Higher viral load and infectivity increase risk of aerosol transmission for Delta and Omicron variants of SARS-CoV-2.

BACKGROUND: Airborne transmission of SARS-CoV-2 is an important route of infection. For the wildtype (WT) only a small proportion of those infected emitted large quantities of the virus. The currently prevalent variants of concern, Delta (B1.617.2) and Omicron (B.1.1.529), are characterized by higher viral loads and a lower minimal infective dose compared to the WT. We aimed to describe the resulting distribution of airborne viral emissions and to reassess the risk estimates for public settings given the higher viral load and infectivity. METHOD: We reran the Monte Carlo modelling to estimate viral emissions in the fine aerosol size range using available viral load data. We also updated our tool to simulate indoor airborne transmission of SARS-CoV-2 by including a CO2 calculator and recirculating air cleaning devices. We also assessed the consequences of the lower critical dose on the infection risk in public settings with different protection strategies. RESULTS: Our modelling suggests that a much larger proportion of individuals infected with the new variants are high, very high or super-emitters of airborne viruses: for the WT, one in 1,000 infected was a super-emitter; for Delta one in 30; and for Omicron one in 20 or one in 10, depending on the viral load estimate used. Testing of the effectiveness of protective strategies in view of the lower critical dose suggests that surgical masks are no longer sufficient in most public settings, while correctly fitted FFP2 respirators still provide sufficient protection, except in high aerosol producing situations such as singing or shouting. DISCUSSION: From an aerosol transmission perspective, the shift towards a larger proportion of very high emitting individuals, together with the strongly reduced critical dose, seem to be two important drivers of the aerosol risk, and are likely contributing to the observed rapid spread of the Delta and Omicron variants of concern. Reducing contacts, always wearing well-fitted FFP2 respirators when indoors, using ventilation and other methods to reduce airborne virus concentrations, and avoiding situations with loud voices seem critical to limiting these latest waves of the COVID-19 pandemic.

Reactions of macrophages exposed to particles <10 microm.

This study describes experiments on cytotoxic effects and the production of oxidative radicals and the proinflammatory cytokine tumor growth factor alpha (TNFalpha) in a cell line of rat lung macrophages exposed to aqueous extracts from ambient air particles <10 microm (PM(10)) collected on Teflon filters. The particles were collected during the four seasons at two urban sites, one rural site, and one alpine site in Switzerland. Cytotoxic effects, determined as a reduction in the metabolic activity, were found in particle extracts from all sites and seasons. Taking together the data from all sites and seasons, a dose-response function was observed between the particle mass on the filter and toxicity (r(2)=0.633, linear regression). The release of the pro-inflammatory cytokine TNFalpha as well as of oxidative radicals was most pronounced in particles collected in spring-summer and autumn. While at Montana (alpine), the stimulation of the cells was positively correlated with the particle mass on the filters, this correlation was negative at the urban sites Zürich and Lugano. It is interpreted that at high PM(10) levels, as in these cities, macrophages are inhibited by increasing air pollution due to toxic effects. Cytotoxic effects and the release of oxidative radicals could be inhibited when the extracts were treated with an endotoxin-neutralizing protein. This suggests that endotoxin, a cell-wall constituent of gram-negative bacteria, is one of the factors which modulates macrophage activity. All together, the experiments indicate that in the PM(10) fraction, water-soluble macrophage-toxic and macrophage-stimulating compounds are present. The data offer an explanation for at least some of the known harmful effects of PM(10), and confirm endotoxin as a possible reactant.

Ambient PM(10) extracts inhibit phagocytosis of defined inert model particles by alveolar macrophages.

We used inert melamin particles that are very well defined in size (diameters: 0.5, 1.9, 6.8 microm) to study the effects of ambient particles of <10 microm (PM(10)) on phagocytosis. Dose-response functions were found between the amount of added melamin particle mass and the toxicity. Fine particles (0.5 microm) were more toxic at low and medium amounts of mass per cell added than the larger particles (1.9, 6.8 microm). However, with regard to particle numbers applied per cell, toxicity is reversed, with the largest particles being the most toxic. In the whole dose range tested, the melamin particles used did not stimulate cells to produce oxidative radicals or tumor necrosis factor-alpha (TNF alpha). Flow cytometric analyses visualized the time-dependent melamin particle uptake, which was inhibited by polyinosinic acid (Poly-I), suggesting that phagocytosis is mediated by scavenger-type receptors. Cells exposed to an aqueous PM(10) extract, which also had a dose-dependent toxic potential, were stimulated to produce oxidative radicals, measured as NO(2)(-), and TNF alpha. Combined exposures with the PM(10) extract followed by the inert melamin particles show that the PM(10) extract inhibits inert particle uptake by alveolar macrophages. It is proposed that ambient PM(10), besides being toxic for alveolar macrophages, stimulates them to produce oxidative radicals and the proinflammotry cytokine TNF alpha. Moreover, it inhibits their particle uptake potential. Thus PM(10) appears to promote inflammation and reduce defense.

Comparison of black smoke and PM2.5 levels in indoor and outdoor environments of four European cities.

Recent studies on separated particle-size fractions highlight the health significance of particulate matter smaller than 2.5 microm (PM2.5), but gravimetric methods do not identify specific particle sources. Diesel exhaust particles (DEP) contain elemental carbon (EC), the dominant light-absorbing substance in the atmosphere. Black smoke (BS) is a measure for light absorption of PM and, thus, an alternative way to estimating EC concentrations, which may serve as a proxy for diesel exhaust emissions. We analyzed PM2.5 and BS data collected within the EXPOLIS study (Air Pollution Exposure Distribution within Adult Urban Populations in Europe) in Athens, Basel, Helsinki, and Prague. 186 indoor/outdoor filter pairs were sampled and analyzed. PM2.5 and BS levels were lowest in Helsinki, moderate in Basel, and remarkably higher in Athens and Prague. In each city, Spearman correlation coefficients of indoor versus outdoor were higher for BS (range rspearman: 0.57-0.86) than for PM2.5 (0.05-0.69). In a BS linear regression model (all data), outdoor levels explained clearly more of indoor variation (86%) than in the corresponding PM2.5 model (59%). In conclusion, ambient BS seizes a health-relevant fraction of fine particles to which people are exposed indoors and outdoors and exposure to which can be assessed by monitoring outdoor concentrations. BS measured on PM2.5 filters can be recommended as a valid and cheap additional indicator in studies on combustion-related air pollution and health.