The questionnaire was distributed to 1171 individuals of a total of 1260. 89 professionals were discarded because they did not meet the inclusion criteria. The response rate was 12.8%, which is lower than in other studies, where the response rate ranged from 86.7% (only 83 individuals were sent the questionnaire)  to 73.8% . It should be taken into account that of the 150 professionals, 15 did not undergo any diagnostic test and therefore could not be classified as a case or control, so there were 135 classified participants. A sample design with random sampling and stratification by sex and type of health professional should have included 107 women (72 were included) and 66 men (60 were included), 84 nurses (compared to 61 in our study), 53 physicians (compared to 38 in our study) and 34 nursing assistants (35 were included). The COVID 19 prevalence obtained in this Hospital was 20.7%. In other studies the seroprevalence ranged from 0.75% (95% CI 0.0-8.13), to 16.4 % in a study done in Spain  or 18.5% in a study done in Ontario . A total of 54.5 % of the healthcare personnel who answered to the questionnaire were women, similar figures were found in the controls (59.6%), but the majority of cases were men (64.3 CI 95% 56.1–72.5), and a statistically significant association was found. Other studies have found different results. In two of them carried out in Spain on health personnel (2020), 23.4 % and 23.8% of the cases, respectively, were men  and another study in Girona showed a percentage of women of 90.1% . Another study in Wuhan had 50.0% of women , another in Canada had 81.7% , another in Buenos Aires had a percentage of women of 71.8%  and another in Germany had a percentage was 73.0%. This difference may have been because many medical officers from the Military Health Corps were commissioned, in reserve status (personnel who cease to be on active duty when they reach 61 years of age). All the personnel were male and with a high age range, which may have increased the number of cases and explained the increase in male cases (41.7 % vs. 28.6 %). The mean BMI (24.9 kg/m2, SD 3.9) of the total and controls was within the normal weight range, while the mean BMI of the cases was elevated (25.3 kg/m2, SD 3.8) and was in the overweight range. One study used data from the UK Biobank (n = 285 817) to show that being overweight increased the risk of COVID-19 by 44.0 % (relative risk [RR] = 1.44; 95 % CI, 1.08–1.92; p = 0.010). Two studies showed that the odds of having COVID-19 were increased by 30 % (OR = 1.30; 95 % CI, 1.09–1.54; p = 0.003) and 38 % (OR = 1.38; p < 0.001) respectively, among overweight/obese individuals. The mechanisms responsible for the increased prevalence of COVID-19 in overweight/obese individuals are unknown. Knowledge of other viral infections, such as influenza, provide insight into how excess weight increases the risk of COVID-19 severity. Understanding why this pathology increases the risk of severe COVID-19 is critical to ensure appropriate preventive and interventional therapies against this novel coronavirus.
The percentage of cases with A + blood type was higher than the other groups. The difference was not statiscally significant, but clinically relevant. Similar data were found in a study done in China where group A was associated with a higher risk of COVID-19 infection compared to AB and O types . In another study authors suggest that while A type may play a role in increasing susceptibility to COVID-19 infection, O type may be somewhat protective. However, when infected, blood type does not appear to influence clinical outcome . Another study done in Changsha First Hospital, China, the proportion of patients with A blood type in the COVID-19 group was significantly higher than that in the control group (36.9 % vs.27.5 %, p = 0.006) and concluded that patients with A blood type had a higher risk of SARS-CoV-2 infection, while O type was associated with a lower risk, indicating that certain ABO blood groups correlated with susceptibility to SARS-CoV-2 .
Concerning the professional group physicians were overrepresented in the cases with respect to the controls, with a statistically significant difference, as was found in another study conducted in a Wuhan Hospital . Different results were found in other studies where the professional groups that presented higher susceptibility were nurses and assistants who spent more time in contact with patients where the values ranged from 20.2–52%. This fact could be due to the performance of bronchoscopies, intubations, or site examinations where exposure levels were higher compared to the care activities performed by nurses and auxiliaries, except for the suctioning activities of ICU staff . Cases with 10 or more years of experience and membership in the military administration were proportional to the total and controls.
The percentage of cases who received the influenza vaccine was very low compared to those who did not, but in the same proportion between cases, controls and totals, and no difference was observed. Several studies show that influenza vaccination does not increase the risk of COVID-19 infection . This study provides trust against speculation that influenza vaccine increases the risk of COVID-19 infection. In addition, other studies claim that influenza vaccination may be associated with lower severity and lower mortality from COVID-19 .
Concerning vitamin D, those with vitamin D deficiency who also took supplements had a lower risk of COVID-19, in the total and both in the cases and controls, with a significant association. The factor that produces positive effects, reducing transmission and improving prognosis, is vitamin D supplementation and not the deficit itself. Several studies reported an association between CRP positivity and vitamin D deficiency. The exact efficacy of vitamin D supplementation for the prevention or as adjunctive treatment of COVID-19 remains to be determined, but several ongoing randomized controlled trials (RCTs) are actively investigating these potential benefits .
Low stress was associated to COVID-19 (low stress level in 89.3 % cases vs. medium stress level 10.7 % cases). These results were not consistent with another study that reported that the presence of psychosocial risks had important consequences for the quality of care and increased the probability of errors .
Likewise, the number of cases among personnel who reported being more exposed was higher than the rest (82.1 % vs. 0.0 %), and no differences were found, since the number of exposed controls was also very high with respect to those not exposed. This fact could suggest that exposure is not as important as nosocomial or community transmission. The same results were not found in a study done in Spain, where greater direct exposure led to a higher number of infections . It would be advisable to maintain a safe distance between professionals, hand hygiene and the use of personal protective equipment. The number of cases among personnel who worked during the morning shift and who had fewer days off than the controls was higher than in other shifts or in the case of the controls, with a statistically significant difference. The morning shift is the one with the greatest work overload and the highest number of health professionals. These two factors, nosocomial transmission and overload, are two important factors to be taken into account for prevention . It would be advisable to increase the number of healthcare personnel during this shift to reduce overload; other studies have obtained similar data .
Continuing with the epidemiological risk antecedents, the cases had fewer days off (1.25 SD 0.7) than the controls (1.44 SD 0.7). In relation to protective equipment, there were hardly any cases among those who reported adequate training in the use of the equipment.
Concerning signs and symptoms, fever, cough and diarrhea were found in at least 50% of the sample with significant differences, and the proportion of cases that also showed other symptoms such as headache or sore throat was not negligible . In a study conducted in Spain , fever and cough were also the most representative signs. Fever was identified more frequently and with significant differences, in men with chronic diseases and without vitamin D deficiency . Sore throat was identified among those who had not been vaccinated against influenza. In a specific analysis, it was observed that health workers with pneumonia were significantly overweight and did not practice any type of sporting activity compared to those who did not have pneumonia. On the other hand, cough was significantly more frequent in healthcare personnel with more than 10 years of experience.
The most important limitation of this study refers to the sample. This difference could have been due to the short time available to the health personnel or to exhaustion after the practice of health care. Likewise, if a sample size calculation had been made with an error percentage of 5 % and a confidence level of 95 %, the number of individuals should have been 173 as opposed to the 150 that made up our study.