According to the WHO, children who tested positive for SARS-CoV-2 acquired the infection more frequently at home, while only few outbreaks were reported in schools. However, evidence is limited above all to studies performed while school were closed and other lockdown measures were on.8 Our study was carried out during the second wave of Covid-19 in Italy. Our results showed a lower probability of infection in children who had a school contact compared to children who had a household contact. This confirms that house-contacts are the main reason for SARS-CoV-2 infection, with 35.8% SARS-CoV-2 positive subjects reporting a contact with a positive case within the household and 14.8% reporting contact with a contact of positive within household, compared to 34.6% of SARS-CoV-2 positive cases reporting a school-contact.
The role of children in the transmission of SARS-CoV-2 is still unclear. According to the WHO, younger children, such as nursery school and primary school children seem to be less involved in the transmission of SARS-CoV-2 when compared to adolescents and adults.8 A high heterogeneity across studies was reported in relation to the transmission of the infection from children to the other family members within the household. Several studies reported few cases of family clusters where children were the index cases. 9,10,11 Two more studies carried out in Greece 12 and South Korea 13 also reported that children were very rarely a cause of secondary transmission of the infection. However, another study carried out in Korea reported positive cases in 11.8% of household contacts,14 and a recent study performed in the US concluded that the transmission of the infection within the household is common, even by children. The study reported an overall secondary infection rate of 53% and a secondary attack rate of 53% (95% CI: 31%-74%), with index cases being aged < 12 years.15 Children who resulted as positive due a school contact in our population had high proportion of family members who subsequently resulted as positive, thus leading to a secondary attack rate of 30.6% (95% CI 20.2–42.5). Moreover, the infection rate, while being similar across the 3 age groups, resulted higher among symptomatic children, in particular within the first few days after the onset of symptoms. This could be explained by a peak of respiratory viral load immediately after symptom onset, followed by a rapid decline in children.16
Three studies carried out in Germany,17 Australia 18 and France 19 report that the transmission of SARS-CoV-2 infection in the school setting or in the kindergarden 20 seems to be relatively uncommon, even though some studies did not consider asymptomatic cases.21 A recent epidemiological investigation on the transmission of SARS-CoV-2 in 41 classes of 36 schools in northern Italy showed an overall secondary attack rate of 3.2%, that reached 6.6% in middle and high schools.22 A study carried out in the USA showed that reducing school attendance to only 2 days per week in combination with strict preventive measures allowed to significatively limit the secondary transmission of SARS-CoV-2 within the schools.23 In our study the probability of being positive resulted almost 5 times lower in children who reported a school contact compared to children who reported a household contact (OR 0.19; 95% CI: 0.07–0.50).
Finally, a recent retrospective database cohort study conducted in a large Health Maintenance Organization in Israel showed that 602 (58.3%) out of 1032 children were infected by a parent and 122 (11.8%) acquired the infection at school 24, and only 6 (5%) of the latter resulted in secondary cases in household. The Authors conclude that children are less likely to be the vector for the infection within the household. However, this study include only a relatively short time since schools were re-opened during a period of low infection rate. In our study, conducted during the second wave of the SARS-CoV-2 infection, 28 children who acquired the infection at school resulted in 22 secondary cases, with a secondary attack rate of 30.6%. Differences in the implementation of the preventive measures against the spreading of the infection at home or a diverse variant of SARS-CoV-2, could explain the different results.
The WHO recommended a set of measures to limit the spreading of the infection, with a particular focus on individual measures, including frequent hand hygiene, social distancing, respiratory etiquette, use of face masks and PPEs if symptomatic or when caring for symptomatic cases.25 Results from different publications showed that the percentage of adherence to these preventive measures outside the households varied across studies, ranging from 67–72%26 to 86–90%.27 The variability might be due to several factors, including differences in the economic and educational level.28 Few studies investigated the adherence to preventive measures at home in case of symptomatic children. When considering the ten recommended quarantine measures, Lou showed that the proportion of families who followed those measures and kept a 1.5 m distance, practiced proper hand hygiene, wore face masks at home, and applied a proper cough etiquette was very low (< 30% for each measure).29 A study by Yun et al showed that the risk of infection of the caregivers in presence of symptomatic children was lower when the caregiver reported using a face mask and practicing hand hygiene, with no positive case among 15 caregivers of children with mild Covid-19 symptoms.30 Our study showed that, despite the 1-year length of the Covid-19 pandemic, and the subsequent continuous dissemination of information on effective preventive measures through all media, the adherence to many of such measures at home is still low, in particular in case of symptomatic children of preschool age.
The present study has some limitations, mainly due to its retrospective design. In particular, a degree of recollection bias can be assumed on information relating to symptoms and other data. However, the time between the swab and the interview was usually very short, and the interviewed parents were very motivated to answer, as they felt that the reason for administering the questionnaire was very relevant to them.
In a recent publication, Munro requested the schools to be reopened, reporting that children are not super spreaders.31 The WHO stated that children and schools are unlikely to be the main drivers of SARS-CoV-2 transmission, when community transmission is low and when appropriate mitigation measures are applied. 8 However, the probability of infection seem to significantly increase when the community transmission rate is higher. During the first wave of the pandemic, a delay in closing the schools was significantly associated to an increase in the incidence rate of infection during the following days.32
In conclusion, our study showed that during the second wave of the pandemic the risk of being positive to SARS-CoV-2 was lower after a school contact when compared to a household contact, and that household contact and school contact contribute in a similar way to the spreading of SARS-CoV-2 infection in children. All children can spread the infection to their family members at home, even nursery school children, and this may be due to a lower adherence to preventive measures. Additional initiatives to increase the adherence to preventive measures at home are needed, to motivate parents and family members to consistently implement these measures, in particular when knowing that the child had been in contact with a positive case.