Combined results of five studies conducted from 2013 to 2016 show the nasal spray, or live attenuated influence vaccine (LAIV4), was less effective against influenza A/H1N1 than the injected, inactivated form of the flu vaccine (IIV) in all pediatric age groups. The meta-analysis "Live Attenuated and Inactivated Influenza Vaccine Effectiveness" will be published in the February 2019 issue of Pediatrics (published online January 7). It documents some of the data, previously reviewed by the American Academy of Pediatrics (AAP) and the Centers for Disease Control and Prevention (CDC), that led to these groups not recommending the nasal spray vaccine during the 2016–2017 and 2017–2018 seasons. Individual US studies had suggested LAIV4 offered less protection against H1N1, compared with IIV, since the 2013–2014 flu season. However, estimated effectiveness by age and prior season vaccination status had been limited by small sample sizes. For this pooled, individual patient-level data analysis of vaccine effectiveness, researchers looked at how many children who received either LAIV4 or IIV developed laboratory-confirmed cases of the flu in these five studies. Of 17,173 patients aged 2–17 years, 4,579 received IIV, 1,979 received LAIV4, and 10,615 were unvaccinated. It was found that vaccine effectiveness against influenza A/H1N1 was 67% (confidence interval 62% to 72%) for IIV and 20% (confidence interval -6% to 39%) for LAIV4. LAIV4 recipients had significantly higher odds of influenza A/H1N1 infection compared with IIV recipients (odds ratio=2.66; confidence interval 2.06 to 3.44). Results were similar when separated by prior season vaccination status. Vaccine effectiveness against influenza A/H3N2 and influenza B were similar for both types of flu vaccine.