Streptococcus pneumoniae: Nasal influenza vaccination, carriage density and transmission in families

The live attenuated influenza vaccine (LAIV) is offered in the United Kingdom to young children, protecting against influenza for those vaccinated and indirect protection for the wider community. It has also been shown to increase carriage density of Streptococcus pneumoniae, to an extent, in children. This study therefore investigates whether the vaccine leads to an increase in density in children and, if so, whether this augments transmission to household contacts. We implemented a randomised control study involving 405 two-year-old children and 958 household contacts. Nasopharyngeal swabs from all participants were taken over 5 visits, each two weeks apart, and tested for pneumococcal carriage. LAIV was given to 205 children at visit 1, and to 200 children at visit 2. We developed regression models to analyse the association between vaccination and whether an increase in pneumococcal density 14 and 28 days later was observed, as well as an increase in the odds of transmission to household members after administering LAIV. From regression analyses, there was a 2.5-fold (95%CI:1.3-4.3, p<0.001) increase in the odds of vaccinated children to have increased pneumococcal density 2 weeks later, compared with unvaccinated children, and a 2.1-fold (95%CI:1.2-3.6, p=0.011) increase in the odds of presumed transmission from children to their household contacts. Our results provide evidence that an attenuated influenza virus infection transiently increases the likelihood of pneumococcal transmission from children who are colonised with the bacterium to their contacts and that this increase is driven by an increase in bacterial abundance triggered by the vaccine.