Serum 25-Hydroxyvitamin D Was Not Associated with Influenza Virus Infection in Children and Adults in Hong Kong, 2009-2010

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-hydroxyvita min 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 similar to 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 (<50 nmol/L compared with >= 50 nmol/L) were not statistically significantly associated with serologic evidence of influenza A(H1N1)pdm09 (RR, 1.18; 95% CI: 0.85, 1.65) or seasonal influenza virus infections [including A(H3N2) and B virus] (RR, 1.13; 95% CI: 0.86, 1.49). In all participants, baseline serum 25(OH)D concentrations were not statistically significantly associated with polymerase chain reaction confirmed influenza virus infection (RR, 1.15; 95% CI: 0.73, 1.83) and influenza-like illness (RR, 1.18; 95% CI: 0.98, 1.43). Conclusions: These findings indicate that lower serum vitamin D concentrations may not contribute to the seasonality of influenza and are not associated with an increased risk of influenza virus infections in persons of all ages in Hong Kong.