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BACKGROUND Some studies have hypothesized that vitamin D may have a role to play in protection against influenza virus infections and illnesses, and that seasonal fluctuation in serum 25-hydroxyvitamin D [25(OH)D] may affect seasonal patterns of influenza virus infections. OBJECTIVE We aimed to investigate whether serum 25(OH)D concentrations were associated with the incidence of influenza virus infections and illnesses in children and adults in Hong Kong. METHODS In 2009-2010, 3030 children and adults of all ages from 796 households in Hong Kong were followed up to identify acute respiratory illnesses. Sera from 2694 participants were collected at baseline and after ∼1 mo, 6 mo, and 12 mo. Influenza virus infections were confirmed by reverse transcriptase-polymerase chain reaction performed on nasal and throat swab samples collected during illness episodes. Serologic evidence of influenza virus infection was measured by hemagglutination inhibition assays in unvaccinated participants. The serum 25(OH)D concentrations were measured after collection of all specimens. Each individual's baseline serum 25(OH)D concentration on 1 January 2010 was predicted by a random-effects linear regression model. RESULTS We found that, in children and adults who had not received a seasonal influenza vaccine, baseline serum 25(OH)D concentrations (

Vitamin D plays an important role in skeletal health throughout life. Some studies have hypothesised that vitamin D may reduce the risk of other diseases. Our study aimed to estimate age-specific and sex-specific serum 25-hydroxyvitamin D (25(OH)D) status and to identify the determinants of serum 25(OH)D status in Hong Kong, a subtropical city in southern China. In 2009–2010, households in Hong Kong were followed up to identify acute respiratory illnesses, and sera from 2694 subjects were collected in three to four different study phases to permit measurement of 25(OH)D levels at different times of the year. A questionnaire survey on diet and lifestyle was conducted among children, with simultaneous serum collection in April and May 2010. The mean of serum 25(OH)D levels in age groups ranged from 39 to 63 nmol/l throughout the year with the mean values in all age groups in spring below 50 nmol/l. Children aged 6–17 years, and girls and women had significantly lower serum 25(OH)D levels than adults, and boys and men, respectively (allP