Introduction

Healthcare workers were heavily exposed to repetitive viral loads during the SARS-CoV-2/COVID-19 pandemic. The duration and effectiveness of post-infection immunity remains an unclear question. The determination of serum IgG anti-S/SARS-CoV-2 (IgG/anti-S) antibodies is the only feasible way to assess immunity against this virus.

Methods

We’ve identified 193 RT-PCR/SARS-CoV-2 positive from a total of 4200 healthcare workers from an university hospital between March-May 2020. Serum IgG/anti-S was determined by chemiluminescence immunoassay in three anonymized quarterly samples to comparison of mean concentrations over the six months of observation (cut-off defined by the manufacturer: ≥1AU/mL). Disease severity was classified as asymptomatic, mild/moderate and severe/very severe.

Results

76.2% were infected women, with mean ( x ¯) age 39.6±11.7 year; men mean age 41.1±13.0 years, and women/men ratio 3.2. 93.8% worked in high-risk areas and 72.5% were nurses or assistants. Regarding the severity, 7.8% had asymptomatic infection and 6.7% had serious illness with inpatient management. 8.3% refused to participate on serologic testing, so 177 participants were included. In the first determination, 73.5% had IgG/anti-S(+ve) (144/166 samples) with x ¯ 12.5±9.1 AU/mL, collected at x ¯ =50.3±15.3 days after symptoms onset. At the second time, 47.9% (80/167) had positive results, with IgG/anti-S x ¯ =2.7±4.9 UA/mL, x ¯ =143.1±43.9 days after symptoms. The third moment had 27.7% positive results, with IgG/anti-S x ¯ =1.3±2.8 AU/mL, x ¯ =241.3±75.5 days after symptoms.

Discussion

A progressive decrease in mean IgG/anti-S titers was evident during the first six months after SARS-CoV-2 infection, in consonance with the available data, with 13% of professionals presenting antibody levels below the threshold of positivity a month and a half after infection, 52% after four and 72% after six months. We also found that even those with positive titers, at six months, approached the cutoff point, showing a downward curve.

Conclusions

These results suggest that natural immunity is weak/scarce and reinforce that infected professionals should be vaccinated against COVID-19. This measure, together with the protective (collective plus individuals) actions, maximize workers protection against new infections, as well as the safety of patients of the institution.

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