Risk of convulsions in children after monovalent H1N1 (2009) and trivalent influenza vaccines: a database study

The monovalent H1N1 (2009) pandemic influenza vaccine used predominantly in the UK in 2009/10 was a split virion vaccine with a novel oil-in-water adjuvant (ASO3). While this was highly immunogenic it was also reactogenic especially for fever in children. There is a paucity of comparative data on reactogenicity of trivalent influenza vaccine (TIV). Using the General Practice Research Database (GPRD) we investigated whether there was an increased risk of convulsions in children vaccinated with monovalent H1N1 influenza vaccine in the 2009/10 season and also the risk after vaccination with the seasonal TIVs using the self-controlled case-series method. A total of 2366 children aged under 10 years with at least one convulsion recorded in the GPRD and who had received at least one influenza vaccine at anytime (2858 doses of TIV and 1895 doses of the monovalent H1N1 influenza vaccine) were identified between May 2000 and April 2010. Over this period these 2366 children had a total of 3846 convulsion episodes. There was no increase in the incidence rate ratio (IRR) in the week after vaccination for either the monovalent H1N1 influenza vaccine (IRR 0.99, 95% CI 0.61-1.60) or the first dose of TIV (IRR 0.89, 95% CI 0.53-1.52). A signal of an elevated risk in the first few days after the second dose of monovalent H1N1 influenza vaccine was seen with an IRR for days 1-3 post vaccination of 3.48 (95% CI 0.86-14.07). This is consistent with findings of increased fever in a clinical trial. These results neither provide evidence of an increased risk of convulsions following TIV over a 10-year surveillance period nor following a single dose of the ASO3 adjuvanted monovalent H1N1 vaccine in 2009/10.