Transmission of COVID-19

(Last update 30 June 2020)

Coronaviruses are mainly transmitted from person to person via respiratory droplets, either by being inhaled or deposited on mucosal surfaces, including aerosols produced when coughing and speaking. The production of aerosols is thought to be increased by aerosol-generating procedures performed in medical settings such as suctioning and endotracheal intubation. Transmission through contact with contaminated fomites is considered possible, although it has not yet been documented for SARS-CoV-2, and cultivable virus has not been detected in real-life situations [1,2]. SARS-CoV-2 has been detected in respiratory and faecal specimens. Viral RNA has also been detected on rare occasions in blood specimens, but there is no evidence of transmission through contact with blood [3-5].

SARS-CoV-2 Transmission and Children

(Last update 26 March 2021)

Children and adolescents have lower susceptibility to SARS-CoV-2 than adults, and children younger than 10-14 years appear to be less susceptible to SARS-CoV-2 than older children and adults (20 years and older). This has been concluded by multiple systematic literature reviews, although heterogeneity in study results has been reported [6,7]. In one of these studies, children (under 20 years of age) had a pooled odds ratio (OR) of being an infected contact of 0.56 (95% CI 0.37-0.85) compared to adults, albeit with substantial study heterogeneity [6]. Further stratification by age showed that the lower susceptibility was limited to those younger than 10-14 years [6]. This finding was also observed for infections by B.1.1.7 variant SARS-CoV-2, which is more transmissible than wild-type SARS-CoV-2 across all age groups [8,9]. There are currently few published studies that have presented data on new variants of concern and their potential transmissibility and disease severity in children, representing a knowledge gap in the field.

Several hypotheses have been formulated to explain the lower infection rates and disease severity observed in children [10]. These include that it could be due to lower case ascertainment as children are more often asymptomatic or mildly symptomatic. However, the lower infection rates in children have been observed also in studies where asymptomatic contacts have been tested [11].

Research from the United Kingdom in September and October 2020 concluded that children, notably those between 12-16 years of age, played a significant role in introducing SARS-CoV-2 infection into households, coinciding with the timing of school reopenings [12].

In assuming that all household contacts have an equal exposure risk to a given household index case, the authors of one systematic review hypothesised that assessing household contacts could provide a clearer indication of SARS-CoV-2 infection susceptibilities. In looking at household contacts only, the pooled OR for secondary infections among child household contacts versus secondary infections among adult household contacts was 0.41 (95% CI: 0.22-0.76) [6]. Another systematic review of household studies found higher secondary attack rates in adult contacts than in child contacts, a difference that held even when all contacts, irrespective of symptoms, were tested [11]. However, these results, as also pointed out by the authors, could not account for the type of household contacts. It is therefore possible that the difference in observed attack rates by age was due to spousal contacts having higher risk of transmission compared to other contacts, rather than a true age-related difference; there appears to be a research gap in understanding transmission risk for child contacts and non-spouse adult contacts [11].

Furthermore, household studies have not generally demonstrated an elevated risk of onward transmission from children. One preprint study of 150 households in Germany found lower secondary attack rates in households where the index cases was younger than 18 years [13]. A large cohort study from Scotland of over 300 000 adults living in households with children found that the risk of testing positive for SARS-CoV-2 was slightly lower for those living with young children (0-11 years old), and that there was no increased risk for such individuals even during periods when nurseries and primary schools were open [14].

In contrast to findings from household studies, a population-based study conducted in Austria seems to indicate that children of all ages have a roughly equivalent infection risk. A representative sample of over 10 000 randomly selected staff and students from 243 primary and secondary school were tested regardless of symptoms in three consecutive periods. For the first three rounds (September 2020, November 2020, and March 2021), there was neither a statistically significant difference between prevalence of SARS-CoV-2 positive tests among primary school students and middle or lower secondary school students, nor between students and teachers [15]. However, the study found that the risk of SARS-CoV-2 positivity in children was substantially higher among schools with higher shares of children from socially disadvantaged backgrounds (OR: 3.58). This study is ongoing and study participants are planned to be tested every three to five weeks at 10 different points during the 2020/2021 academic year.

A population based cohort in the United Kingdom among 9 334 392 adults aged 65 years and under found that in contrast to wave 1, evidence existed of increased risk of reported SARS-CoV-2 infection and covid-19 outcomes among adults living with children of any age during wave 2 (hazard ratio 1.06 (95% confidence interval 1.05 to 1.08). However, this did not translate into a materially increased risk of covid-19 mortality, and absolute increases in risk were small [16].

Children tend to have less severe COVID-19 outcomes than adults, meaning that children positive for SARS-CoV-2 may be under-represented in case-based reporting as well as in studies that have not tested asymptomatic contacts [17]. Population-based studies, such as the ones using representative sampling of individuals with COVID-19 compatible symptoms or studies that include testing regardless of symptoms, may help to address this gap.

SARS-CoV-2 in Educational Settings

(Last update 26 March 2021)

Although there is some heterogeneity in the findings from the literature noted above, there is general consensus from research from several European settings that schools do not act as amplifiers of community transmission of SARS-CoV-2, and rather that case rates in schools are following background levels of community transmission [17, 28].

In educational settings, notably primary schools, where appropriate infection prevention and control measures are in place, SARS-CoV-2 transmission from children does not appear to occur very frequently [18-22].

There is evidence that there is an increased risk of SARS-CoV-2 transmission to teachers when secondary schools are open. A study from Sweden compared onward transmission from students in lower-secondary schools (typical student age 14-16 years) that remained open with onward transmission from students in upper-secondary schools (typical student age 17-19 years) that were closed during the study period. The study found a small increase in the risk of SARS-CoV-2 infection in parents of lower-secondary school students (OR: 1.17, 95% CI: 1.03-1.32), but double the risk for lower-secondary school teachers versus upper-secondary school teachers (OR: 2.01, 95% CI: 1.52-2.67) [23].

It has been observed that the effect of school closures for reducing SARS-CoV-2 transmission was small in the second wave compared to the first wave in Europe, suggesting that schools were able to operate more safely in the second wave through having implemented measures such as symptom screening, asymptomatic testing, contact tracing, sanitisation, ventilation, distancing, reducing group sizes and preventing the mixing of groups [24]. Nonetheless, in the third wave, widespread community transmission of SARS-CoV-2 variants of concern (with high transmissibility) increases the possibility of onward transmission in schools and subsequently household settings [17,25], particularly in the absence of appropriate in-school mitigation measures. This may lead to the need for school closures, either in response to school-specific outbreaks or to alleviate current or anticipated risk of or increase in community transmission and the pressure on healthcare systems [26].

School closures have a negative impact on children and adolescents’ health and wellbeing, as well as on their long term educational outcomes [27], so keeping schools open when possible is a key objective [28]. School closures, if deemed necessary, should initially be arranged for children in older age groups. An age-structured modelling study from the Netherlands, based on the virus characteristics of previously dominant SARS-CoV-2 strains, concluded that the largest impact on community transmission was achieved by reducing contacts in secondary schools [29]. Another modelling study from Denmark which assumed that children under 10 years of age were 50% less susceptible to SARS-CoV-2 infection than adults, indicates that, all other measures being constant, opening only primary schools (grades 0-4) in February 2021 would not lead to a substantial increase in new cases or hospitalisations, provided that the transmissibility of variant B.1.1.7 increases by 40% compared to the previously circulating SARS-CoV-2 viruses. Case rates increased slightly, while hospital admissions remained stable through February and March [30].

Transmission risks in different settings

(Last update 10 August 2020)*

Several outbreak investigation reports have shown that COVID-19 transmission can be particularly effective in crowded, confined indoor spaces [31] . Transmission can be linked with to specific activities, such as singing in a choir [32]. In a 2.5 hour choir practice in Washington, US, there were 32 confirmed and 20 probable secondary COVID-19 cases among 61 participants (85.2%)[32]. The duration of the indoor activity and the increased production of respiratory droplets through loud speech and singing, likely increased the risk of transmission.

Poor ventilation in confined indoor spaces is associated with increased transmission of respiratory infections and COVID-19 in particular [33]. In a restaurant outbreak of 10 cases in three families in Guangzhou, China, transmission was attributed to the spread of respiratory droplets carrying SARS-CoV-2 by the airflow generated by the air-conditioning [34]. Similarly, two other outbreaks from China in January 2020 attribute air conditioning systems using a re-circulating mode as a likely aid to transmission [35]. Well-maintained, heating, ventilation, and air conditioning systems may have a complementary role in decreasing transmission in indoor spaces by increasing the rate of air change, decreasing recirculation of air and increasing the use of outdoor air [33]

Occupational settings

(Last update 11 August 2020)*

Multiple outbreaks of COVID-19 have been observed in several occupational settings within and beyond the EU/EEA and the UK, including slaughterhouses, meat processing plants, mines and building sites [31,36,37]. Possible factors contributing to clusters and outbreaks in occupational settings are:

  • Working in confined indoor spaces: Studies have shown that in Europe >80% of working time is spent indoors and that variations in the socioeconomic and demographic status lead to different work-day patterns indoors [38]. Participating in meetings and sharing the same office space has been reported in literature as a risk factor for contracting COVID-19 [39,40].
  • Lack of social distancing: Outbreaks in different workplaces have been described when there were difficulties maintaining the recommended distance of at least two metres [39,41]. Sharing facilities (e.g. canteen and dressing rooms), transport and accommodation may also contribute to transmission [42].
  • Close/direct contact with COVID-19 cases: Healthcare workers are known to be at greater risk of occupational exposure to biological agents, particularly infectious pathogens such as TB, influenza, SARS, measles etc. [43,44]. In a UK study of more than 120 000 employed persons, the risk of healthcare workers testing positive for COVID-19 was over seven times higher than for non-essential workers, and those in social care had a risk that was three times higher [45]. Further specific occupations which are probably at risk of exposure to COVID-19 include transport workers (taxi and bus drivers), sales people, postal/package delivery workers and domestic cleaners, due to the fact that they are exposed to multiple clients. A study from Sweden that looked at cases of COVID-19 diagnosed in different occupations found the highest risk among taxi drivers, with a relative risk of being diagnosed with COVID-19 that was 4.8 times higher than in all other professions (95% confidence interval 3.9-6) followed by bus and tram drivers (RR 4.3, 95% CI 3.6–5.1) [46].
  • Insufficient or incorrect use of protective personal equipment (PPE): Some work sites where outbreaks have occurred have been slow to implement appropriate infection control and hygiene standards or have done so inadequately [47]. Insufficient access to PPE has been identified as an additional risk factor [48]. A systematic review and meta-analysis of 172 observational studies both in healthcare settings and the community, that looked into the effect of distance from the source patient and the use of respiratory and eye protection in the risk of transmission of SARS-CoV, MERS-CoV and SARS-CoV-2, concluded that physical distancing of at least one metre, use of face masks and eye protection were associated with a much lower risk of transmission [49].
  •  ‘Presenteeism” (i.e. reporting to work despite being symptomatic for a disease): Fear of losing their job or inability to reduce their working hours in order to stay home may lead to continued commuting and working, even when the employee or a family member are exhibiting symptoms compatible with COVID-19 [47].

Further information on outbreaks in occupational settings in the EU/EEA can be found in ECDC’s technical report on this topic [50].

 

*ECDC no longer updates this section