Objectives:
Many studies on COVID-19 have reported diabetes to be associated with severe disease and mortality, however, the data is conflicting. Therefore, this review article has been conducted.

Does diabetes increase risk of COVID-19 infection?

Study design:
This review article included 33 case-control studies with 16,003 patients (8,849 (55%) were reported from Mainland China, 7,030 (44%) from USA and 124 (1%) from France). The pooled mean age was 52.6 ± 17.4 years.

Objectives:
What are the risk factors of COVID-19?

Study design:
This review article included 21 clinical studies with a total of 47,344 patients (24,419 male and 22,925 females).
The number of cases in the selected studies varied by approximately 3722-fold and ranged from 12 to 44,672 cases.
The sex ratio (male to female) was 1.06 and the overall average age of the subjects was greater than 40 years.

Objectives:
Multiple sclerosis (MS) is a demyelinating and disabling inflammatory disease of the central nervous system. Multiple sclerosis is triggered by complex environmental factors which mostly affect genetically the susceptible young people. Emerging data has suggested that changes of homocysteine (Hcy), vitamin B12 and folate serum levels may be associated with multiple sclerosis. However, previous findings are not always consistent. Therefore, this review aricle has been conducted.

Objectives:
What are the risk factors and laboratory abnormalities of COVID-19?

Study design:
This review article included 43 studies involving 3,600 patients.

Results and conclusions:   
The investigators found among COVID-19 patients, fever [83.3%, 95% CI = 78.4 to 87.7], cough [60.3%, 95% CI = 54.2 to 66.3] and fatigue [38.0%, 95% CI = 29.8 to 46.5] were the most common clinical symptoms.

Objectives:
The emergence of coronavirus disease 2019 (COVID-19) caused by SARS-CoV-2 made imperative the need for diagnostic tests that can identify the infection. Although Nucleic Acid Test (NAT) is considered to be the gold standard, serological tests based on antibodies could be very helpful. However, individual studies are usually inconclusive. Therefore, this review article has been conducted.

What is the specificity and sensitivity of all available antibody-tests for SARS-CoV-2 (COVID-19) infection?

Objectives:
Older people (≥60 years old) are particularly vulnerable to influenza virus infection and vaccine is effective in reducing the disease burden in this population. However, it remains obscure whether their antibody response is lower than those of younger adults (18-60 years old). Therefore, this review article has been conducted.

Objectives:
Does liver disease or chronic kidney disease increase risk of severity and mortality in COVID-19 patients?

Study design:
This review article included 22 studies with a total of 5,595 COVID-19 patients, of which 2,045 (36.55%) were female.
Where reported, 147/5,305 (2.77%) and 83/5,038 (1.65%) had comorbidities of liver diseases and chronic kidney disease, respectively.
The mean ±SD (range) of the sample sizes of all included studies was 254.32 ± 385.76 (29-1,591).

Objectives:
The coronavirus disease 2019 (COVID-19) has been found to be caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, comprehensive knowledge of COVID-19 remains incomplete and many important features are still unknown. Therefore, this review article has been conducted.

Objectives:
Conflicting recommendations exist related to whether masks have a protective effect on the spread of respiratory viruses. Therefore, this review article has been conducted.

Do face masks prevent spreading of laboratory-confirmed respiratory virus transmission?

Objectives:
Epidemiologic studies focusing on the association between 1-carbon metabolism-related vitamins (ie, folate, vitamin B6, vitamin B2, vitamin B12) and breast cancer risk have reported inconsistent findings. Therefore, this review article (meta-analysis) has been conducted.