During the pandemic spread, increasing evidence has highlighted the potential role of hypovitaminosis D as a modifiable risk factor for SARS-CoV-2 infection and worse acute COVID-19 [3, 13,14,15,16]. Furthermore, lower vitamin D levels were also recently associated to an increased risk for the occurrence of Long COVID syndrome [17].

Due to the well-known actions of vitamin D in regulating the immune response and immunocompetence [18, 19] previous studies have investigated its role also in influencing the humoral response after different vaccinations [1, 5]. In our study, the long-term humoral response to anti-SARS-CoV-2 vaccination clearly appeared to be influenced by baseline 25(OH)D levels. In fact, we found that poor vitamin D status, as defined by 25(OH)D levels <20 ng/mL [20], before the immunization was associated with reduced anti-RBD-S-IgTot levels 5 and 9 months after vaccination. Differently from most of the previously published studies, our findings were controlled in nature since in our work stringent inclusion and exclusion criteria were used enrolling a homogenous population of healthcare workers immunized in the same time period with the same vaccine type, and including only individuals who were not supplemented with vitamin D and naïve for SARS-CoV-2 infection and without breakthrough infections during the study-period. Furthermore, subjects were followed-up for a much longer time period as compared to the previously available studies. This allowed us to focus not only on the impact of vitamin D levels on peak humoral response but also on the protracted and long-term efficacy of anti-SARS-CoV-2 vaccination. Clinically, this may be a relevant information since subjects with reduced antibody persistence should likely require more frequent additional booster vaccinations to preventing COVID-19 worse outcomes. Interestingly, we observed that an adequate vitamin D status at the beginning of the immunization cycle was associated with a significantly more sustained anti-SARS-CoV-2 antibody response.

The retrospective nature of our study did not allow us to assess the protective effects of the persistence of higher antibody titers against either SARS-CoV-2 infection or severe acute COVID-19. In fact, on one hand, anti-SARS-CoV-2 antibody titers are tightly linked to degree of protection at least against severe forms of COVID-19 [21, 22]. On the other hand, in fragile and high-risk populations, such as elderly, patients with cancers or under dialytic treatment, characterized by an immune response impairment with reduced humoral response, despite poor seroconversion in comparison to healthy subjects, the protective effect of the vaccine seems to be maintained [23,24,25]. Nevertheless, in support of clinical relevance of our data, it is also widely recommended in these fragile populations to administer extra- and booster vaccine doses to grant them an adequate protection against the severe infection [26,27,28,29] suggesting that it is not the short-term but the persistency of the response to the vaccine to be impaired.

It could be also hypothesized that hypovitaminosis D may represent a marker of limited outdoor activities and fragility, possibly in turn being per se, at least in part, responsible for the lower observed antibody levels [30]. However, no one of our study subjects could be defined “fragile” or with intrinsic high-risk for impaired immune response, since we included in our cohort only healthy and relatively young health hospital workers. Also, the lack of 25(OH)D levels re-evaluation at different study-timepoints could represent a limitation of the study, although the possible effect of seasonality on 25(OH)D levels should apply to all subjects included in the study due to the very short time-window of enrollment. Moreover, other data, beyond BMI, about the metabolic features of our subjects were not available since not included in main study protocol. Finally, our data were obtained in a very selected relatively young cohort without a high prevalence of hypovitaminosis D and therefore could not be automatically extended to the general population in which however the reported scenario may even be worsened in case of more generalized and severe vitamin D deficiency.

Besides these limitations, our data showed that people with vitamin D deficiency were at risk of a less sustained immune response to COVID-19 vaccine. Vitamin D deficiency was also extensively reported as an important modifiable risk factor for severe COVID-19 increasing the risk of worse outcomes [3, 13,14,15,16]. Severe COVID-19 was reportedly associated with post-infection elevated peak of antibodies against SARS-CoV-2 [31, 32] which has been hypothesized, although not yet proven, to occur regardless of vitamin D status [33]. Interestingly, a limited influence of hypovitaminosis D on this short-term humoral response is also observed in our data and in the previous published reports, supporting the hypothesis that vitamin D levels may be associated with the long-term humoral response to COVID-19 vaccine rather than to the acute seroconversion, elicited by the infection and/or vaccination.

In conclusion, in a cohort of highly selected relatively young subjects with limited prevalence of hypovitaminosis D, low levels of circulating 25(OH)D were associated with impaired long-term response to COVID-19 vaccine. Therefore, based on our data it can be hypothesized that at least in populations at higher risk of both hypovitaminosis D and COVID-19 such as the elderly [34], assessing vitamin D levels and eventually improving a poor vitamin D status before the vaccination could be a possible strategy to optimize the vaccination campaigns to prevent severe COVID-19. This hypothesis needs to be tested in controlled clinical trials assessing the effect of vitamin D supplementation in subjects with hypovitaminosis D on the long-term vaccine protection against worse COVID-19 outcomes and the requirement for vaccine boosts which may have both clinical and economic favorable implications.