Different age group are generally susceptible to SARS-CoV-2. Du et al. estimated that the basic reproduction number (R0 value) is 2.56 [13]. Since December 2019, there have been COVID-19 confirmed cases in all provinces and cities nationwide in China, and in many other countries. As of 6 February 2020, at least 230 COVID-19 cases in children (≤ 18 years) have been reported in China [14], suggesting that the SARS-CoV-2 has a strong transmission capacity in the special population (i.e., neonate and children). The present study indicates that infected preschool children have different clinical symptoms and CT imaging findings from those of adults. Symptoms may not be apparent in young children. Under the current epidemic conditions, cluster outbreaks in families have occurred. If children, especially preschool children, are not paid attention to, they may contribute to the spread of infection and may not be treated in a timely manner.
The incubation period of the novel coronavirus infection in children can be as short as 1 day or as long as 14 days, with an estimated mean of 5.2 days [15]. Of the eight cases in this study, two subjects had a travel history to Wuhan for surgery; however, these two cases were in Wuhan prior to the outbreak of the COVID-19. Based on the history of exposure, their incubation periods were greater than 14 days. Thus, it is reasonable to think that the COVID-19 already existed in Wuhan before January 2020, or that the two cases were all postoperative cases with low immunity who were infected with the SARS-CoV-2 after they returned home. Findings from a recent study also indicate that SARS-CoV-2 infections in children were occurring early in the epidemic [16]. Of the six remaining cases, two mild cases had a history of family members (mothers) infected, and thus were familial outbreaks. In four cases, the parents denied or were uncertain concerning the history of contact with the epidemic area.
Epidemiologically, viral pneumonia is an inhalation infection that is directly transmitted from human-to-human through sputum and droplets. The infection is mainly caused by pulmonary lesions due to the downward spread of upper respiratory virus. Since preschool children tend not to actively enter high-traffic places, the possibility of transmission through droplets is markedly lower than that in adults. The infection may be “second-generation” or “third-generation”. Therefore, epidemiology is important in the diagnosis of COVID-19 in preschool children, although the final diagnosis remains dependent on the nucleic acid test.
Our study revealed that the clinical manifestations of COVID-19 in preschool children are different from those in adults. The main clinical manifestations of COVID-19 in adults are fever and cough [7]. Of the eight cases in our study, only one exhibited high fever. Clinical symptoms of COVID-19 in preschool children are relatively mild, and most are diagnosed as mild or moderate. This may be related to the non-susceptible physiological basis of children. The prevalence of severe acute respiratory syndrome in children is also lower than that in adults. Considering the common characteristics of coronaviruses, children may be relatively less susceptible based on their cellular structure or immunity.
The present findings indicate that the chest CT features of COVID-19 pneumonia in preschool children differ from those in adults. The early-stage imaging features of preschool children are not typical, mainly presenting consolidations, while the most common early-stage CT feature in adult patients with COVID-19 pneumonia is the GGO distributed along the bronchovascular bundle or the dorsolateral and subpleural part of the lungs [12]. Pulmonary consolidations were smaller in the present cases and showed changes that included small patches or nodules with unclear margins. There were no pure GGO, or “crazy paving sign”, or other imaging features that are typical findings in adult patients at the early stage [17–19]. The chest CT manifestations of the eight cases were mostly mild or moderate, with better outcomes after treatment. The lesions were significantly absorbed in 6–12 days. This is different from changes in adult that pulmonary interstitial fibrosis is common after treatment. In our study, there was one critical case who was only 8 months old and had a previous history of surgery and combined with mycoplasma infection, suggesting that infants can also be infected with the SRAS-CoV-2, and that a previous surgical history and comorbidity may aggravate the progression of COVID-19. Asthmatic bronchitis- and bronchial pneumonia-like changes are also characteristics of COVID-19 pneumonia in preschool children, which were found in four cases in our study. In addition, the COVID-19 pneumonia in preschool children can manifest as small airway lesions, with uneven lucency of the regional lung lobe or multiple small cystic lucency shadows in the bilateral lower lungs.
The epidemic of COVID-19 began during the winter season in China. Common diseases seen in young children in winter are mycoplasma pneumonia, influenza A (H1N1) with pneumonia, and adenovirus pneumonia. Therefore, the CT features of COVID-19 pneumonia need to be differentiated from those infectious diseases. The most common CT manifestations of mycoplasma pneumonia are bilateral peribronchial perivascular interstitial infiltrations in central and middle lung zones [20]. One case in our study was confirmed to have mycoplasma and SARS-CoV-2 infection at the same time. Adenoviral pneumonia mostly occurs in children, mainly involving the middle and inner zones of bilateral lungs and presenting hilar enlargement, pleural effusion, pneumothorax, mediastinal emphysema, subcutaneous emphysema, while involvement of subpleural areas is rare [21]. The most common imaging features of H1N1 pneumonia are unilateral or bilateral GGO with or without associated focal or multifocal areas of consolidation, and the GGO and areas of consolidation had a predominant peribronchovascular and subpleural distribution [22, 23], which is difficult to distinguish from CT findings of COVID-19 pneumonia.