Trivalent inactivated influenza vaccine (TIV) is effective in preventing infection and illness associated with influenza A and B viruses in children during seasons when the vaccine components closely match circulating strains. On the basis of evidence from ecological studies, intervention trials, and simulation models, some health authorities have recommended vaccination of school‐age children against seasonal and pandemic influenza, not only to directly protect those children, but also to confer indirect protection on the general community by reducing transmission. Approximately one‐third of transmission is thought to occur within households, and children are more likely than adults to transmit infection to household contacts. However, there have been few detailed individual‐based studies of the indirect benefits to household contacts of vaccinating children. We therefore designed a randomized controlled trial to assess whether vaccinating children against seasonal influenza protects their household contacts. We began a pilot study of 120 households in 2008–2009, to be followed by a main study of 800 households in 2009–2010.
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The pandemic (H1N1) 2009 virus emerged in North America and rapidly spread worldwide. There has been intense interest in the effect of seasonal influenza vaccination on the risk of pandemic influenza infection, following results from a Canadian study that suggested that seasonal vaccine was associated with increased risk of pandemic influenza. Other studies have suggested that seasonal vaccine may confer no protection or partial protection against the pandemic virus. Taking advantage of the pilot study that was implemented during 2008–2009, and which thus covered the first wave of the pandemic, we investigated the effect of vaccination against seasonal influenza on the risk of pandemic influenza infection. The results for the original research question concerning direct and indirect benefits of vaccinating children in the household setting will be separately reported on completion of the main phase.
Recruitment and follow‐up of participants. Invitation letters were distributed via schools located within 3 km of our study clinic in Kowloon and to the families of members of a local birth cohort. Households who expressed an interest in the study were assessed for their eligibility to participate and were invited to our study clinic. Eligible households included at least 1 child aged 6–15 years who did not have any contraindications against injection of inactivated influenza vaccine, including allergy or hypersensitivity to eggs or other substances contained in the vaccine. Children who were prescribed immunosuppressive treatment or were otherwise immunocompromised were excluded.
One eligible child from each household was randomized to receive either a single dose of TIV (0.5 mL Vaxigrip; Sanofi Pasteur) or 0.5 mL of saline solution intramuscularly. The 2008–2009 TIV used in our study included the strains A/Brisbane/59/2007(H1N1)‐like, A/Brisbane/10/2007(H3N2)‐like, and B/Florida/4/2006. We hypothesized that vaccine‐naive children aged 6–8 years in Hong Kong may be more influenza‐experienced than those in Western temperate climates and thus might only require 1 vaccine dose, because a prior influenza infection could have already primed their immune system.