Objectives:
Currently, the number of patients with coronavirus disease 2019 (COVID-19) has increased rapidly, but relationship between comorbidity and patients with COVID-19 is still not clear. Therefore, this review article has been conducted.

Do medical masks and N95 respirators offer similar protection against viral respiratory infection including coronavirus?

Study design:
This review article included 4 RCTs.
The studies had a low certainty evidence.

Objectives:
Previous studies have demonstrated some associations between dietary vitamin A intake and ovarian cancer risk with an inconsistent relationship. Therefore, this review article (meta-analysis) has been conducted.

Does higher dietary intake of vitamin A reduce ovarian cancer risk?

Study design:
This review article included 10 case-control studies and 5 cohort studies, involving 4,882 cases (persons with ovarian cancer) among 443,179 participants.

Objectives:
Does a high LDL cholesterol level (bad cholesterol) increase risk of Alzheimer's disease (AD)?

Study design:
This review article included 26 case-control studies involving 7,033 participants (2,266 Alzheimer's disease patients and 4,767 non-dementia controls).

Objectives:
Do fermented dairy foods and probiotics supplementation improve risk factors (such as metabolic syndrome, type 2 diabetes, cholesterol) of cardiovascular diseases?

Study design:
This review article included 20 prospective cohort studies and 52 RCTs.

Results and conclusions:
The investigators found in prospective cohort studies, that fermented milk was associated with a 4% reduction in risk of stroke, ischemic heart disease and cardiovascular mortality [RR = 0.96, 95% CI = 0.94 to 0.98].

Objectives:
Is there a difference in platelet count between COVID-19 patients with or without severe disease and is thrombocytopenia associated with severe COVID-19?

Study design:
This review article included 9 studies with 1,779 COVID-19 patients, of which 399 (22.4%) with severe disease.

Objectives:

Objectives:
Is there a causal relationship between dietary intake of dairy products and a positive effect on bone mineral density (BMD) in healthy postmenopausal women?

Objectives:
An epidemic of Coronavirus Disease 2019 (COVID-19) began in December 2019 in China leading to a Public Health Emergency of International Concern (PHEIC). Clinical, laboratory, and imaging features have been partially characterized in some observational studies. However, no systematic reviews on COVID-19 have been published to date. Therefore, this review article has been conducted.

What is the prevalence of symptoms of COVID-19 infection?

Objectives:
Does fish consumption decrease risk of cardiovascular disease, depression, mortality and cancer?
 
Study design:
This review article included a total of 34 meta-analyses of prospective observational studies, reporting summary relative risks (SRR) for 40 different outcomes obtained from 298 primary prospective cohort studies.

The summary relative risk (SRR) for each meta-analysis was recalculated by using a random-effects model.

Objectives:
A growing body of literature on the 2019 novel coronavirus (SARS-CoV-2) is becoming available, but a synthesis of available data has not been conducted. Therefore, this review article has been conducted.

What are the symptoms of COVID-19 or SARS-CoV-2 infection?

Study design:
This review article included a total of 61 studies with 59,254 patients.