It is now more than evident that COVID-19 is a complex disease, capable of manifesting itself in a rather varied manner in the population, affecting many organs and systems with varying degrees of severity. The importance of our study lies in the inclusion of individuals without significant comorbidities who showed a statistically significant decline in lung function indices (FVC, FEV1) during follow-up spirometry, which persisted over the medium to long term.

Our study revealed subclinical changes in the lung function of the subjects included in the sample. Since a physiological decline in lung function indices is expected over a three-year period, in order to highlight the role of COVID-19 infection, we compared the loss of FEV1, FVC and FEV1/FVC in the group of subjects who had the infection in comparison to those who did not become infected. Although no significant differences were found between the two groups overall when considering COVID-19 infection as a whole, the analysis of the different years showed that infection with in 2020 and 2021 (time of spread of the first variant strains) was associated with a slight but significant reduction in FVC. Previous studies had already found a reduction in lung function indices in patients with COVID-19. [10]

Referring to another international study, the COMMUNITY study assessed long-term immunity in healthcare workers at a Swedish hospital who had a mild form of the disease during the acute phase [11]. The study had an 8-month follow-up until early 2021. Participants reported symptoms through questionnaires and had periodic antibody titer evaluations through blood tests. Results showed that many low-risk subjects with mild SARS-CoV-2 infection experienced diverse long-term symptoms, negatively impacting their lives. However, the analysis has limitations, such as relying on subjective symptom evaluation and omitting objective data like spirometric indices. Nevertheless, this adds to the hypothesis that COVID-19 is not solely an acute disease, and our study, involving young and healthy individuals with a history of mild acute disease, supports the theory of Long COVID possibility.

To test the hypothesis that previous vaccination with Comirnaty (BNT162b2) is protective with respect to worsening lung function indices, we entered this vaccination into a multiple regression model, considering age, gender, BMI and smoking as covariates. Vaccination was statistically associated with a lower loss of FVC emphasising the importance of vaccination in protecting even from subclinical effects of COVID-19 infection.

It is necessary to specify that our investigation was primarily limited by the relatively small sample size (321 operators), which could have led to falsely negative results in determining additional variables of functional loss. However, given the correlation between SARS-CoV-2 positivity and changes in functional indices, it is reasonable to assume that this result was not influenced by experimental limitations.

There is a notable heterogeneity in the number of COVID-19 cases that developed within our sample over different years. This is another limitation of our study, attributable to the epidemiological characteristics of the infection.

Certainly, further studies are required to extend the follow-up period, allowing for a better evaluation of the significance of the reductions in spirometric indices recorded by us, which are still in the subclinical phase. As a result, the prognostic significance of our analysis results remains limited.

Undoubtedly, the evidence of unfavorable functional outcomes in a population represented by subjects without risk factors and with a history of mild disease can contribute to increasing the perception of COVID-19-related risk. The lack of risk perception resulting from the disease has been responsible for a significant increase in vaccine hesitancy, even among healthcare workers.

Despite the aforementioned limitations, our work demonstrates the effectiveness of the vaccination campaign in preventing unfavorable outcomes on functional indices, a factor to consider in defining vaccination strategies for populations at higher risk of infection, such as healthcare workers.

The results of our study shed important light on the impact of subclinical infection in 2020 and 2021, which coincides with the circulation of the alpha, beta and delta variants of SARS-CoV-2. Notably, we observed that such subclinical infections were associated with a deterioration of key respiratory parameters, specifically FVC and FEV1. This suggests that even mild or asymptomatic infections during the delta variant-dominated period could have significant implications for respiratory health.

On a positive note, our research also revealed a promising trend concerning vaccination. We found that individuals who received vaccination appeared to be better protected against the adverse respiratory outcomes associated with subclinical infections. Even in healthy subjects who had experienced an infection, the presence of vaccination seemed to exert a protective effect on respiratory function, potentially mitigating the impact of the infection on lung health. In the literature, we can see that vaccination is associated with a reduction in inflammation in the short and long term, which could partly explain the reduced impact of COVID-19 on lung function parameters in vaccinated subjects [12].

These findings underscore the crucial importance of widespread vaccination, especially among healthcare workers. As frontline heroes battling the COVID-19 pandemic, their health and well-being are of paramount importance. By ensuring universal vaccination of healthcare workers against COVID-19, we can take proactive measures to safeguard their respiratory health and reduce the risk of potential long-term consequences resulting from subclinical infections.

In summary, our study not only emphasizes the significance of addressing subclinical infections during the delta variant surge but also highlights the encouraging role of vaccination in mitigating the adverse respiratory effects of such infections. Implementing universal vaccination among healthcare workers is a vital step towards safeguarding their health and the well-being of the broader community in the ongoing fight against the COVID-19 pandemic.