Table 1 Categorization of data with definitions

Age

Newborn: up to 1 month

Infants: 2 months to 2 years

Children: 3–10 years

Adolescent: 11–18 years

Adults: ≥ 19 years

Elderly: ≥ 60 years

At-risk population

Populations at risk [27]:

1. Individuals with primary immunodeficiencies

2. Individuals with secondary immunodeficiencies caused by disease of the immune system: leukemia and other hematologic malignancies, human immunodeficiency virus infection

3. Critically sick intensive care unit patients

4. Recipients of drugs suppressing the immune system: anti-CD52, anti-CD20, anti-CD25, anti-tumor necrosis factor

General population: general population (eg, city or country)

Not at risk: population without any risk factor

Sex

Male: men

Women of reproductive age: defined as pregnant women, mothers of infants, reproductive-age women, women of childbearing age, teenage and adult women (not elderly). The reproductive age might be defined as age range 15 to 49 years [28]. This definition was used in this review, but if the publications included different age ranges for women of childbearing age or pregnant women, these data were included as well

Social status

As defined by authors of included study

Education level

As defined by authors of included study

Race/ethnicity

As defined by authors of included study

Developed or developing country

As categorized by the International Monetary Fund [29]