Out of 1,402 patients finally included in the study, the mean age (± SD) was 51.64 (± 21.433). The study population consisted of 678 male (48.4%) and 724 (51.6%) female participants. Of those, 228 tested positive for Influenza A (16.2%), 196 for SARS-CoV-2 (14%), 64 for Influenza B (4.6%), 61 for RSV (4.4%), and 11 for adenovirus (0.8%). Due to the small number of adenovirus-positive patients, they were not analyzed further.

Hospitalized VS Non-hospitalized overall

Among 248 patients, approximately 20% required hospitalization for respiratory infections, while the remaining patients were discharged with home care instructions. Males (N = 140) were significantly more likely to be hospitalized than females (N = 108), with an odds ratio of 1.48 (95% CI: [1.13, 1.96], p = 0.005). The mean age of hospitalized patients was 70.60 years (± 16.87, 95% CI: [68.5, 72.7]), significantly higher than non-hospitalized patients (47.56 ± 20.08, 95% CI: [46.4, 48.8], p p = 0.003). The average duration of symptoms before hospital admission was 4.79 ± 3 days for hospitalized patients, compared to 4.29 ± 4.58 days for non-hospitalized patients, with no statistically significant difference. Among hospitalized patients, 38 (15.3%) tested positive for Influenza A, 20 (8.1%) for SARS-CoV-2, 26 (10.5%) for RSV, 1 (0.4%) for Influenza B, and 1 (0.1%) for adenovirus. The remaining hospitalized patients tested negative for these specific viruses but were admitted due to the severity of their symptoms.

Hospitalization rates

Hospitalization rates varied significantly across viruses, with the highest involving RSV (42.6%), followed by Flu-A (16.6%), SARS-CoV-2 (10.2%), and Flu-B (rare, with one case). Gender differences emerged: women had higher hospitalization rates for RSV and SARS-CoV-2, while men were slightly more affected by Flu-A. Flu-B hospitalizations were rare in both sexes. Specifically, 13.55% of hospitalized women had Flu-A, 0% had Flu-B, 13.68% had SARS-CoV-2, and 44.4% had RSV. Among men, 20% had Flu-A, 3.22% had Flu-B, 6.90% had SARS-CoV-2, and 40% had RSV (Fig. 1).

Symptoms/Signs

Symptoms analysis based on virus type

Among Flu-A patients, hospitalized individuals had significantly higher rates of sputum production (95% CI: [1.08, 4.42], p = 0.042) and dyspnea (95% CI: [4.77, 23.65], p p p p = 0.008), suggesting that higher fever may be linked to the need for inpatient care in RSV infections. As in Flu-A, URTI symptoms were linked to outpatient management (p 2).

Symptoms/Signs analysis between hospitalized patient groups

When grouped into systemic symptoms (including fever, myalgias/arthralgias, headache, and fatigue) or URTI symptoms (congestion and sore throat), no statistically significant differences were observed between the hospitalized patient groups.

However, symptom-specific differences did emerge. Cough prevalence differed significantly between groups (p = 0.014), with post-hoc analysis showing higher cough frequency in RSV compared to SARS-CoV-2 (p = 0.008). Sputum production also varied by group (p = 0.028), being significantly more common in RSV than in SARS-CoV-2 (p = 0.018), and in Flu-A compared to SARS-CoV-2 (p = 0.025). Wheezing was most frequently observed in RSV cases (~ 60%) and occurred significantly more often than in Flu-A (p = 0.038) and SARS-CoV-2 (p p = 0.005), and magnesium sulfate, a marker of severe bronchospasm, was administered only in RSV cases (p = 0.009).

Comparative analysis of symptoms/signs between Non-Hospitalized patient groups

Analysis of non-hospitalized patients infected with Flu-A, Flu-B, RSV, or SARS-CoV-2 showed that systemic and upper respiratory tract (URTI) symptoms were common across all groups, with modest variation in frequency (Fig. 3). Systemic symptoms were reported in > 90% of Flu-A, Flu-B, and SARS-CoV-2 cases and somewhat less often in RSV (~ 80%). URTI symptoms were also frequent overall; Flu-B showed the highest proportion (85.7%), followed by SARS-CoV-2 (73.9%), RSV (62.9%), and Flu-A (61.6%).

Among individual symptoms, several pairwise differences reached statistical significance (Fig. 3). Fever was most common in influenza cases (Flu-A 83.2%, Flu-B 77.8%), and less common in SARS-CoV-2 (63.6%), and RSV cases (42.9%). Similarly, dyspnea occurred more often in RSV patients (31.4%) compared to all other viruses, where the prevalence remained below 15%. In contrast, cough was prevalent across all viruses, exceeding 84% in Flu-A, Flu-B, and RSV, with somewhat lower rates in SARS-CoV-2 (64.2%). Sputum production tended to be higher in RSV and Flu-B (45.7% and 47.6% respectively) than in SARS-CoV-2 (25%), while Flu-A patients showed intermediate rates (33.7%). Notably, no statistically significant differences were observed between Flu-A and Flu-B across the examined symptoms, underscoring their overlapping clinical presentations in outpatient settings.

Influenza A

Among Influenza A-positive patients, one in six required hospitalization, with the majority being male. Hospitalized patients were significantly older (75.68 ± 13.29 years, 95% CI: [71.3, 80.1) compared to non-hospitalized patients (45.1 ± 18.4 years, 95% CI: [42.5, 47.8]), with a large effect size (r = − 0.52, 95% CI: [–0.65, − 0.39], p p p = 0.049, r = − 0.13, 95% CI: [–0.26, − 0.001]). Radiological evidence of pneumonia was strongly associated with hospitalization (p 

Multivariate binary logistic regression revealed that age (aOR = 0.896, p = 0.001), diabetes (aOR = 5.12, p = 0.034), unilateral pneumonia (aOR = 529.01, p p = 0.039) were strong predictors of hospitalization. The overall model was statistically significant (χ²(15) = 148.60, p p = 0.805), suggesting that comorbidities and disease severity were the primary drivers.

Influenza B

Out of 64 Influenza B-positive patients, only one required hospitalization. This patient was a 72-year-old male, obese, with multiple comorbidities, including COPD, coronary artery disease, and diabetes mellitus. He delayed seeking medical attention for approximately one week before presenting to the emergency department, subsequently developing unilateral pneumonia, likely due to a secondary bacterial infection. Despite prior influenza vaccination, hospitalization was required. In contrast, the majority of Influenza B-positive patients (30 males and 33 females, mean age 34.49 ± 16.17 years) sought medical care after an average of 3.43 ± 1.87 days and did not require hospitalization.

Seasonal influenza vaccination

The following analysis explores the relationship between influenza vaccination and outcomes in Flu-A positive patients during the 2023–2024 season. As this is an observational study, associations are reported without implying causation, and confounding factors—particularly age and comorbidity—are considered in both unadjusted and adjusted analyses.

Out of the total study population of 1,402 individuals, 1 out of 4 (N = 352) were vaccinated against Influenza. Among the 292 Flu-positive patients, only 39 (13.4%) had received the vaccine, with 33 cases attributed to the Flu-A strain and six to the Flu-B strain.

Given that only one Flu-B case resulted in hospitalization, the hospitalization analysis focused solely on Flu-A cases. Among Flu-A positive patients, 38 required hospitalization, of whom 11 had been vaccinated, representing 28.9% of hospitalized cases. In contrast, among the 190 non-hospitalized Flu-A patients, 22 were vaccinated (11.6%). When analyzing hospitalization rates based on vaccination status, 33.3% (11 out of 33) of vaccinated Flu-A patients were hospitalized, compared to 13.8% (27 out of 195) of non-vaccinated Flu-A patients. This difference was statistically significant, with a p-value of 0.01, indicating, at first glance, that vaccinated individuals had a higher observed higher hospitalization rate than their non-vaccinated counterparts.

Also, the calculated odds ratio of hospitalization,

indicate that, in unadjusted analysis, the vaccinated participants had approximately three times the odds of being hospitalized compared to unvaccinated individuals. However, this association must be interpreted with caution as significant confounding factors, such as age, comorbidities, timing of vaccination, and flu strain mismatch, could be influencing the results.

The data indicate a notable age difference between the two groups, with vaccinated Flu-A patients having a mean age of 70.55 years, while unvaccinated Flu-A patients had a mean age of 46.79 years. This difference is statistically significant (p 

Moreover, the prevalence of comorbidities was considerably higher in the vaccinated group. Among vaccinated individuals, 87.9% had at least one comorbidity, compared to only 40.5% in the unvaccinated group (p p p = 0.004). Given that these conditions independently increase the risk of severe outcomes, their uneven distribution between the two groups could be contributing to the observed difference in hospitalization rates.

The initial unadjusted analysis indicated that the vaccinated participants had 3.11 times higher odds of hospitalization compared to the unvaccinated counterparts. After adjusting for the aforementioned confounding factors using multivariate logistic regression, vaccination status was no longer significantly associated with hospitalization (aOR = 1.178, p = 0.805), indicating that vaccination was not an independent predictor of hospitalization risk. This finding suggests that the initially observed higher hospitalization rate among vaccinated individuals was likely driven by baseline differences in age and health status rather than the effect of vaccination itself. Instead, age, diabetes, pneumonia, and dyspnea emerged as the strongest predictors of hospitalization among FluA-positive patients.

We further calculated the vaccine effectiveness (VE) for Flu-A using the standard formula:

The attack rate in unvaccinated individuals was 0.1857 (18.57%), while the attack rate in vaccinated individuals was 0.0938 (9.38%).

Thus, the unadjusted estimated vaccine effectiveness was 49.5%, suggesting that vaccination was associated with a nearly 50% lower risk of Flu-A infection compared to unvaccinated individuals, indicating a moderate level of protection in our study population. However, this estimate should be interpreted with caution given the observational nature of the data and the potential for residual confounding.

SARS-COV-2

Among SARS-CoV-2 positive patients, 10% (20/196) required hospitalization, with the majority being female. Hospitalized patients were significantly older (79.35 ± 10.51 years, [95% CI: 72.0–85.0]) compared to non-hospitalized patients (60.17 ± 20.31 years, [95% CI: 60.0–69.0], p p = 0.005). Among SARS-CoV-2–positive patients, hospitalization was significantly associated with chronic pulmonary disease (OR = 3.23, [95% CI: 1.17–8.89], p = 0.026), particularly COPD (OR = 6.00, [95% CI: 1.58–22.71]), as well as chronic heart disease (OR = 3.78, [95% CI: 1.21–11.75]; p = 0.017). Hospitalized patients also had a longer symptom duration (5.35 ± 3.35 days, [95% CI: 3.0–8.0]) before seeking medical care compared to non-hospitalized patients (3.04 ± 2.27 days, [95% CI: 2.0–3.0], p = 0.002). Radiological findings, including unilateral consolidative infiltrates, were strongly linked to hospitalization (OR = 87.5, [95% CI: 4.43–1729.99], p p = 0.004).

Multivariate binary logistic regression identified age (p = 0.017), longer symptom duration before seeking care (p = 0.020), and dyspnea (p = 0.049) as significant predictors of hospitalization. The overall model was statistically significant (χ²(10) = 51.56, p p = 0.393).

SARS-COV-2 vaccination

In the study, 76.5% of the total population (1072/1402) were vaccinated against SARS-CoV-2, with most receiving three doses (52.1%), followed by two doses (26.2%) and four or more doses (15.1%). Only 6.5% received one dose. Among SARS-CoV-2 positive patients, 83.2% (163/196) were vaccinated. Of the 20 hospitalized SARS-CoV-2 patients, 18 were vaccinated and 2 unvaccinated, with no significant association between vaccination status and hospital admission (p = 0.537). Vaccination distribution among positive patients included 3.1% with one dose, 12.9% with two doses, 55.8% with three doses, and 28.2% with four or more doses.

Respiratory syncytial virus

Four out of ten RSV patients required hospitalization, with in-patients significantly older (71.3 ± 14.1 years, 95% CI: [66.0–82.0]) than non-hospitalized ones (45.5 ± 20.0 years, 95% CI: [32.0–55.97], p p = 0.005) and those over 75 had 24 times higher odds (95% CI: 4.6-124.9, p p = 0.022). Chronic heart disease, in particular, was strongly associated with hospitalization (OR = 5.40, 95% CI: [1.77–16.47], p 

Multivariate logistic regression identified age (OR = 0.909, p = 0.002,) and fever (OR = 9.959, p = 0.011,) as significant predictors of hospitalization, with febrile patients having nearly 10 times higher odds of hospitalization. The model demonstrated excellent model fit (Omnibus test χ² = 77.703, p p = 1.000).

Comparative analysis of hospitalized patients with Influenza A, RSV, and SARS-CoV-2

Among hospitalized patients with Influenza A, RSV, and SARS-CoV-2, the average age was over 70, with SARS-CoV-2 patients being slightly older. Most had at least one comorbidity, commonly chronic heart disease. Symptom duration before admission was longest in SARS-CoV-2 cases, though not significantly. Unilateral pneumonia occurred in about 25%, while bilateral pneumonia was more frequent in Influenza A. Respiratory failure affected around half the patients, with conventional oxygen therapy most used. Hospital stays were longest for Influenza A, but differences were not significant. Mortality was low overall, highest in SARS-CoV-2 patients (Table 1.)

The analysis of laboratory findings among hospitalized patients with Influenza A, RSV, and SARS-CoV-2 revealed significant differences in white blood cell (WBC) count and lymphocyte (LYM) count, while C-reactive protein (CRP) levels did not differ significantly, despite higher CRP levels in RSV patients. WBC count was significantly higher in RSV patients compared to Influenza A (p = 0.016). Lymphocyte count varied significantly, with RSV patients having higher counts than Influenza A patients (p = 0.036), while SARS-CoV-2 patients had significantly lower lymphocyte levels than both Influenza A (p = 0.006) and RSV patients (p 1.)

Comparative analysis of Non-Hospitalized patients with Influenza A/B, RSV, and SARS-CoV-2

Non-hospitalized Flu-A patients were significantly older than Flu-B patients (p p p = 0.006).

SARS-CoV-2 patients had the highest comorbidity burden, with nearly 65% having at least one comorbidity, followed by RSV (60%) and Flu-A (38.9%), while Flu-B had the lowest (28.6%). Chronic heart disease was most common in SARS-CoV-2 patients (51.1%), and RSV patients had the highest rate of chronic lung disease (28.6%). Diabetes was more prevalent in SARS-CoV-2 and Flu-A patients but lower in Flu-B and RSV. RSV patients had the longest symptom duration before seeking medical attention, averaging 4.4 days, significantly longer than Flu-A (p p = 0.045), and SARS- CoV-2 (p p = 0.019).

Among the remaining patients, the majority of Flu-A, Flu-B, and SARS-CoV-2 patients experienced symptom relief within 2–5 days. However, RSV cases had a more even distribution between those recovering within 2–5 days and those needing more than 5 days. The overall distribution of symptom resolution time did not differ significantly among virus groups (p = 0.171) (Table 2.)