Asymptomatic SARS-CoV-2-infected children attending hospital with non-COVID-19 diagnoses, March 2020-February 2021.

We read with interest the article by de Paul et al., which highlighted gastrointestinal manifestations of SARS-CoV-2 infection in children. Most children infected with SARS-CoV-2 exhibit COVID-19 symptoms, but about 20–30% may be truly asymptomatic, who may then pose an undiagnosed infection hazard to other hospital staff and patients - especially as children are not yet eligible for COVID-19 vaccination.

This may become a seasonal problem, as we have seen with other respiratory viruses, such as respiratory syncytial virus (RSV) and influenza. Indeed, children with asymptomatic SARS-CoV-2 infection can show higher nasopharyngeal viral loads than hospitalised adults with severe disease, and can shed virus for up to 3 weeks.

Throughout the COVID-19 pandemic in the UK during 2020, asymptomatic SARS-CoV-2 infections in children were difficult to assess directly, as only symptomatic children were tested during the first wave of the pandemic, and only if they required hospitalisation. Most community SARS-CoV-2 testing was stopped after 12 March 2020 for both adults and children.

From June 2020, UK national guidance mandated that all new hospital admissions undergo screening for COVID-19. This universal screening policy allowed us to monitor SARS-CoV-2 infection rates in children who were both symptomatically and asymptomatically infected with SARS-CoV-2, with some of the latter group being admitted for other medical problems.

Our Children's Hospital serves a paediatric population of 233,796 throughout Leicester, Leicestershire and Rutland, seeing over 60,000 children in the children's Emergency Department (ED), annually. We performed a 1-year retrospective surveillance audit to determine the incidence of asymptomatic paediatric SARS-CoV-2 infections admissions.

Inclusion criteria: all under-18-year olds who had been seen and swabbed (nasopharyngeal) in ED or their destination ward, within 72 h of admission, who tested SARS-CoV-2 PCR positive, during 1 March 2020 to 21 February 2021.

Exclusion criteria: all swabs taken during this same study period, by other teams or by other referring hospitals, including any repeat positive swabs from the same patient and/or positive tests from samples taken from beyond their first 72 h of admission (based on the average incubation period of 5–6 days for SARS-CoV-2 infection).

Using hospital electronic patient records, children with symptomatic SARS-CoV-2 infection had their symptoms classified as COVID-19-compatible or not, according to the World Health Organization symptom list of COVID-19 symptoms.

An ‘unclear’ COVID-19 status was assigned when a patient presented with at least one COVID-19-compatible symptom but who also had a concurrent illness with overlapping symptom patterns, e.g. a child admitted with fever and abdominal pain, who had surgically proven appendicitis, but who was also found to be SARS-CoV-2 positive.

Out of a total of 11,793 nasopharyngeal swabs, 202 (1.71%) were SARS-CoV-2 PCR positive. Of these, swabs from 80 patients met our inclusion criteria for laboratory-confirmed SARS-CoV-2 infection. Of these 80 cases, 68 were swabbed in ED (85%) and 11 (13.75%) by their destination inpatient ward and 1 (1.25%) by the mortuary following an out-of-hospital cardiac arrest admitted via ED.

Table 1 shows the trends of SARS-CoV-2 infections amongst the 1427 new paediatric admissions during the audit period, with 5.61% (80/1427) being infected with the virus.

Table 1

Trends of SARS-CoV-2 infections in new paediatric admissions during March 2020 to February 2021, Leicester Children's Hospital, Leicester, UK.

Month	Total number of monthly admissions	SARS-CoV-2 positive cases		Symptomatic SARS-CoV-2 infections		Asymptomatic SARS-CoV-2 infections		Unclear group SARS-CoV-2 infections
		n	%	n	%	n	%	n	%
Mar-20	132	3	2.27%	2	1.52%	1	0.76%	0	0
Apr-20	46	14	30.43%	12	26.09%	0	0	2	4.35%
May-20	82	1	1.22%	1	1.22%	0	0	0	0
Jun-20	111	2	1.80%	1	0.90%	1	0.90%	0	0
Jul-20	96	3	3.13%	2	2.08%	1	1.04%	0	0
Aug-20	104	1	0.96%	1	0.96%	0	0	0	0
Sep-20	147	7	4.76%	4	2.72%	3	2.04%	0	0
Oct-20	125	2	1.60%	1	0.80%	1	0.80%	0	0
Nov-20	187	14	7.49%	7	3.74%	4	2.14%	3	1.60%
Dec-20	177	11	6.21%	9	5.08%	1	0.56%	1	0.56%
Jan-21	103	14	13.59%	8	7.77%	2	1.94%	4	3.88%
Feb-21	117	8	6.84%	4	3.42%	2	1.71%	2	1.71%
Total	1427	80	5.61%	52	3.64%	16	1.12%	12	0.84%

The majority of these children were of preschool (53/80, 66.25%), then secondary school (21/80, 26.25%), then primary school (6/80, 7.5%) age. The highest proportions of new SARS-CoV-2 cases occurred in April 2020 (14/46, 30.43%) and January 2021 (14/103, 13.59%), immediately following the government implementation of school closures (23 March 2020 and 5 January 2021.) (Fig. 1 )

Fig. 1

Percentage of monthly asymptomatic SARS-CoV-2 infections admitted during March 2020 to February 2021, to Leicester Children's Hospital, Leicester, UK. First school closures: national lockdown 23 of March to 31 of May 2020; Second school closures: local lockdown 4 of July 2020 to 31 August 2020; Third school closures: national lockdown 5th January to 7th March 2021.

In terms of clinical presentation, of the 80 SARS-CoV-2-infected cases, 52/80 (65%) had COVID-19-compatible symptoms, 16/80 (20%) were asymptomatic and in 12/80 (15%) it was unclear. The 20% asymptomatic infections reported here in this Leicester, UK cohort is similar to the 22% (20/91) figure reported in a South Korean paediatric cohort by Han et al. 2021.

Of the 52 children displaying COVID-19 symptoms; 23/52 (44.23%) had fever only, 13/52 (25%) had fever and a respiratory symptom, 11/52 (21.15%) had respiratory symptoms only, 2/52 (3.85%) presented with febrile seizures, 2/52 (3.85%) with headaches alone and 1/52 (1.92%) with skin discoloration of their extremities.

Stratifying by school stage, the symptomatic and asymptomatic infections were, respectively: preschool: 38/80 (47.5%), 8/80 (10%); primary school: 2/80 (2.5%), 2/80 (2.5%); secondary school: 12/80 (15%), 6/80 (7.5%).

The overall percentage of new paediatric admissions with asymptomatic SARS-CoV-2 infection remained at or below 1.04%, until September 2020 when it rose to 2.04%, then further increased to 2.14% in November 2020. In addition, whilst schools remained open throughout the 2020 autumn term, it was noticeable that dips in asymptomatic SARS-CoV-2 case numbers occurred in October 2020 (0.80%) and December 2020 (0.56%), which coincided with the school half-term and Christmas holidays (Fig. 1). This may have been due to an overall reduction in social contacts between children during these school breaks.

Conversely, and compatible with this explanation, during the 5 months that the schools were open (June, September-December), there was a 1.65-fold increase (1.29% vs. 0.78%) in the mean percentage of new asymptomatic paediatric SARS-CoV-2 infections admitted, compared to the 7 months when the schools were closed (March-May, July-August, January-February). Thus, the trend of asymptomatic paediatric SARS-COV-2 infections appears to follow the timing of the school terms.

Asymptomatic SARS-CoV-2 infections presenting to hospital for other, non-COVID-19-related medical reasons may pose a nosocomial transmission risk to other patients and staff, as has been seen with other seasonal respiratory viruses , . Even where bedside rapid diagnostic tests are available, these can still take 30–60 min to complete, and patient waiting areas can still allow some degree of close-contact mixing, particularly with active young children. This risk may increase during school term times, as the percentage of asymptomatic paediatric SARS-CoV-2 infections being seen in hospital rises.

We therefore urge paediatric ED and outpatient teams to be particularly vigilant for potentially asymptomatic SARS-CoV-2-infected children during school terms, particularly as SARS-CoV-2/COVID-19 becomes more endemic and seasonal, and whilst children are still not eligible for COVID-19 vaccinations.