Maternal Influenza Vaccination and the Risk of Laboratory-Confirmed Influenza Among Household Contacts Under the Age of Five in Mali

In 2008, there were approximately one million influenza-associated severe acute lower respiratory infections among children under five worldwide; an estimated 180,000 cases occurred in Africa.1,2 However, these estimates are based on severely limited surveillance data on influenza incidence in Africa, particularly in sub-Saharan Africa. There is limited understanding of the burden of influenza in Africa, aside from surveillance data implemented in response to the H1N1 pandemic outbreak of 2009.3 In Mali, serial serosurveys pre- and post-pandemic influenza found 13% of serum samples became positive for H1N1.4 Household transmission is one of the primary contributors to the spread of influenza with approximately 42% of all infections estimated to occur in the household5 and the risk of infection due to contact with an infected household member (secondary infection rate [SIR]) is estimated at 38%.6 Studies on household influenza transmission have primarily been conducted in North America, Europe, and Asia,5–10 with a few studies in South Africa.11 However, data from low-income countries are of particular interest because the rate of household transmission is thought to depend on certain household attributes6 which differ substantially from those found in higher-income countries. Although immunization against influenza protects the vaccinated individual from illness, the impact on household transmission is unclear. Few studies report significant protection when only a few individuals in the household are vaccinated, suggesting lower vaccine effectiveness against infection in the household setting.6,7,10 Cocooning, or vaccinating adults to protect neonates in the household, has been recommended to protect high-risk populations against pertussis and influenza12; however there is no strong evidence to support this recommendation in the case of influenza. Studies in North America have found that vaccination of children younger than five years or school-aged children can protect household contacts and community members from influenza-like illnesses (ILIs).13,14 One study of influenza cocooning found that neonates in households with high vaccination rates were significantly less likely to experience acute respiratory infections than neonates in unvaccinated households.15 However, data are lacking on the impact of influenza vaccination of a single adult on the risk of influenza in young children, who are at greater risk of disease. Studies on household-level effects of individual vaccination are especially important to inform vaccine introduction in areas with minimal pre-existing vaccine coverage such as sub-Saharan Africa. Using data from a clinical trial of maternal influenza vaccination in Bamako, Mali, we aimed to examine the effect of vaccination of a single adult within the household (the pregnant woman) on the risk of influenza among household contacts younger than 5 years, and the effect of vaccination on the household SIR.