammatory and profibrotic CCR2 ⁺ CD163 ⁺ monocytes, coupled with elevated serum-soluble CD163, that may be linked to the late gadolinium enhancement on cardiac MRI, which can persist for months after vaccination. Together, our results demonstrate up-regulation in inflammatory cytokines and corresponding lymphocytes with tissue-damaging capabilities, suggesting a cytokine-dependent pathology, which may further be accompanied by myeloid cell–associated cardiac fibrosis. These findings likely rule out some previously proposed mechanisms of mRNA vaccine–-associated myopericarditis and point to new ones with relevance to vaccine development and clinical care.

Background: Incidence estimates of post-acute sequelae of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, also known as long COVID, have varied across studies and changed over time. We estimated long COVID incidence among adult and pediatric populations in 3 nationwide research networks of electronic health records (EHRs) participating in the RECOVER (Researching COVID to Enhance Recovery) Initiative using different classification algorithms (computable phenotypes). Methods: This EHR-based retrospective cohort study included adult and pediatric patients with documented acute SARS-CoV-2 infection and 2 control groups: contemporary coronavirus disease 2019 (COVID-19)–negative and historical patients (2019). We examined the proportion of individuals identified as having symptoms or conditions consistent with probable long COVID within 30–180 days after COVID-19 infection (incidence proportion). Each network (the National COVID Cohort Collaborative [N3C], National Patient-Centered Clinical Research Network [PCORnet], and PEDSnet) implemented its own long COVID definition. We introduced a harmonized definition for adults in a supplementary analysis. Results: Overall, 4% of children and 10%–26% of adults developed long COVID, depending on computable phenotype used. Excess incidence among SARS-CoV-2 patients was 1.5% in children and ranged from 5% to 6% among adults, representing a lower-bound incidence estimation based on our control groups. Temporal patterns were consistent across networks, with peaks associated with introduction of new viral variants. Conclusions: Our findings indicate that preventing and mitigating long COVID remains a public health priority. Examining temporal patterns and risk factors for long COVID incidence informs our understanding of etiology and can improve prevention and management.

IMPORTANCE: Nirsevimab, a long-acting monoclonal antibody, demonstrated efficacy against respiratory syncytial virus (RSV)–associated lower respiratory tract infections (LRTI) in clinical trials. Postlicensure monitoring is essential to confirm these benefits in clinical settings.OBJECTIVE: To estimate the effectiveness of nirsevimab against medically attended RSV infections in infants and to assess how effectiveness varies by disease severity, dosage, and time since immunization.DESIGN, SETTING, AND PARTICIPANTS: This test-negative case-control study utilized inpatient, outpatient, and emergency department data from the Yale New Haven Health System. Nirsevimab-eligible infants who were tested for RSV using polymerase chain reaction between October 1, 2023, and May 9, 2024, were included. Infants with RSV-positive results were cases and infants with RSV-negative results were controls.EXPOSURE: Nirsevimab immunization, verified through state immunization registries.MAIN OUTCOMES AND MEASURES: Effectiveness was estimated using multivariable logistic regression, adjusting for age, calendar month, and potential confounders. Separate models examined estimated effectiveness by clinical setting, dosage, time since immunization, and severity (defined as needing high-flow oxygen or intensive care unit admission). Broader outcomes were also analyzed, including all-cause LRTI and all-cause LRTI-associated hospitalization.RESULTS: The analytic sample included 3090 infants (1722 male [57.3%]; median [IQR] age at testing, 6.7 [3.6-9.7] months), with 680 (22.0%) RSV-positive cases and 2410 (78.0%) RSV-negative controls. Nirsevimab uptake was 10.7% (330 patients), with 21 RSV-positive cases and 309 RSV-negative controls immunized. Adjusted effectiveness was 68.4% (95% CI, 50.3%-80.8%) against medically attended RSV infection, 61.6% (95% CI, 35.6%-78.6%) against outpatient visits, and 80.5% (95% CI, 52.0%-93.5%) against hospitalizations. The highest estimated effectiveness (84.6%; 95% CI, 58.7%-95.6%) was observed against severe RSV disease. Although estimated effectiveness against RSV infections declined from 79.3% (95% credible interval, 63.4%-90.6%) at 2 weeks postimmunization to 54.8% (95% credible interval, 16.3%-74.7%) at 14 weeks postimmunization, it remained significant. Estimated effectiveness did not vary substantially by dosage. During peak RSV season, nirsevimab appeared effective against all-cause LRTI (49.4%; 95% CI, 10.7%-72.9%) and all-cause LRTI-associated hospitalizations (79.1%; 95% CI, 27.6%-94.9%). From February to May 2024, when most LRTIs were caused by other viruses, its estimated effectiveness against these broader outcomes was negligible.CONCLUSIONS AND RELEVANCE: In this case-control study, nirsevimab provided substantial protection against RSV-associated outcomes. These findings support its continued use and provide evidence that may help boost public confidence in the immunization program.

Background: Risk stratification is a cornerstone of the Pediatric Infectious Diseases Society COVID-19 treatment guidance. This systematic review and meta-analysis aimed to define the clinical characteristics and comorbidities associated with critical COVID-19 in children and adolescents. Methods: Two independent reviewers screened the literature (Medline and EMBASE) for studies published through August 31, 2023, that reported outcome data on patients aged ≤21 years with COVID-19. Critical disease was defined as an invasive mechanical ventilation requirement, intensive care unit admission, or death. Random-effects models were used to estimate pooled odds ratios (OR) with 95% confidence intervals (CI), and heterogeneity was explored through subgroup analyses. Results: Among 10,178 articles, 136 studies met the inclusion criteria for review. Data from 70 studies, which collectively examined 172,165 children and adolescents with COVID-19, were pooled for meta-analysis. In previously healthy children, the absolute risk of critical disease from COVID-19 was 4% (95% CI, 1%–10%). Compared with no comorbidities, the pooled OR for critical disease was 3.95 (95% CI, 2.78–5.63) for the presence of one comorbidity and 9.51 (95% CI, 5.62–16.06) for ≥2 comorbidities. Key risk factors included cardiovascular and neurological disorders, chronic pulmonary conditions (excluding asthma), diabetes, obesity, and immunocompromise, all with statistically significant ORs > 2.00. Conclusions: While the absolute risk for critical COVID-19 in children and adolescents without underlying health conditions is relatively low, the presence of one or more comorbidities was associated with markedly increased risk. These findings support the importance of risk stratification in tailoring pediatric COVID-19 management.

Congenital cytomegalovirus (cCMV) is the most common congenital infection and is associated with sensorineural hearing loss, developmental delays, and visual impairment. The clinical presentation of cCMV is variable, and the majority (80%–90%) of newborns will never manifest any clinical symptoms. Given the clinical heterogeneity of cCMV infection, it is challenging to identify which newborns may benefit from testing. Recently, certain states have implemented a targeted screening program in which newborns who fail the newborn hearing screen are tested for cCMV. Clinicians and legislative bodies have been propelled into debates about the ethical and moral permissibility of a targeted cCMV screening approach. Those who oppose this screening approach describe undue burden on patients, families, and the health care system because the majority of newborns who fail the newborn hearing screen and have cCMV will not go on to have any sequelae related to cCMV, including hearing loss. However, those who support this screening approach cite the importance of early detection and ongoing surveillance for hearing loss and developmental delays in this high-risk group of newborns. This debate will be considered by experts in the field.