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Objectives: The immune response in children elicited by SARS‐CoV‐2 Omicron infection alone or in combination with COVID‐19 vaccination (hybrid immunity) is poorly understood. We examined the humoral and cellular immune response following SARS‐CoV‐2 Omicron infection in unvaccinated children and children who were previously vaccinated with COVID‐19 mRNA vaccine. Methods: Participants were recruited as part of a household cohort study conducted during the Omicron predominant wave (Jan to July 2022) in Victoria, Australia. Blood samples were collected at 1, 3, 6 and 12 months following COVID‐19 diagnosis. Humoral immune responses to SARS‐CoV‐2 Spike proteins from Wuhan, Omicron BA.1, BA.4/5 and JN.1, as well as cellular immune responses to Wuhan and BA.1 were assessed. Results: A total of 43 children and 113 samples were included in the analysis. Following Omicron infection, unvaccinated children generated low antibody responses but elicited Spike‐specific CD4 and CD8 T‐cell responses. In contrast, vaccinated children infected with the Omicron variant mounted robust humoral and cellular immune responses to both ancestral strain and Omicron subvariants. Hybrid immunity persisted for at least 6 months post infection, with cellular immune memory characterised by the generation of Spike‐specific polyfunctional CD8 T‐cell responses. Conclusion: SARS‐CoV‐2 hybrid immunity in children is characterised by persisting SARS‐CoV‐2 antibodies and robust CD4 and CD8 T‐cell activation and polyfunctional responses. Our findings contribute to understanding hybrid immunity in children and may have implications regarding COVID‐19 vaccination and SARS‐CoV‐2 re‐infections. [ABSTRACT FROM AUTHOR]

Background: Data available for RSV and influenza infections among children < 2 years in Mongolia are limited. We present data from four districts of Ulaanbaatar from April 2015 to June 2021. Methods: This study was nested in an enhanced surveillance project evaluating pneumococcal conjugate vaccine (PCV13) impact on the incidence of hospitalized lower respiratory tract infections (LRTIs). Our study was restricted to children aged < 2 years with arterial O2 saturation < 93% and children with radiological pneumonia. Nasopharyngeal (NP) swabs collected at admission were tested for RSV and influenza using qRT‐PCR. NP swabs of all patients with radiological pneumonia and of a subset of randomly selected NP swabs were tested for S. pneumoniae (S.p.) by qPCR and for serotypes by culture and DNA microarray. Results: Among 5705 patients, 2113 (37.0%) and 386 (6.8%) had RSV and influenza infections, respectively. Children aged 2–6 months had a higher percentage of very severe RSV infection compared to those older than 6 months (42.2% versus 31.4%, p‐value Fisher's exact = 0.001). S.p. carriage was detected in 1073/2281 (47.0%) patients. Among S.p. carriage cases, 363/1073 (33.8%) had S.p. and RSV codetection, and 82/1073 (7.6%) had S.p. and influenza codetection. S.p. codetection with RSV/influenza was not associated with more severe LRTIs, compared to only RSV/influenza cases. Conclusion: In Mongolia, RSV is an important pathogen causing more severe LRTI in children under 6 months of age. Codetection of RSV or influenza virus and S.p. was not associated with increased severity. [ABSTRACT FROM AUTHOR]

Preterm birth is associated with aberrant pulmonary development and increased susceptibility to a range of chronic lung diseases. Even in healthy preterms, the prevalence of physician‐diagnosed asthma is far higher than in infants born at term. While physiological, environmental, and genetic factors have been studied extensively, few studies have investigated the immunological factors underpinning this increased susceptibility. Lower rates of atopy and allergic sensitization in preterm compared to term infants suggests non‐allergic mechanisms may be driving asthma development in preterms. Preterm infants are more likely to develop severe RSV and HRV disease and have altered microbiomes compared to term infants. Therefore, investigating the differences in immunological interactions (e.g., response to viral infections, microbiome) between children born preterm and term will aid in understanding the immunological basis for their increased susceptibility to asthma development. This is critical to inform the development of interventions to reduce the burden of asthma in this highly vulnerable demographic. [ABSTRACT FROM AUTHOR]

Background: Household studies are crucial for understanding the transmission of SARS‐CoV‐2 infection, which may be underestimated from PCR testing of respiratory samples alone. We aim to combine the assessment of household mitigation measures; nasopharyngeal, saliva, and stool PCR testing; along with mucosal and systemic SARS‐CoV‐2–specific antibodies, to comprehensively characterize SARS‐CoV‐2 infection and transmission in households. Methods: Between March and September 2020, we obtained samples from 92 participants in 26 households in Melbourne, Australia, in a 4‐week period following the onset of infection with ancestral SARS‐CoV‐2 variants. Results: The secondary attack rate was 36% (24/66) when using nasopharyngeal swab (NPS) PCR positivity alone. However, when respiratory and nonrespiratory samples were combined with antibody responses in blood and saliva, the secondary attack rate was 76% (50/66). SARS‐CoV‐2 viral load of the index case and household isolation measures were key factors that determine secondary transmission. In 27% (7/26) of households, all family members tested positive by NPS for SARS‐CoV‐2 and were characterized by lower respiratory Ct values than low transmission families (Median 22.62 vs. 32.91; IQR 17.06–28.67 vs. 30.37–34.24). High transmission families were associated with enhanced plasma antibody responses to multiple SARS‐CoV‐2 antigens and the presence of neutralizing antibodies. Three distinguishing saliva SARS‐CoV‐2 antibody features were identified according to age (IgA1 to Spike 1, IgA1 to nucleocapsid protein (NP)), suggesting that adults and children generate distinct mucosal antibody responses during the acute phase of infection. Conclusion: Utilizing respiratory and nonrespiratory PCR testing, along with the measurement of SARS‐CoV‐2–specific local and systemic antibodies, provides a more accurate assessment of infection within households and highlights some of the immunological differences in response between children and adults. [ABSTRACT FROM AUTHOR]

Background Human respiratory syncytial virus (RSV) is an important community and nosocomial pathogen in developed countries but data regarding the importance of RSV in developing countries are relatively scarce. Methods During a 1-year surveillance study in 2010, we took serial samples from children admitted to the Emergency Unit of the Respiratory Ward of Children's Hospital 1 in Ho Chi Minh City, Vietnam. RSV was detected within 72 hours of admission to the ward in 26% (376/1439; RSV A: n = 320; RSV B: n = 54; and RSV A and B: n = 2). Among those negative in the first 72 hours after admission, 66% (25/377) acquired nosocomial RSV infection during hospitalization (RSV A: n = 22; and RSV B: n = 3). Results Children with nosocomial RSV infection were younger (P = 0001) and had a longer duration of hospitalization (P < 0.001). The rate of incomplete recovery among children with nosocomial RSV infection was significantly higher than among those without (P < 0.001). Phylogenetic analysis of partial G gene sequences obtained from 79% (316/401) of positive specimens revealed the co-circulation of multiple genotypes with RSV A NA1 being predominant (A NA1: n = 275; A GA5: n = 5; B BA3: n = 3; B BA9: n = 26; and B BA10: n = 7). The RSV A GA5 and RSV B BA3 genotypes have not been reported from Vietnam, previously. Conclusion Besides emphasizing the importance of RSV as a cause of respiratory infection leading to hospitalization in young children and as a nosocomial pathogen, data from this study extend our knowledge on the genetic diversity of RSV circulating in Vietnam. [ABSTRACT FROM AUTHOR]