Introduction:

The aim was to estimate the evolution of the levels of anti-SARS-CoV-2 antibodies, the associated factors, and the incidence of new infections during the follow-up period.

Method:

Prospective cohort study of a representative sample of workers at the General University Hospital of Castellon 8 months after receiving the second dose of Pfizer-BioNTech vaccine against SARS-CoV-2, by determining IgG-S, IgG-NP, follow-up and response to a questionnaire. The results were compared with those at the start of the cohort in February 2021. Multivariate linear regression and Poisson regression were used.

Results:

A total of 253 workers participated out of the 275 in the start of the cohort. All had detectable levels of IgG-S, median 691% AU/ml, decreasing by 93.3% compared with the first study. The decline of IgG-S increased with age and obesity; and decreased with a COVID-19 previous history, regular exercise, and in smokers. IgG-NP was positively associated with a history of COVID-19, taking vitamin D, and decreased from 4.4% to 1.2%. There were 4 new cases of COVID-19 in the cohort, with and incidence rate of 1.7%. One death occurred in a participant with immunosuppressive treatment, only one case was asymptomatic and no reinfections occurred.

Conclusions:

A general decrease of IgG-S and IgG-NP antibodies after the second dose of Pfizer-BioNTech vaccine was observed in the cohort, as well as with new SARS-CoV-2 infections. Booster doses, maintaining protective measures and further determination of the protection threshold of vaccination are recommended.

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