Most clinical cases in this cohort of HCW were mild-moderate COVID-19, with 24 hospitalized, and 64 presenting with sequelae. Median age was 49 years (IQR 41-58), 137 were females, and there were 13 smokers and 31 ex-smokers.
We did not detect a significant decline in antibody levels as a function of time since symptoms onset (Figure 1). The percentage of seropositivity 149-270 days after symptoms onset combining RBD and S antigens was 60.69% for IgM, 76.30% for IgA, and 90.17% for IgG, consistent with the expected longer duration of the latter isotype. Unexpectedly, seropositivity was quite sustained also for IgM and IgA, considered to be isotypes of shorter duration. Computing all immunoglobulins, seroprevalence 5-9 months after the initial COVID-19 episode was as high as 92.49%, indicating very stable persistence of responses. Furthermore, 64 of 173 HCW not yet vaccinated were tested in January and April 2021, and the overall percentage of seropositivity up to 322-379 days after onset of symptoms in this subset was still as high as 96.88% (IgG 95.31%, IgA 82.81%, IgM 25.0%). These 64 HCW had a seropositivity of 98.44% at 5-9 months after the initial COVID-19 (IgG 95.31%, IgA 87.50%, IgM 37.50%).
There were four suspected reinfections (Table 1). Before the second positive polymerase chain reaction (PCR) diagnosis, two symptomatic cases were seronegative, one asymptomatic was seropositive with low antibodies, and one had unknown serostatus. We attempted to recover the viral RNA from the first episodes for genome sequencing and demonstration of different strains but unfortunately it was not stored. This data set provides some indication of the frequency of reinfection in 173 primary infections with three likely reinfections (interval >90 days as per CDC guidelines) and one suspected reinfection (<90 days between primary and reinfection). Therefore, there was a minimal overall rate of symptomatic re-infection of 2/173 (1.16%). This rate contrasts with what we found in another HCW cohort that we followed up for 7-month seroprevalence in which no reinfections were detected [19, 5]. It could be that primary HCW are more at risk of reinfection than hospital-based HCW, although it should be pointed out that is based on limited sample size. The study also provides some evidence that a lack of S antibody response is a risk factor for symptomatic reinfection while positive serology leads to asymptomatic reinfection (Table 1). This is relevant due to the strong correlation (rho=0.9) between antibody levels to S and RBD with neutralizing function that are thought to confer protection .
Stepwise multivariable regression analyses showed that the baseline factors most consistently and significantly associated with higher levels of antibodies 5-9 months after infection were having been admitted to hospital, presenting fever (n=131), anosmia and/or hypogeusia (n=106), and having had previous allergies (n=24) (Table 2). Specifically, for anti-S IgG, HCW with fever had 2.5 times higher levels, patients with anosmia and/or hypogeusia had 2.6 times higher levels, and those with allergies had 1.9 times higher levels, than patients without those conditions. Baseline factors associated with lower levels of IgA and IgG included being a nurse (n=68) or a physician (n=70) compared to other occupation categories working in primary health care centers including customer and social services staff (n=35), and smoking. For anti-S IgA, physicians had 34.84% and nurses 45.67% lower levels than the other jobs, and smokers had 46.17% less than non-smokers (Table 2). Nurses included eight auxiliary nurses, and physicians included one dentist. Other factors were associated with only certain isotypes. Presenting with sputum and/or hemoptysis (n=13) was associated with higher IgM levels, and shivers (n=86) were associated with higher IgAs. Of note, hospitalized patients had 2.1 times higher IgM levels to RBD than non-hospitalized. Age correlated positively with IgGs, having 1.39% higher antibody levels to RBD with each year of age older (Table 2). Higher IgGs (and IgAs less strongly) positively correlated with duration of symptoms (median 24 days, IQR 13-36; S rho=0.229 P=0.002; RBD rho=0.246, P=0.001) and number of symptoms (median 10, IQR 6-12; S rho=0.351 P<0.001; RBD rho=0.364, P<0.001). All other variables, symptoms, or sequelae, were either not statistically significantly associated with antibody levels or weakly in univariable models.
Previous acute phase studies showed that COVID-19 severity was associated with higher antibody responses. Here, hospitalization was associated with higher immunoglobulin levels many months after convalescence, suggesting that severity does not affect stability of memory B cell and plasma cells producing antibodies [2–4, 20]. Common symptoms like fever and highly specific symptoms like alteration in smell and taste were also associated with higher antibodies. Interestingly, having previous allergies also correlated positively with higher antibody levels, which to our knowledge has not been reported. This could be related to disease exacerbation and increased risk of respiratory infections associated with some allergies  although this relationship remains unclear. Lower antibody levels in nurses and physicians than other HCW could indicate a lower exposure due to PPE use and higher awareness of risks . Smoking had previously been associated with lower antibody responses [22, 23] and we show that this effect persists after several months primarily affecting IgA, the main mucosal antibody.
In conclusion, despite the large heterogeneity in antibody levels induced by SARS-CoV-2 infection, most HCW patients remained seropositive for anti-S antibodies up to 12.5 months after COVID-19. The findings that after PCR reversion, 2 out of 13 seronegatives had another symptomatic episode, and that one low responder had a second (asymptomatic) infection, are consistent with a protective role for antibodies . Considering that antibody levels achieved by COVID-19 immunization are usually higher than those elicited following natural infection, based on this study it could be speculated that immune memory induced by first generation vaccines could also be long-lasting, therefore reducing the probability that periodical boosters might be required to sustain protective immunity, at least within the first year. Furthermore, data indicates that naïve people should be prioritized for vaccination over those who had suffered COVID-19 since the latter maintain antibodies for at least a year.