Table 2.

Summary of the main outcomes

Study	Main outcome
Anand et al., 2021 [22]	There was a highly significant association between some PD parameters and the risk of COVID-19 : plaque index (OR 7.01; 95% CI, 1.83 -26.94), gingival inflammation (OR, 17.65; 95% CI, 5.95 - 52.37), CAL (OR, 8.46; 95% CI, 3.47 to 20.63), periodontitis (OR, 11.75; 95% CI, 3.89 to 35.49).yet, there were no significant differences between the two groups in terms of missing teeth, carious teeth, and calculus scores.
Marouf et al., 2021 [25]	A significant association between periodontitis and COVID-19 complications including death (OR = 8.81, 95% CI 1.00–77.7), ICU admission (OR = 3.54, 95% CI 1.39–9.05) and the need for assisted ventilation (OR = 4.57, 95% CI 1.19–17.4). Furthermore, white blood cells, D-dimer and CRP levels were significantly higher in COVID-19 patients with PD.
Larvin et al 2020 [29]	There was no any association between painful/bleeding gum and the risk of COVID-19 (OR = 1.10, 95% CI = 0.72–1.69) or risk of hospital admission (OR = 0.90, 95% CI = 0.59–1.37). However, there was a significant association between PD and the mortality rate; participants with painful/bleeding gum had twofold higher mortality rate than those with healthy gum (OR = 1.71, 95% CI = 1.05–2.72). There was no a significant association between loose teeth with COVID-19 parameters: risk of COVID-19 (OR = 1.15, 95% CI = 0.84–1.59); hospital admission (OR = 1.55, 95% CI = 0.87–2.77); or mortality (OR = 1.85; 95% CI = 0.92–2.72)
Gupta et al. 2021 [23]	There is a significant association between PD and COVID-19 - outcomes. Higher severity of PD was associated with 7.45 odds of requiring assisted ventilation, 36.52 odds of hospital admission, 14.58 odds of death, and 4.42 odds of COVID-19 -related pneumonia
Holt et al. 2021 [40]	The results showed a significant increase in the risk of COVID-19 in PD subjects (OR: 1.20). However, after controlling all potential confounding factors, no significant association was observed between the two conditions.
Donders et al 2022 [31]	The results showed a significant association between PD (alveolar bone loss [OR: 5.60; 95%CI: 1.21; 25.99; P = 0.028] and tooth loss [OR: 1.04; 95%CI: 1.00; 1.09; P = 0.047]) with severity of COVID-19. However, such associations disappeared after adjusting for all potential confounders (P more than 0.05).
Larvin et al 2021 [30]	The risk for COVID-19 infection was not associated with periodontal disease across the 3 BMI categories: normal weight (OR, 0.97; 95% CI, 0.88 to 1.07), overweight (OR, 1.06; 95% CI, 0.98 to 1.15), and obese (OR, 1.08; 95% CI, 0.99 to 1.17). The risk of hospital admission for people with periodontal disease was 38% higher in participants who were overweight (HR, 1.38;95% CI, 1.02 to 1.87) and 124% higher in those who were obese (HR, 2.24; 95% CI, 1.66 to 3.03) compared to those of normal weigh. In addition, for participants with obesity, the mortality rate was much higher (hazard ratio, 3.11; 95% CI, 1.91 to 5.06) in participants with periodontal disease than those with healthy periodontium.
Costa et al., 2022 [37]	Periodontitis was significantly associated with ICU admission [IRR = 1.44 (95%CI = 1.07–1.95); p = 0.017], critical symptoms [IRR = 2.56 (95%CI = 1.44–4.55); p = 0.001], and risk of death [IRR = 2.05 (95%CI = 1.12–3.76); p = 0.020]
Kaur et al 2022 [42]	The risk of severe periodontal disease was 6.32 times higher in COVID-19 patients with severe symptoms (P= 0.024) as compared to mild COVID cases.
Mishra et al 2022 [44]	There was a significant association between periodontitis and severity of COVID-19 symptoms. The adjusted odds ratio (OR) of having severe COVID-19 in periodontitis patients was 2.8133 (0.4077–19.7523 at 95% CI, p = 0.004).
Kamel et al. 2021 [24]	There was a significant association between poor oral health and the severity of COVID-19. Additionally, poor oral health was significantly associated with delayed recovery time and higher CRP values.
Sirin & Ozcelik 2021 [47]	There was a significant association between dental disease including alveolar bone loss and severity of COVID-19 complications. Individuals with severe dental diseases (including alveolar bone loss) had more severe COVID-19 symptoms and higher mortality rate than those with no/mild dental diseases.
Koppolu et al., 2023 [43]	There was a statistically significant link between gingivitis and periodontitis. The majority of mild gingivitis cases (63%) was associated with the COVID-19-negative group, whereas the majority of severe gingivitis groups (85.7%) was associated with the COVID-19-positive group (χ2 = 9.94; P = 0.007). Similarly, the majority of Stage 1 periodontitis (62.9%) was associated with COVID-19-negative participants, whereas the majority of Stage 4 periodontitis (P = 0.007) was associated with COVID-19-positive groups (χ2 = 22.51; P = 0.047).
Sari et al., 2023 [46]	A higher prevalence of periodontitis compared with gingivitis and periodontal health was detected in the test group (p=0.003). In line with this, all clinical periodontal parameters related to periodontal disease severity were higher in the test group than in the control group. In particular, GI, BOP (%), PD, CAL, and the number of missing teeth were statistically significantly higher in the test group than in the control group (p < 0.05), except PI, which did not reach statistical significance (p = 0.052).
Said et al., 2023 [27]	Risk analysis of COVID-19 complications revealed that while periodontitis stage 2–4 (regardless of treatment) was associated with higher risk of complications (i.e. need for mechanical ventilation [AOR = 3.32, 95% CI 1.10–10.08, p = 0.034]), subjects with treated periodontitis had a lower risk than the non-treated ones. Adjusted OR analysis comparing treated and non-treated periodontitis (stages 2–4) revealed that treated patients were at lower risk of complications; however, this was not statistically significant.
Alnomay et al., 2023 [35]	Patients with periodontitis were 3-times 182 more likely to have COVID-19 complications than those without periodontitis (p = 0.025).
Guardado-Luevanos et al., 2022 [38]	A statistically significant difference (p value < 0.001) was observed, showing that positive self-RPD (n = 95, 85.1%) was often higher in SARS-CoV-2-positive individuals than in the controls (n = 66, 56.4%), with an OR of 3.3 (1.8–6.0) for SARS-CoV-2 infection in people with self-reported periodontal disease.
Gujar et al., 2022 [39]	Participants with COVID-19 had significantly higher mean values of plaque scores, number of mobile teeth, gingival bleeding scores, PD, REC, and CAL compared to the controls. The mean percentages of inter-proximal sites with PD ≥ 4 mm, PD ≥ 5 mm, CAL ≥ 3 mm, CAL ≥ 4 mm, and CAL ≥ 6 mm were also significantly higher in the case group than in the control group.
Wadhwa et al. 2022 [26]	Covid-19 infected patients had significantly greater alveolar bone loss and missing teeth than controls. Additionally, missing teeth and bone loss were associated with more hospitalization.
Poyato-Borrego et al., 2023 [45]	A significant association has been found between the severity of COVID-19 and the periodontal disease (p= 0.04).
Kalsi et al., 2023 [41]	The percentage of patients suffering from severe COVID disease was least (8.3%) in stage 1 of periodontitis and was highest (62.5%) in stage 4 of periodontitis. Whereas out of eight patients tested positive in stage 1 of periodontitis, 7 had only mild disease. Out of total of 8 patients with stage 4 category of periodontitis, two remain uninfected, five developed severe form of COVID, and only one patient had moderate COVID disease.
Bemquerer et al., 2023 [36]	In the COVID-19 group, plaque index was worse in patients with periodontitis than in patients without periodontitis (p < 0.001). Among individuals with COVID-19, periodontitis was associated with more hospitalization (p = 0.009), more days in the ICU (p = 0.042), admission to the SICU (p = 0.047), and higher need for oxygen therapy (p = 0.042). However, there was no difference in total hospitalization length, need for ICU, need for mechanical ventilation, or death between individuals with and without periodontitis in the COVID-19 group. Individuals with periodontitis were 1.13 times more likely to be hospitalized than individuals without periodontitis (CI = 1.01–1.26; p = 0.028).

PD periodontal disease, OR odds ratio, CI confidence interval, MR Mendelian randomization, IVW Inverse-variance weighted, CRP C-reactive protein