Healthcare workers as 'canaries' for acute respiratory infections and pathogens during the COVID-19 pandemic.

Sir,

During the coronavirus disease (COVID-19) pandemic, there was a decline in the detection of respiratory viruses including influenza during winter in temperate regions [1,2]. Prior to 2020, influenza viruses circulated year-round in Singapore, with bimodal peaks corresponding to the winter seasons in the Northern Hemisphere (December to February) and Southern Hemisphere (May to July) [3]. In healthcare workers (HCWs), acute respiratory infection (ARI) could be acquired in the course of work or from the community during social activities. As such, HCWs may serve as sentinels for the surveillance of ARI and circulating respiratory pathogens in the community.

We described the changing epidemiology of ARI among HCWs during the COVID-19 pandemic, between May 6th, 2020 and March 6th, 2021 (epidemiological week (e-week) 19 in 2020 to e-week 9 in 2021), using data collected for ARI surveillance in HCWs at Tan Tock Seng Hospital (TTSH) in Singapore. The surveillance included the monitoring of self-reported fever and ARI symptoms through the online staff health surveillance system (S3), and medical attendances at the hospital's occupational health clinic (OHC) for fever or ARI. At the OHC, these HCWs had nasal and throat swabs taken for SARS-CoV-2 polymerase chain reaction (PCR) test and respiratory multiplex PCR test (BioFire Respiratory Panel 2.1 System) which tested for common respiratory pathogens.

From e-week 19, 2020 to e-week 9, 2021, there were 1650 ARI visits by 1260 HCWs at the OHC. Of 1422 swabs taken, rhinovirus was detected in 231 samples (16%). Fourteen swabs (1%) were positive for other respiratory pathogens (adenovirus (N = 3), coronavirus 229E (N = 1), coronavirus NL63 (N = 3), respiratory syncytial virus (N = 1), parainfluenza 3 virus (N = 2), parainfluenza 4 virus (N = 1), human metapneumoviruses (N = 3)). SARS-CoV-2 and influenza viruses were not detected in any sample.

In Singapore, the weekly ARI incidence per 10,000 staff declined across the ‘Circuit Breaker’ period, Phase 1 and first eight weeks of Phase 2 Reopening (Figure 1 ). During the Circuit Breaker period, control measures implemented included closure of non-essential workplaces and schools, social activity restrictions, mandatory use of face masks in public areas, and safe distancing measures. During Phase 1 Reopening, two family visitors per household were allowed each day [4]. In Phase 2 Reopening, social gatherings were limited to five people.

Figure 1

Weekly incidence of acute respiratory infection (ARI) per 10,000 staff and rhinovirus and other respiratory viruses positivities (%) from epidemiological week 19, 2020 to epidemiological week 9, 2021 (SARS-CoV-2 and influenza viruses were not detected).

From August 4th, 2020, while still in Phase 2 Reopening, the maximum number of guests for marriage solemnizations was increased to 50 people, and for wakes to 30 people [4]. This was followed by an increase in ARI incidence and rhinovirus activity till e-week 38, 2020, coinciding with post-September school holidays, after which it declined to high baseline (Figure 1). Rhinovirus activity peaked again during the year-end school holidays from e-week 48, 2020 before declining in e-week 52, 2020. Further reopening in the community termed Phase 3 was implemented on 28 December 2020, e-week 53 [5]. Since then, ARI incidence among HCWs increased, with a sharp rise in e-week 7, 2021 corresponding to the period of COVID-19 vaccination in HCWs. By contrast, rhinoviral positivity peaked in e-week 3, 2021 and then declined.

The persistent detection of rhinovirus is likely because rhinovirus is a non-enveloped virus and therefore not susceptible to soap and sanitizers [2]. Furthermore, asymptomatic rhinoviral infections enhance its circulation [2]. The overall rhinoviral positivity of 16% observed in our HCWs was higher than that reported in a study in Japan (5.8%) conducted from March to May 2020 [6]. Australia also reported the resurgence of rhinovirus with school reopening post lockdown during the COVID-19 pandemic, supporting the notion that school-going children were reservoirs and transmission agents for rhinovirus [1]. The sharp increase in ARI incidence in e-week 7, 2021, with contrasting decline in rhinoviral positivity, was likely due to (i) ARI symptoms post second dose of COVID-19 vaccination among HCWs, and (ii) Chinese New Year holiday, a major public holiday in Singapore which falls on e-week 6.

The non-detection of influenza viruses throughout the study period is consistent with the low influenza activity in Singapore and other temperate regions during the COVID-19 pandemic, most likely due to travel restrictions and social distancing [7]. All travellers were required to serve a 14-day stay-home notice (SHN) upon arrival in Singapore [8]. Respiratory infection, if any, would have resolved by the end of SHN or have been medically treated. The close reflection of influenza activity in HCWs and in the community makes HCWs potential sentinels for any surge in influenza and other respiratory infections.

PCR testing of respiratory samples yields aetiological evidence on the likely reservoir and transmission, allowing early detection of ARI clusters. Active ARI surveillance in HCWs could also evaluate the effectiveness of COVID-19 control measures both in the hospital and in the community. In short, rhinovirus continues to circulate during the COVID-19 pandemic and close surveillance of circulating respiratory pathogens is essential as countries prepare to reopen their borders in the coming months.

Conflict of interest statement

None declared.

Funding sources

None.