By using longitudinal data, our study provides robust data on the incidence of new SARS-CoV-2 infection among HCWs in a paediatric hospital during the pre- and post-vaccinations periods, encompassing the second and third wave of the COVID-19 pandemic in Poland.
During the ten months of the study period, approximately one fifth (19.4%) of the HCWs susceptible to primary infection became infected with SARS-CoV-2.
It is worth noting that almost half of the HCWs who had laboratory confirmed SARS-CoV-2 infection in our study did not report symptoms, suggesting that without the implementation of proactive universal screening a significant proportion of infection among HCWs would have remained undetected and that they would likely have continued working while unaware of their status, therefore presenting a risk of transmission to patients and co-workers. Although this study was not designed to address the issue of nosocomial transmission reduction through asymptomatic testing, we assume that the universal screening programme of HCWs applied in our hospital (as part of a bundle of intensified infection control measures) reduced the number of SARS-CoV-2 infections by early identification and isolation of SARS-CoV-2–positive individuals. There are also some issues associated with massive asymptomatic testing that must be underlined. First, it requires additional operational and testing costs and a large laboratory capacity to provide rapid turnaround for testing. Second, the results of the PCR-based screening are valid only for the day of the test, which can cause a false sense of confidence13. Furthermore, the resources required to identify a single asymptomatic case are substantial and may not be cost-effective during a low prevalence period14. Thus, testing strategies should be implemented after careful consideration of resources, infrastructure capacity, and logistical issues15. In addition, the testing strategy must be guided by the local epidemiology, vaccination coverage, and efficacy among HCWs and in the community.
In line with some other studies conducted after the first wave of the pandemic, the changes in the incidence rate of SARS-CoV-2 infection among HCWs in our cohort were closely followed by community infection rates8,16,17. Especially in the period before vaccination (study phase 1), both the dynamic and the magnitude of new SARS-CoV-2 infections were similar. On the other hand, after deployment of the HCW vaccination programme, a reduction in the weekly incidence of new cases among HCWs was observed. This downward trend initially was in parallel with the incidence decline in the community but persisted up to epi week 11, 2021, while in the general population a rapid increase in SARS-CoV-2 incidence was observed attributed to the SARS-CoV-2 Alpha variant domination (corresponding to the third pandemic wave). The incidence rate among HCWs also increased afterwards, but the observed peak was relatively lower. This finding is consistent with the anticipated protective effect of the COVID-19 vaccine.
When considering the two phases of the present study, the cumulative incidence of new SARS-CoV-2 infections decreased from 16.6% before vaccination to 6.3% after the implementation of the vaccination programme at the CMHI (study phase 2).
Due to the observational design of our study, we were unable to assess the extent to which HCW vaccination contributed to the observed reduction in the incidence of HCW SARS-CoV-2 infections. However, as the uptake of vaccination among HCWs at the CMHI was high (88.9%), we hypothesise that this impact could be significant, especially when the short-term effect is considered.
In addition, this hypothesis was supported by the high vaccine effectiveness observed in our cohort. Our findings indicate that the effectiveness of the vaccine was 95% and 79% in fully and partially vaccinated HCWs, respectively. These results are comparable to previous reports including those from clinical trials and other real-world studies6,7,18. In line with a previous study conducted among HCWs, we did not observe significant differences in vaccine effectiveness by age or occupational groups7,18, although we observed slightly lower vaccine efficacy among HCWs in the ‘other without direct patient contact’ group. This finding could reflect lower awareness, behaviour change, and a misbelief that vaccination allows infection control measures to be relaxed.
However, despite high vaccine efficacy and high vaccination coverage, we still observed new SARS-CoV-2 infections among HCWs at a rate from 2.9% in the fully vaccinated to 39.7% in those unvaccinated.
These numbers are higher than those reported previously. In a systematic review of eighteen studies, the pooled proportion of SARS-CoV-2 infection was 1.3%, 2.3%, and 10.1% among fully vaccinated, partially vaccinated, and unvaccinated HCWs, respectively19.
Variability in demographic and occupational factors, as well as case definitions, could explain this difference. Furthermore, the fact that we excluded HCWs with a previous SARS-CoV-2 infection from our analysis may have affected the higher infection rates observed in our cohort (the absence of the protective effect of post-infection immunity).
Although rare, these infections might diminish HCWs’ belief in vaccination effectiveness, especially in those becoming infected with SARS-CoV-2 after initial vaccination. It is worth noting that, at the time of manuscript preparation, we faced a low rate (approximately 5%) of second booster dose uptake among HCWs at the CMHI. This led to concerns when considering waning vaccine immunity over time and the emergence of new variants of concern (VOC).
A systematic review by Biswas et al. revealed that vaccine acceptance varied widely between countries and ranged from 4.3–72%20. Individuals who were men, older, physicians, or well educated had a lower hesitancy to receive the COVID-19 vaccine. Other factors associated with higher vaccine acceptance were: higher income, medical risk, chronic disease history, not being infected with SARS-CoV-2 in the past, knowledge of COVID-19, and a belief that vaccines may protect friends, family, and community members20. Although the present study was not designed to assess the attitude to vaccination, these findings are consistent with our study. We observed that older individuals, physicians, those working in clinical settings, and those previously uninfected were more likely to be vaccinated. These observations might inform tailored communication strategies to be implemented to increase the uptake rate of COVID-19 vaccines among HCWs.
Reliable data on risk factors, SARS-CoV-2 incidence, and the proportion of HCWs who remain naïve (had no history of SARS-CoV-2 infection and/or unvaccinated) are crucial to inform infection control strategies. Previous studies reported widely varying estimates of incidence and risk factors for infection among HCW21, and only a few studies assessed HCW infection and risk longitudinally17.
When considering potential risk factors of SARS-CoV-2 infection among HCWs in our study, we did not find a significant difference in the infection rates among HCWs working in wards involved and not involved in the treatment of COVID-19 patients, clinical or non-clinical areas, or across different wards and these findings applied to both phases of the study. Our findings are in contrast to some previous studies, especially those studies performed during the early pandemic stage22, which reported higher infection rates among HCWs working in COVID-19 wards with direct patient contact, but our findings are in line with the emerging literature pointing out that, except for breaches in PPE, the main risks to HCWs come from outside of work factors (in the community and household)17,23–26. In our cohort, before implementation of the vaccination programme (study phase 1), nurses had the highest adjusted likelihood of being infected, while physicians had a lower likelihood. Surprisingly, in the post-vaccination study period, we did not find any association between risk of SARS-CoV-2 infection and professional category. The only factor significantly associated with SARS-CoV-2 infection in the multivariate analysis was vaccination status. The increased risk of SARS-CoV-2 infection for nurses was previously reported in many observational studies27–29. The higher risk among nursing staff has been consistently explained by their more frequent contact with and longer contact times with COVID-19 patients when compared with physicians28,30,31. However, in our study we did not observe an increase in the number of cases on the wards involved in the treatment of patients with COVID-19. Furthermore, only a small percentage of HCWs (with available data on the possible source of exposure) reported exposure at work, and even fewer had high-risk contact with a COVID-19 patient. A recent case-control study from Ireland, investigating the impact of demographic and work-related factors on the risk of SARS-CoV-2 infection after in-work exposure to a confirmed case of COVID-19, revealed that male sex, Eastern European nationality, exposure location, PPE use, and vaccination status all impact the likelihood of SARS-CoV-2 infection throughout the first, second, and third pandemic waves32. In this study, no individual job role was determined to have a consistently higher risk of infection after documented nosocomial exposure. It is likely that the increased risk of SARS-CoV-2 infection among nurses observed in our and other studies may be an indication of the cumulative risk of certain nursing staff roles, which have an increased intensity of contact with patients and other HCWs over time, and this increases the probability of infection. In addition, nurses make up a significant proportion of the total hospital staff (approximately one-third in our cohort), thus resulting in over-representation, which may influence the interpretation of risk32.
Together, these observations highlight the need for awareness of non-patient care exposure risk that contributes to infection in HCWs and should be considered in infection control measures (i.e. universal masking, reinforcement of hand hygiene, and distancing).
The limitations of our data should be borne in mind. First, the retrospective and observational design of the study is subject to missing and incomplete data and thus to unmeasured confounding bias. Second, although our study cohort was stratified by different occupational categories, there may still be different exposure risks within the categories. Third, we were unable to adjust our analysis for other potential confounders that may affect the risk of infection (such as comorbidities, contact time with COVID-19 cases, movement of HCWs between high- and low-risk wards, compliance with PPE, household size) due to data limitations, and therefore the results may not generalise to other settings with different characteristics. Fourth, the clinical data regarding symptoms for each HCW were retrieved from an unstandardized database generated by the ICP team for epidemiological purposes and were not consistently available. In addition, we relied on self-reported symptoms and community/household exposures. Exposures in the workplace were revealed through epidemiological investigations, including contact tracing, but are still subject to recall bias. Data on exposure was missing for almost 40% of the HCWs, therefore it was unknown whether SARS-CoV-2 PCR positive cases resulted from infections in the workplace or were acquired in the community or household. On the other hand, the exact source and direction of infection may only be inferred from epidemiological data combined with viral genomic data, otherwise assessing the source is often subjective. Fifth, the lack of detailed information on SARS-CoV-2 exposure and the use of PPE, prevented us from analysing the association of adherence to PPE over time and across different occupational categories with the risk of infection. Six, we attempted to include only new cases of SARS-CoV-2 infection, excluding HCWs with records of a positive test by PCR or anti-nucleocapsid IgG. However, as systematic testing by PCR was not performed before the study period and not all HCWs had participated in a previous serology screening programme, we cannot rule out that we included some previous positive cases in our incidence analysis. Seventh, testing in study phase 2 was less dense as the universal screening programme ended on May 6, 2021, thus HCWs with asymptomatic SARS-CoV-2 infection might be underrepresented. Eight, although our estimated vaccine effectiveness aligns with the results provided in other reports, this estimate is based on a relatively short follow-up period. Finally, our study took place prior to the emergence of the Delta variant of SARS-CoV-2, thus it might not be generalizable to an epidemic situation with a domination of another VOC (variant of concern), however, the evidence from the early period of the pandemic may inform the infection control policy in a vulnerable population with regard to other respiratory viruses.
To our knowledge, this is the only study of the SARS-CoV-2 infection among HCWs in Poland and one of the few studies based on robust data obtained mainly from proactive universal screening PCR testing. The size and diversity of the cohort (all HCWs, including nurses, physicians, and other personnel providing direct and indirect patient care, as well as non-clinical staff) together with longitudinal data collected in the period before and after vaccination implementation in HCWs, meant that it was possible to examine risk factors of the new SARS-CoV-2 infection by demographic, occupational, and vaccination status (which was determined based on vaccination records instead of self-reporting, to mitigate recall bias).
In summary, our analysis provides information on the incidence of SARS-CoV-2 infection in the pre- and post-vaccination periods. We found that after the implementation of vaccination, the risk of infection among HCWs remained relatively high despite the high coverage of vaccination and the high effectiveness of the vaccine. Although rare, breakthrough infections are challenging, as they may pose a risk to the vulnerable population. Furthermore, there are concerns about the decrease in vaccine effectiveness over time and during the emergence of new VOCs.
Thus, continued efforts to promote infection control measures and vaccination (including booster vaccine doses) and distancing remain necessary during the ongoing COVID-19 pandemic.