may have saved lives. A 2010 study found a significantly enhanced immune response against the 2009 pandemic H1N1 in study participants who had received vaccination against the swine flu in 1976. The 2009 H1N1 “swine flu” outbreak resulted in the rapid approval of pandemic influenza vaccines. Pandemrix was quickly modified to target the circulating strain and by late 2010, 70 million people had received a dose. Eight years later, the BMJ gained access to vaccine pharmacovigilance reports compiled by GSK (GlaxoSmithKline) during the pandemic which the BMJ reported indicated death was 5.39 fold more likely with Pandemrix vs the other pandemic vaccines.. Quadrivalent vaccines for seasonal flu A quadrivalent flu vaccine administered by nasal mist was approved by the FDA in March 2012. Fluarix Quadrivalent was approved by the FDA in December 2012. In 2014, the Canadian National Advisory Committee on Immunization (NACI) published a review of quadrivalent influenza vaccines. Starting with the 2018–2019 influenza season most of the regular-dose egg-based flu shots and all the recombinant and cell-grown flu vaccines in the United States are quadrivalent. In the 2019–2020 influenza season all regular-dose flu shots and all recombinant influenza vaccine in the United States are quadrivalent. In November 2019, the FDA approved Fluzone High-Dose Quadrivalent for use in the United States starting with the 2020–2021 influenza season. In February 2020, the FDA approved Fluad Quadrivalent for use in the United States. In July 2020, the FDA approved both Fluad and Fluad Quadrivalent for use in the United States for the 2020–2021 influenza season. Medical uses The U.S Centers for Disease Control and Prevention (CDC) recommends the flu vaccine as the best way to protect people against the flu and prevent its spread. The flu vaccine can also reduce the severity of the flu if a person contracts a strain that the vaccine did not contain. It takes about two weeks following vaccination for protective antibodies to form. A 2012 meta-analysis found that flu vaccination was effective 67percent of the time; the populations that benefited the most were HIV-positive adults aged 18 to 55 (76percent), healthy adults aged 18 to 46 (approximately 70percent), and healthy children aged six months to 24 months (66percent). The influenza vaccine also appears to protect against myocardial infarction with a benefit of 15–45%. Effectiveness A vaccine is assessed by its efficacy – the extent to which it reduces risk of disease under controlled conditions –