4. Discussion
Our findings show that in 2020, during the first and second outbreak of SARS-CoV-2 across Northern Italy, overall childhood asthma exacerbations were inhibited, as compared to the same calendar period of the previous 5 years: during lockdown-1, PED referrals incurred a significant decline by 85%, and in the following months by 80%, reflecting the slow, although incomplete return to almost regular activities during summer. An increase in PED referrals for acute asthma re-started in August, continued in September, with a peak in October, anticipating the so-called “September asthma epidemics”, the common burst of exacerbations generally observed with the start of the school year and due to a combination of infectious, allergic, environmental, and climatic triggers23. The peak in October was less pronounced than the previous years and this decreasing trend continued in November and December, when only 8 asthma referrals were registered, likely reflecting the strict hygiene measures and social distancing adopted as a consequence of lockdown-2. We also observed a prevalent reduction trend of hospital admissions and short-stay observations during the pandemic.
Our PED and many others, reported a substantial activity decrease by ca. 70-80% during lockdown-1, with drastic reductions in referrals for a wide spectrum of diagnoses1,2. In other studies, this was attributed to the introduction of new guidance for primary care (e.g. phone selection triage procedure, more tele-health) and fear of exposure to SARS-CoV-2 infection, leading to higher risk of severe illness from delayed diagnosis1,2,24. In contrast, the reduction of PED referrals for asthma exacerbations was associated with a significant reduction by 50-60% of high-priority cases, thus, the reluctance to seek hospital care cannot be the only explanation. A major role is likely played by a combination of factors.
As reported in other studies, social distancing and stepped-up hygiene measures determined a concomitant reduction in the diffusion of respiratory pathogens, leading to decreased respiratory tract infections3,25–27. Moreover, the feeling of the emergency period might have enhanced adherence to controller medications among asthmatic population27,28.
In addition, the reduced exposure to environmental factors – i.e. air pollution and pollen - may have played a role in the “anomalous” pattern of asthma exacerbations in 2020. With regard to changes in air quality during the pandemic, the gaseous pollutants NO2and C6H6, generally associated with fossil fuel combustion and directly correlated with vehicular traffic in urban context, showed substantially decreased concentrations during lockdown periods. In densely populated areas, this clearly mirrored a reduction of traffic-related sources in concomitance with the restrictive measures, consistent with observations at a larger scale in Italy and mobility drop of 75%29.
Unlike NO2 and C6H6, PM mass concentration was not significantly affected by lockdown measures. This can be explained by the complex emission patterns of PM in urbanized environment, encompassing both natural and anthropogenic, primary and secondary sources, thus leading to mass concentrations driven to a large extent by non-urban and non-traffic sources20. By contrast, a substantial reduction in the contribution of specific chemical components of traffic-related PM was clearly shown during lockdown-1, i.e. black carbon (BC) and the TRAP contribution tracer.
It is worth noting that traffic-related PM may gain relevance per unit mass of PM because of its higher oxidative potential and smaller size, leading to deposition in the deepest tracts of respiratory system16. Thus, these observations point to PM less enriched in ultrafine particles and those co-emitted components and micropollutants (e.g. BC, heavy metals and polycyclic aromatic hydrocarbons), which are recognized as specific asthma triggers, able to drive a cascade of pro-inflammatory and oxidative responses that increase the risks of acute pulmonary diseases30. This effect was likely amplified by less time spent outdoor and less movements during traffic rush-hours, reducing risk associated with proximity of children to strong local sources (exhaust and non-exhaust vehicular traffic). Moreover, a higher impact is expected in inner-city urban areas, where vehicular traffic presents a higher share of emissions. This evidence would suggest the recommendation for adopting more specific chemical tracers as air quality indicators, to elucidate associations between air pollution and respiratory diseases, despite the commonly used PM mass concentration.
On the contrary, pollen emissions are not impacted by changes in human behaviors and social restrictions, deriving from known natural sources. Indeed, during lockdown periods, pollen emissions did not present substantial variations beyond their interannual variability. However, the restrictive measures likely influenced exposure profiles of children in other ways, such as by reducing exposure to outdoor seasonal allergens.
In conclusion, several factors may explain the substantial decrease in the number of acute asthma PED referrals during the SARS-CoV-2 pandemic in 2020, with the concomitant reduction of high-priority codes. The restrictive measures adopted to limit the pandemic entailed a reduced exposure to viruses, outdoor environmental allergens, and air pollution, the latter notably exhibiting weakened inflammatory/oxidative properties.
Further investigations are needed to clarify in a more quantitative manner the role of each confounding trigger, driving synergistically variation in the clinical course of asthma. However, due to the peculiarity of the lockdown periods, which acted as a big experiment of anthropogenic air pollution cleansing and radical changes of human lifestyle, this study represented a unique opportunity to observe these hypotheses using real data from a severely SARS-CoV-2 affected urban area, reflecting what occurred in many densely populated areas around the world.