Discussion

This paper describes the estimated seroprevalence of SARS-CoV-2 antibodies in a cohort of 676 HCWs working in a referral hospital for contagious diseases from the beginning to the peak of the COVID-19 outbreak in Milan. The Wantai total Ab assay used has previously been found to perform well and to have an acceptable positive predictive value even in populations with a low seroprevalence.10 11

The estimated seroprevalence of SARS-CoV-2 increased from 0.5 (95% CI 0.1 to 1.7) in March to 5.4 (95% CI 3.6 to 7.9) in April and reached a plateau in May, in line with the course of the epidemic in Milan. Two previous studies have cross-sectionally evaluated the prevalence of SARS-CoV-2 infection among HCWs working in Milan hospitals14 15: the study by Lombardi et al 14 was conducted in a very early phase (12 February–31 March) and found that 139 (8.8%) of 1573 HCWs had a positive NSP test; the study by Sandri et al 15 was carried out between 28 April and 16 May and found a similar prevalence (8%) in 2872 employees of three hospitals tested for SARS-CoV-2 antibodies. Both of these findings are higher than ours. The only available data concerning the general population of Milan come from a cross-sectional study of anti-SARS-CoV-2 IgM/IgG using a lateral flow immunoassay in healthy asymptomatic blood donors, which indicated a seroprevalence of 7.2%.16 The reported rates of SARS-CoV-2 infection in HCWs in Europe range from 3% to 45%14 15 17–23 and depend on the testing approach (ie, testing only symptomatic or both symptomatic and asymptomatic subjects), the diagnostic method (ie, PCR tests of NPS or serology, or both) and, mainly, the intensity of the local epidemic at the time. Even the data concerning Lombardy (the Italian region most involved by the epidemic) range widely (from 3% to 43%) and are strictly related to the epidemic burden in the area.15

The lower seroprevalence observed in our study may be explained by the fact that our hospital is the reference centre for infectious diseases and bioemergencies in Northern Italy and has the largest number of ID beds in Lombardy and an active biocontainment unit in which ID and ICU staff are continuously trained to use different types of PPE. As demonstrated during the SARS outbreak and the COVID-19 epidemic in China, the appropriate use of masks, gloves, gowns and hand hygiene is effective in preventing virus transmission in clinical settings,24–27 and so it is likely that the availability of PPE and confidence in using it, together with the early implementation of preventive measures throughout the hospital (including the use of surgical masks in COVID-free areas and by colleagues not involved in treating patients with COVID-19) played a crucial role in containing the spread of the virus. It must be acknowledged that differently from other clinical settings and territorial health facilities, our hospital did not experience severe shortage of PPE at any time of the outbreak. It should also be noted that we did not find any difference in seroprevalence between the HCWs working in medical wards converted into COVID-19 wards and those working in the ID department (data not shown), which indicates that negative-pressure rooms are not essential if PPE is used and suitable infection control measures are taken.

In line with the findings of other recent studies,18 28 we found that previous contacts with COVID-affected relatives or friends and a history of symptoms consistent with COVID-19 were strongly associated with an increased likelihood of testing positive for SARS-CoV-2 antibodies, whereas there was no significant association with the type of occupation or having had contacts with hospitalised patients with COVID-19. Notably, while at the time of first data collection, out-of-hospital contacts with both relatives and friends had been reported; over the remaining study period, the same were prevalently referred to relatives cohabitating with the HCW as the result of the strict lockdown measures imposed to Italy. Although our study is not designed to define the dynamics of infection within family clusters, the fact that the probability of seroconversion among HCWs at the different time points was consistent with the incidence of infection in the general population suggests an out-of-hospital source of the infection. Despite the number of seroconversions finally occurred in our cohort was small, the awareness of factors associated with a higher risk of acquiring SARS-CoV-2 in a potentially risk environment is crucial to indicate the correct behaviours to adopt inside and outside the working setting, and it might be helpful to cope with the ongoing second wave of the epidemic.

Moreover, and again in line with the findings of other studies,14 15 17–20 23 29 30 we observed a considerable proportion of seropositive subjects (38%) with asymptomatic or paucisymptomatic infection. During the COVID-19 outbreak in Milan, asymptomatic HCWs were not screened in order to reduce the risk of outbreaks at work, and PCR testing of NPS was only used in the presence of symptoms; from our findings, it cannot be inferred if the benefits of periodic screening for SARS-CoV-2 among HCWs, in the case of well-trained personnel and a continuous supply of PPE, outweigh the costs.

Finally, it is interesting to note that a large proportion (40%) of our seronegative study participants reported symptoms compatible with SARS-CoV-2 infection at some time during the study, a finding that is similar to that of Brandstetter et al.31 This may have been due to the simultaneous spread of respiratory infections other than SARS-CoV-2 during the winter/spring, seasonal allergic disorders and/or psychological distress, but a number of false negative results cannot be excluded. Moreover, it has recently been demonstrated that subjects with a history of asymptomatic or mild COVID-19 show SARS-CoV-2 specific T cell responses in the absence of specific antibodies,32 thus suggesting the wider spread of infection despite the absence of a detectable humoural response.

Our study has some limitations. Although representative of our hospital’s occupational categories, the sample was not randomly enrolled and therefore our findings may not reflect true prevalence as a potential selection bias cannot be excluded; however, the longitudinal study of a large number of voluntary participants does provide important information concerning the dynamics of SARS-CoV-2 seroconversion among HCWs and the factors associated with it.

Second, it is difficult to follow subjects exposed to the intense workload, shift work and stressful environment characterising the peak phase of the epidemic in our hospital (and many others); however, only a few of the participants with gaps in their follow-up (4/71) were hospitalised (one) or quarantined (three), and this is unlikely to have introduced biases in the seroprevalence data.

Finally, although the performance of the ELISA we used is good, the accuracy of commercially available SARS-CoV-2 antibody tests requires further validation in the setting of paucisymptomatic and asymptomatic SARS-CoV-2 infection.

In conclusion, the longitudinal sampling of HCWs in a referral hospital for patient with COVID-19 in Milan revealed an increase in the seroprevalence of SARS-CoV-2 antibodies at the time of the peak of the epidemic, but this was less than that observed in other city hospitals14 15 and probably reflects the prompt and effective reorganisation of our hospital and the implementation of infection control measures as the seroconversions seemed to be related to extrahospital contacts. The detection of SARS-CoV-2 antibodies in a relatively high number of asymptomatic HCWs highlights the need for the regular surveillance of asymptomatic HCWs in order to protect both staff and patients from possible nosocomial transmission, particularly if strict adherence to infectious diseases control measures and continuous training in the use of PPE cannot be ensured. Further longitudinal serological studies would help to clarify the quality and persistence of SARS-CoV-2 specific immunity among HCWs.