Exhaled aerosols among PCR-confirmed SARS-CoV-2-infected children

The study aimed to understand the role of children in SARS-CoV-2 transmission and whether aerosol measurements could identify superspreaders. Initial understanding of SARS-CoV-2 transmission focused on large droplets and fomites, but airborne transmission via aerosols is now recognized as crucial, particularly explaining superspreading events and indoor/outdoor transmission differences. Aerosol particle behavior is influenced by size, density, shape, temperature, humidity, and air circulation. While aerosol emission varies interindividually and with activity (increased during singing or shouting), there's limited data on children. Children often experience mild or asymptomatic COVID-19, possibly due to lower ACE2 receptor expression, which allows viral entry. This study investigated aerosol emissions in children and adolescents to assess their contribution to SARS-CoV-2 spread.

The literature review highlighted the evolving understanding of SARS-CoV-2 transmission, initially focusing on large droplets and fomite transmission before shifting to recognize the significant role of airborne transmission via aerosols. The review discussed the factors influencing the physical behavior of exhaled aerosol particles and existing knowledge gaps regarding aerosol emission in children. The limited data available on children's role in SARS-CoV-2 transmission and the hypothesis of lower ACE2 receptor expression in children impacting their susceptibility to infection were also discussed.

This monocentric prospective study included 250 children and adolescents (2-17 years) divided into three age groups (2-5, 6-11, 12-17 years). Participants were recruited from pediatricians' offices, schools, and sports clubs. Aerosol concentrations and size distribution were measured using the Resp-Aer-Meter (Palas GmbH), detecting particles between 145 nm and 10 µm. Before measurement, participants or guardians provided information on health status, medical history, allergies, medication, vaccination status, and past SARS-CoV-2 infection. Participants underwent PCR testing for SARS-CoV-2. During measurement, participants breathed orally through a mouthpiece connected to the Resp-Aer-Meter, with nasal breathing prevented by a nose clip. A washout phase removed initial environmental aerosols before a 60-90 second measurement phase. Statistical analysis employed linear regression models, initially bivariate and then multiple, incorporating age, sex, BMI, symptoms, vaccination status, and past infection. Model selection used the Akaike Information Criterion (AIC).

The study included 105 SARS-CoV-2-positive and 145 SARS-CoV-2-negative children and adolescents. The median age was 9 years. The median particle count for all participants was 79.55 particles/liter. In the bivariate model, SARS-CoV-2 PCR status was not predictive of exhaled aerosol particle count (t=0.82, p=0.415). However, in the optimized multiple regression model controlling for age, BMI, COVID-19 vaccination status, and past SARS-CoV-2 infection, SARS-CoV-2 status was a significant predictor (t=2.81, p=0.005). There was a tendency for older children (12-17 years) to exhale more particles (98 p/L) than younger children (2-5 years; 79 p/L). Past SARS-CoV-2 infection was significantly associated with increased particle counts in the multiple regression model (t=2.26, p=0.025). COVID-19 vaccination status was a highly significant predictor of exhaled aerosol particle count (p<0.001), with a median of 133 p/L in vaccinated subjects compared to 74.8 p/L in unvaccinated subjects. No significant differences were found based on sex, cough, rhinitis, sore throat, fever, allergies, tobacco exposure, or pre-existing conditions. A significant difference in the size distribution of exhaled aerosols was found between SARS-CoV-2 positive and negative subjects (p=0.041), with a narrower distribution in the positive group, predominantly below 0.5 µm in both groups.

The findings showed substantially lower particle concentrations in children compared to healthy adults reported in previous studies using the same method. This might be due to differences in respiratory tract structure, respiratory maneuvers, surfactant production, and respiratory minute volume. Contrary to some previous studies during Delta variant prevalence, this study found no significant difference in exhaled aerosol particles between SARS-CoV-2-positive and -negative children during Omicron variant dominance. The study found no evidence of superspreading events among children. The unexpectedly high aerosol particle counts in vaccinated individuals warrants further investigation, potentially related to altered surfactant production during or after COVID-19. The different variants might account for the discrepancies found when compared with other studies.

This study demonstrates that exhaled aerosol particle measurement is not suitable for identifying SARS-CoV-2 infection in children and adolescents, unlike in adults. While children exhale fewer particles than adults, the results provide valuable insight into aerosol production in this age group. Future research could investigate the influence of other factors on aerosol production and explore longitudinal measurements to examine the dynamics of aerosol production during infection.

The study's limitations include the inclusion of only mildly ill, non-hospitalized children, the potential for higher aerosol counts in severely ill individuals, and the time lag between aerosol measurement and PCR testing. The study did not test for other respiratory pathogens, nor did it include longitudinal measurements or control for all environmental factors that might influence aerosol production or measurement. The majority of participants were infected with the Omicron variant, limiting the generalizability of the findings.