Our study revealed a cumulative incidence of SARS-CoV-2 Omicron infection among hospital staff from Chengdu Women’s and Children’s Hospital to be 86.4% (95% CI 85.2%-87.7%), which was higher than the reported rate of 74.3% among the general population in the same province [18]. However, since our study was conducted more than one week later, it is uncertain whether hospital staff faced a greater infection risk compared to individuals in the community or if the timing difference influenced the documentation of cumulative incidence rates during a rapid and widespread Omicron outbreak. In addition, our rate was much higher than that previously reported for healthcare workers in other studies, most of which had an even longer time frame than our research [19,20,21,22,23]. According to a study conducted in Japan, the cumulative incidence of infection among healthcare workers increased substantially from 2.0% in June 2021 (pre-Delta) to 39.0% in December 2022 (Omicron variant-predominant period) [19]. Similarly, the infection rate of healthcare workers in China remained low before the emergence of the Omicron variants [24]. No previous studies have reported the epidemiology of COVID-19 infection among healthcare workers during the Omicron outbreak in mainland China. A study from Hong Kong revealed that nearly one in five healthcare workers were infected between December 2021 and May 2022, when the Omicron sublineages caused outbreaks in the community [8]. The extremely high infection rate of hospital staff observed in our study was in line with the surge of COVID-19 cases in China around the end of 2022, when the ‘dynamic zero-COVID policy’ was eased and Omicron subvariant BA.5 was dominant [25].
We observed a high prevalence of systemic symptoms, including fatigue and fever, among our participants who had either a confirmed COVID-19 diagnosis or highly suspected symptoms. These clinical manifestations were different from those reported in other studies [15, 26]. For instance, a registry-based observational study in Japan revealed that when the omicron subvariants BA.2 and BA.5 were prevalent, upper respiratory symptoms such as cough (62.7%), sore throat (60.7%) and nasal discharge (44.3%) were more common than fever (38.8%) and severe fatigue (26.8%) [26]. A large proportion of the disparities in clinical presentation may be attributed to differences in previous infection status. A history of previous SARS-CoV-2 infection has been reported to be inversely associated with the risk of systemic symptoms [26, 27]. During the implementation of the ‘dynamic zero-COVID strategy’, China witnessed an exceptionally low infection rate. with no cases detected among our hospital staff. A lack of innate immune protection following SARS-CoV-2 infection, irrespective of vaccine-induced immunity, potentially contributes to the heightened prevalence of fatigue and fever in our study participants. In addition, the high prevalence of systemic symptoms in the present study aligns with the findings of a previous survey conducted on 328 healthcare workers infected with the Omicron variants, in which 83.5% reported experiencing fever [28]. The self-reporting of healthcare workers, who possess sufficient knowledge and awareness to monitor their own body temperature, may have played a significant role. Furthermore, considering the escalated disease burden of COVID-19 in China, including within work environments, it is plausible that the participants were highly conscious of potential symptoms, which could have influenced their reporting.
In our study, female staff members had a greater risk of SARS-CoV-2 infection than did their male colleagues. This finding adds to the evidence of gender disparities in COVID-19 susceptibility among the prime working-age population. Accumulating epidemiological evidence has shown similar infection rates of SARS-CoV-2 between males and females in general communities [29]. Nevertheless, a study using data reported by health authorities in Canada revealed that working-age women were more vulnerable to infections than were similar-aged men during the COVID-19 pandemic [30]. Women’s predominant roles as caregivers both in families and workforces may expose them to an elevated risk of infection [31].
Consistent with some previous research [20, 32], we found that older individuals were less likely to become infected. It has been widely recognized that older COVID-19 patients, especially those aged over 65 years, have strikingly greater mortality rates than younger individuals [33, 34]. Thus, to reduce possible adverse consequences, older hospital staff may have higher compliance with the implementation of preventive measures.
Being a clinical practitioner greatly increased the risk of contracting COVID-19 compared with working as a cleaner, secure guard or maintenance worker in hospitals, who have little direct contact with patients or medical professionals. During the emerging outbreaks at the end of 2022, hospitals in Chengdu were overwhelmed with patients seeking medical care. Clinical practitioners inevitably have a large number of contacts with COVID-19 patients, leading to high levels of stress and workloads. These staff may then reduce their compliance with protective measures and consequently a high rate of SARS-CoV-2 infection. Moreover, no significant differences in infection rates were found among clinical practitioners, nurses, medical technicians and administrative/logistic personnel. The intensive cooperation of relevant departments within the hospital might contribute to explaining these findings. Since there was frequent interdepartmental support during the surge of COVID-19 patients, we did not further divided the clinical practitioners and nurses into specific departments.
COVID-19 vaccination is a safe and effective way to prevent related hospitalizations and deaths. By May 2022, three Chinese COVID-19 vaccines, two inactivated and one recombinant, had been listed by the WHO for emergency use [35]. Nine out of ten staff members in our hospital had received at least three doses of COVID-19 vaccines, but 88.3% of them had the latest dose administered more than 6 months before the change in the COVID-19 containment policy. A study from Hong Kong showed a significant decrease in neutralizing antibodies 4 months after vaccination in a group of people who received the inactivated CoronaVac vaccine [35]. Therefore, it was not surprising to find that a three-dose vaccination, with the most recent dose administered within 3 months before 7 December 2022, was associated with a reduced risk of SARS-CoV-2 infection in our study.
Among the SARS-CoV-2 infection patients, all of whom were infected for the first time, the incidence of long COVID-related fatigue or brain fog was 4.27%. It was comparatively lower than that observed in the majority of other studies on long COVID among both the general population and healthcare workers [13, 36]. The disparities were likely attributed to our inclusion of only two symptoms. For instance, the corresponding rate was 8.89% in a study conducted during the Omicron BA.2 outbreak in Shanghai, China [37]. However, it is important to note that the Shanghai study encompassed 18 pre-specified long COVID symptoms. In another study of 679 healthcare workers who tested positive for COVID-19, nearly one-third reported having suffered from at least one of the 40 long COVID symptoms assessed [38]. In addition, previous studies have shown that no vaccination, severe COVID-19 illness during the acute phase, advanced age and obesity are common factors associated with an increased risk of long COVID [36, 39]. Thus, the relatively low incidence of long COVID symptoms in our study may also be partially attributed to the high prevalence of three-dose vaccination (90.2%), low incidence of hospitalization (0.9%), and low prevalence of advanced age (≥ 60 years) and obesity (≥ 28 kg/m2) (both < 4%) among our SARS-CoV-2 infection patients.
Our study has several limitations. First, data on confirmed SARS-CoV-2 infection were unavailable since large-scale nucleic acid testing was overhauled and there was a national shortage of COVID-19 antigen test kits at the end of 2022. The use of self-reported data may introduce some misclassification. However, since there were no flu or other respiratory disease (e.g., respiratory syncytial virus) waves in Chengdu throughout the last winter, the number of misclassified cases should be extremely small. Second, long COVID includes a broad range of symptoms; however, due to logistic reasons, we only focused on fatigue and brain fog. This may lead to an underestimation of the risk of long COVID. In addition, symptom data are subject to variance in individual perceptions. Some participants may be more prone to report the presence of symptoms than others. Nevertheless, it was unlikely that significant recall errors occurred in our study, as the baseline information was collected within a short period of the outbreak, with follow-up interviews conducted between five and six months after the initial infection. The telephone follow-up interviews were conducted by trained research assistants to ensure a standardized data collection process and minimize potential bias. Furthermore, a recent meta-analysis highlighted the occupational exposures associated with COVID-19 infection among healthcare workers, such as inadequate or lack of personal protective equipment and performing tracheal intubation [10]. However, the present study did not gather data on these specific factors, potentially leading to unmeasured confounding that cannot be adjusted for in the regression analyses. Finally, since we recruited participants from a single hospital, our sample may not be representative of the target population in China.
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